HOME

TheInfoList




The human digestive system consists of the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
plus the accessory organs of digestion (the
tongue The tongue is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Brita ...

tongue
,
salivary gland The salivary glands in mammals are exocrine glands that produce saliva through a system of Duct (anatomy), ducts. Humans have three paired major salivary glands (Parotid gland, parotid, Submandibular gland, submandibular, and sublingual gland, sub ...
s,
pancreas The pancreas is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ ...

pancreas
,
liver The liver is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's t ...

liver
, and
gallbladder In vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotro ...

gallbladder
).
Digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such a ...
involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the
cephalic phase The nervous system, and endocrine system collaborate in the digestive system to control gastric secretions, and motility associated with the movement of food throughout the gastrointestinal tract, including peristalsis, and segmentation contractio ...
, the gastric phase, and the intestinal phase. The first stage, the cephalic phase of digestion, begins with gastric secretions in response to the sight and smell of food. This stage includes the mechanical breakdown of food by
chewing Chewing or mastication is the process by which food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to liv ...
, and the chemical breakdown by digestive enzymes, that takes place in the
mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

mouth
.
Saliva Saliva (commonly referred to as spit) is an extracellular fluid In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including t ...
contains
digestive enzyme Digestive may refer to: Biology *Digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of c ...
s called
amylase An amylase () is an enzyme Enzymes () are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one lo ...

amylase
, and
lingual lipase Lingual lipase is a member of a family of digestive enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemist ...
, secreted by the
salivary gland The salivary glands in mammals are exocrine glands that produce saliva through a system of Duct (anatomy), ducts. Humans have three paired major salivary glands (Parotid gland, parotid, Submandibular gland, submandibular, and sublingual gland, sub ...
s and
serous glands Serous glands secrete serous fluid. They contain serous acini, a grouping of serous cells that secrete serous fluid, isotonicity, isotonic with blood plasma, that contains enzymes such as alpha-amylase. Serous glands are most common in the parotid ...
on the tongue. The enzymes start to break down the food in the mouth. Chewing, in which the food is mixed with saliva, begins the mechanical process of digestion. This produces a bolus which can be swallowed down the
esophagus The esophagus (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United State ...

esophagus
to enter the
stomach The stomach is a muscular, in the of humans and many other animals, including several s. The stomach has a dilated structure and functions as a vital organ. In the digestive system the stomach is involved in the second phase of digestion, ...

stomach
. The second stage of digestion then begins in the stomach with the gastric phase. Here the food is further broken down by mixing with
gastric acid Gastric acid, gastric juice, or stomach acid, is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of for ...
until it passes into the
duodenum The duodenum is the first section of the small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from foo ...

duodenum
, the first part of the small intestine. The third stage begins in the duodenum with the intestinal phase, where partially digested food is mixed with a number of
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s produced by the
pancreas The pancreas is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ ...

pancreas
. Digestion is helped by the chewing of food carried out by the
muscles of mastication There are four classical muscles of mastication. During mastication, three muscles of mastication (''musculi masticatorii'') are responsible for adduction of the jaw, and one (the lateral pterygoid) helps to Abduction (kinesiology), abduct it. Al ...
, the tongue, and the
teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. ...
, and also by the
contractions
contractions
of
peristalsis Peristalsis is a radially symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take ...

peristalsis
, and
segmentation Segment or segmentation may refer to: Biology *Segmentation (biology), the division of body plans into a series of repetitive segments **Segmentation in the human nervous system *Internodal segment, the portion of a nerve fiber between two Nodes of ...
.
Gastric acid Gastric acid, gastric juice, or stomach acid, is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of for ...
, and the production of
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists o ...

mucus
in the stomach, are essential for the continuation of digestion. Peristalsis is the rhythmic contraction of
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s that begins in the esophagus and continues along the wall of the stomach and the rest of the gastrointestinal tract. This initially results in the production of
chyme Chyme or chymus (; from Ancient Greek, Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenumsmall intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...

small intestine
is absorbed as
chyle Chyle (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximat ...
into the
lymphatic system The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the circulatory system and the immune system. It is made up of a large network of lymph, lymphatic vessels, lymph nodes, lymphatic or lymphoid organs, an ...

lymphatic system
. Most of the digestion of food takes place in the small intestine. Water and some
minerals In geology Geology (from the Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers ...

minerals
are reabsorbed back into the blood in the colon of the
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human dig ...

large intestine
. The waste products of digestion (
feces Feces ( or faeces) is the solid or semi-solid remains of food that was not digested in the small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where mos ...

feces
) are defecated from the
rectum The rectum is the final straight portion of the large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the re ...

rectum
via the
anus The anus (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

anus
.


Components

There are several organs and other components involved in the digestion of food. The organs known as the accessory digestive organs are the
liver The liver is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's t ...

liver
,
gall bladder In vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotrop ...
and
pancreas The pancreas is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ ...

pancreas
. Other components include the
mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

mouth
,
salivary gland The salivary glands in mammals are exocrine glands that produce saliva through a system of Duct (anatomy), ducts. Humans have three paired major salivary glands (Parotid gland, parotid, Submandibular gland, submandibular, and sublingual gland, sub ...
s,
tongue The tongue is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Brita ...

tongue
,
teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. ...
and
epiglottis The epiglottis is a leaf-shaped flap in the throat In vertebrate anatomy, the throat is the front part of the neck, internally positioned in front of the vertebra, vertebrae. It contains the Human pharynx, pharynx and larynx. An important secti ...

epiglottis
. The largest structure of the digestive
system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purp ...
is the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
(GI tract). This starts at the mouth and ends at the
anus The anus (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

anus
, covering a distance of about nine metres. The largest part of the GI tract is the colon or
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human dig ...

large intestine
. Water is absorbed here and the remaining waste matter is stored prior to
defecation frame, Human anatomy of the ''anorecturm'' (anus and rectum">anus.html" ;"title="Human anatomy of the ''anorecturm'' (anus">Human anatomy of the ''anorecturm'' (anus and rectum) Defecation (or defaecation) is the final act of digestion, by whi ...
. Most of the digestion of food takes place in the
small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...

small intestine
which is the longest part of the GI tract. A major digestive organ is the
stomach The stomach is a muscular, in the of humans and many other animals, including several s. The stomach has a dilated structure and functions as a vital organ. In the digestive system the stomach is involved in the second phase of digestion, ...

stomach
. Within its
mucosa A mucous membrane or mucosa is a membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes (ou ...
are millions of embedded
gastric glands The gastric glands are located in different regions of the stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure a ...
. Their secretions are vital to the functioning of the organ. There are many specialised
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
of the GI tract. These include the various cells of the gastric glands, taste cells, pancreatic duct cells,
enterocyte Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biol ...
s and
microfold cell Microfold cells (or M cells) are found in the gut-associated lymphoid tissue (GALT) of the Peyer's patches in the small intestine The small intestine or small bowel is an organ in the gastrointestinal tract The gastrointestinal tract, (GI t ...
s. Some parts of the digestive system are also part of the
excretory system The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more gener ...
, including the large intestine.


Mouth

The
mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

mouth
is the first part of the upper gastrointestinal tract and is equipped with several structures that begin the first processes of digestion. These include salivary glands, teeth and the tongue. The mouth consists of two regions; the vestibule and the oral cavity proper. The vestibule is the area between the teeth, lips and cheeks, and the rest is the oral cavity proper. Most of the oral cavity is lined with
oral mucosa The oral mucosa is the mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers of Epithelium, epithelial c ...

oral mucosa
, a
mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers of Epithelium, epithelial cells overlying a layer of loose connect ...
that produces a lubricating
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists o ...
, of which only a small amount is needed. Mucous membranes vary in structure in the different regions of the body but they all produce a lubricating mucus, which is either secreted by surface cells or more usually by underlying glands. The mucous membrane in the mouth continues as the thin mucosa which lines the bases of the teeth. The main component of mucus is a
glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
called
mucin Mucins () are a family of high molecular weight A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical ph ...

mucin
and the type secreted varies according to the region involved. Mucin is viscous, clear, and clinging. Underlying the mucous membrane in the mouth is a thin layer of
smooth muscle tissue Smooth muscle is an involuntary non-striated muscle Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. The presence of sarcomeres manifests as a series of bands visible along the muscle fibers, ...

smooth muscle tissue
and the loose connection to the membrane gives it its great elasticity. It covers the cheeks, inner surfaces of the
lip Lips are a visible body part at the mouth of many animals, including humans. Lips are soft, movable, and serve as the opening for food intake and in the articulation of sound and speech. Human lips are a tactile sensory organ, and can be an ...

lip
s, and floor of the mouth, and the mucin produced is highly protective against
tooth decay Tooth decay, also known as dental caries or cavities, is the breakdown of teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the ...
. The roof of the mouth is termed the
palate The palate () is the roof of the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ...

palate
and it separates the oral cavity from the nasal cavity. The palate is hard at the front of the mouth since the overlying mucosa is covering a plate of
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
; it is softer and more pliable at the back being made of muscle and connective tissue, and it can move to swallow food and liquids. The
soft palate The soft palate (also known as the velum, palatal velum, or muscular palate) is, in mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was origin ...
ends at the
uvula The palatine uvula, usually referred to as simply the uvula, is a conic projection from the back edge of the middle of the soft palate The soft palate (also known as the velum, palatal velum, or muscular palate) is, in mammal Mammals (from ...
. The surface of the
hard palate The hard palate is a thin horizontal bony plate made up of two bones of the facial skeleton The facial skeleton comprises the ''facial bones'' that may attach to build a portion of the skull The skull is a bone A bone is a Stiffness, ri ...
allows for the pressure needed in eating food, to leave the nasal passage clear. The opening between the lips is termed the oral fissure, and the opening into the throat is called the fauces. At either side of the soft palate are the
palatoglossus muscle The palatoglossus or palatoglossal muscle is a muscle of the soft palate and extrinsic muscle of the tongue. Its surface is covered by oral mucosa and forms the visible palatoglossal arch. Structure Palatoglossus arises from the palatine aponeuro ...
s which also reach into regions of the tongue. These muscles raise the back of the tongue and also close both sides of the fauces to enable food to be swallowed. Mucus helps in the mastication of food in its ability to soften and collect the food in the formation of the bolus.


Salivary glands

There are three pairs of main
salivary gland The salivary glands in mammals are exocrine glands that produce saliva through a system of Duct (anatomy), ducts. Humans have three paired major salivary glands (Parotid gland, parotid, Submandibular gland, submandibular, and sublingual gland, sub ...
s and between 800 and 1,000 minor salivary glands, all of which mainly serve the digestive process, and also play an important role in the maintenance of dental health and general mouth lubrication, without which speech would be impossible.Ten Cate's Oral Histology, Nanci, Elsevier, 2013, page 275-276 The main glands are all
exocrine gland Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( ...
s, secreting via ducts. All of these glands terminate in the mouth. The largest of these are the
parotid gland The parotid gland is a major salivary gland The salivary glands in mammals are exocrine glands that produce saliva through a system of Duct (anatomy), ducts. Humans have three paired major salivary glands (Parotid gland, parotid, Submandibular ...
s—their secretion is mainly
serous In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physica ...
. The next pair are underneath the jaw, the
submandibular gland The paired submandibular glands (historically known as submaxillary glands) are major salivary gland The salivary glands in mammals are exocrine glands that produce saliva through a system of Duct (anatomy), ducts. Humans have three paired major ...

submandibular gland
s, these produce both
serous fluid In physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...
and
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists o ...
. The serous fluid is produced by
serous gland Serous glands secrete serous fluid. They contain serous acini, a grouping of serous cells that secrete serous fluid, isotonicity, isotonic with blood plasma, that contains enzymes such as alpha-amylase. Serous glands are most common in the parotid ...
s in these salivary glands which also produce
lingual lipase Lingual lipase is a member of a family of digestive enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemist ...
. They produce about 70% of the oral cavity saliva. The third pair are the
sublingual gland The paired sublingual glands are major salivary glands in the mouth. They are the smallest, most diffuse, and the only unencapsulated major salivary glands. They provide only 3-5% of the total salivary volume. There are also two other types of sal ...
s located underneath the tongue and their secretion is mainly mucous with a small percentage of saliva. Within the
oral mucosa The oral mucosa is the mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers of Epithelium, epithelial c ...

oral mucosa
, and also on the tongue, palates, and floor of the mouth, are the minor salivary glands; their secretions are mainly mucous and they are innervated by the
facial nerve The facial nerve (the labyrinthine segment) is the seventh Cranial nerves, cranial nerve, or simply CN VII. It emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste sensation ...

facial nerve
( CN7).Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012, p. 157 The glands also secrete
amylase An amylase () is an enzyme Enzymes () are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one lo ...
a first stage in the breakdown of food acting on the carbohydrate in the food to transform the starch content into maltose. There are other
serous gland Serous glands secrete serous fluid. They contain serous acini, a grouping of serous cells that secrete serous fluid, isotonicity, isotonic with blood plasma, that contains enzymes such as alpha-amylase. Serous glands are most common in the parotid ...
s on the surface of the tongue that encircle
taste bud Taste buds contain the taste receptor cells, which are also known as gustatory cells. The taste receptor A taste receptor is a type of Receptor (biochemistry), cellular receptor which facilitates the sensation of taste. When food or other su ...

taste bud
s on the back part of the tongue and these also produce lingual lipase.
Lipase A lipase (, ) is any enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into different molecules known as . Almost all ...
is a
digestive enzyme Digestive may refer to: Biology *Digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of c ...
that catalyses the
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord fo ...

hydrolysis
of
lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
s (fats). These glands are termed
Von Ebner's glands Von Ebner's glands, also called Ebner's glands or gustatory glands, are exocrine gland Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Gr ...
which have also been shown to have another function in the secretion of histatins which offer an early defense (outside of the immune system) against microbes in food, when it makes contact with these glands on the tongue tissue. Sensory information can stimulate the secretion of saliva providing the necessary fluid for the tongue to work with and also to ease swallowing of the food.


=Saliva

=
Saliva Saliva (commonly referred to as spit) is an extracellular fluid In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including t ...
moistens and softens food, and along with the chewing action of the teeth, transforms the food into a smooth bolus. The bolus is further helped by the lubrication provided by the saliva in its passage from the mouth into the esophagus. Also of importance is the presence in saliva of the digestive enzymes
amylase An amylase () is an enzyme Enzymes () are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one lo ...
and
lipase A lipase (, ) is any enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into different molecules known as . Almost all ...
. Amylase starts to work on the
starch Starch or amylum is a polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance ...
in
carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
s, breaking it down into the simple
sugars Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides o ...
of
maltose} Maltose ( or ), also known as maltobiose or malt sugar, is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disacchari ...

maltose
and
dextrose Glucose is a simple sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glyco ...

dextrose
that can be further broken down in the small intestine. Saliva in the mouth can account for 30% of this initial starch digestion. Lipase starts to work on breaking down
fat In nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided ...

fat
s. Lipase is further produced in the
pancreas The pancreas is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ ...

pancreas
where it is released to continue this digestion of fats. The presence of salivary lipase is of prime importance in young babies whose pancreatic lipase has yet to be developed. As well as its role in supplying
digestive enzyme Digestive may refer to: Biology *Digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of c ...
s, saliva has a cleansing action for the teeth and mouth. It also has an
immunological Immunology is a branch of biology that covers the study of immune systems in all organisms. Immunology charts, measures, and contextualizes the Physiology, physiological functioning of the immune system in states of both health and diseases; malf ...
role in supplying antibodies to the system, such as
immunoglobulin A File:3D structure of secretory IgA1.png, Two views, one rotated 90 degrees with respect to the other, of the amino acid chains comprising secretory IgA1. Colors are: H-chains (blue and light blue), L-chains (red and light red), J-chain (magenta) an ...
. This is seen to be key in preventing
infection An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host A host is a person responsible for guests at an event or for providing hospitality during it. Host may ...

infection
s of the salivary glands, importantly that of parotitis. Saliva also contains a
glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
called haptocorrin which is a binding protein to vitamin B12. It binds with the vitamin in order to carry it safely through the acidic content of the stomach. When it reaches the duodenum, pancreatic enzymes break down the glycoprotein and free the vitamin which then binds with
intrinsic factor Intrinsic factor (IF), also known as gastric intrinsic factor (GIF), is a glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within or ...
.


Tongue

Food enters the mouth where the first stage in the digestive process takes place, with the action of the
tongue The tongue is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Brita ...

tongue
and the secretion of saliva. The tongue is a fleshy and
muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the Uni ...
sensory organ A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to Stimulus (physiology), stimuli. (For example, in the human body, the brain receives signals from the senses ...
, and the first sensory information is received via the taste buds in the papillae on its surface. If the taste is agreeable, the tongue will go into action, manipulating the food in the mouth which stimulates the secretion of saliva from the salivary glands. The liquid quality of the saliva will help in the softening of the food and its enzyme content will start to break down the food whilst it is still in the mouth. The first part of the food to be broken down is the starch of carbohydrates (by the
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...
amylase An amylase () is an enzyme Enzymes () are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one lo ...
in the saliva). The tongue is attached to the floor of the mouth by a ligamentous band called the and this gives it great mobility for the manipulation of food (and
speech Speech is human vocal communication Communication (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''c ...

speech
); the range of manipulation is optimally controlled by the action of several muscles and limited in its external range by the stretch of the frenum. The tongue's two sets of muscles, are four intrinsic muscles that originate in the tongue and are involved with its shaping, and four extrinsic muscles originating in
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
that are involved with its movement.


=Taste

=
Taste The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, ph ...

Taste
is a form of
chemoreception A chemoreceptor, also known as chemosensor, is a specialized sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system In Biology, biology, the nervous system is a Complex system, highly complex ...
that takes place in the specialised
taste receptor A taste receptor is a type of cellular receptor In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and Signal_transduction, transduce signals that may be integrated into biological systems. T ...
s, contained in structures called
taste buds Taste buds contain the taste receptor cells, which are also known as gustatory cells. The taste receptors are located around the small structures known as lingual papillae, papillae found on the upper surface of the tongue, soft palate, upper e ...

taste buds
in the mouth. Taste buds are mainly on the upper surface (dorsum) of the tongue. The function of taste perception is vital to help prevent harmful or rotten foods from being consumed. There are also taste buds on the
epiglottis The epiglottis is a leaf-shaped flap in the throat In vertebrate anatomy, the throat is the front part of the neck, internally positioned in front of the vertebra, vertebrae. It contains the Human pharynx, pharynx and larynx. An important secti ...

epiglottis
and upper part of the
esophagus The esophagus (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United State ...

esophagus
. The taste buds are innervated by a branch of the facial nerve the
chorda tympani The chorda tympani is a branch of the facial nerve The facial nerve (the labyrinthine segment) is the seventh cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (f ...
, and the
glossopharyngeal nerve The glossopharyngeal nerve (), known as the ninth cranial nerve Cranial nerves are the nerve A nerve is an enclosed, cable-like bundle of nerve fibers called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerv ...
. Taste messages are sent via these
cranial nerves Cranial nerves are the nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, ...
to the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

brain
. The brain can distinguish between the chemical qualities of the food. The five
basic tastes The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, ph ...
are referred to as those of
saltiness The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, ph ...
,
sourness The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory re ...
, bitterness,
sweetness Sweetness is a basic taste most commonly perceived Perception (from the Latin ''perceptio'', meaning gathering or receiving) is the organization, identification, and interpretation of Sense, sensory information in order to represent ...

sweetness
, and
umami Umami ( from ja, 旨味 ), or savoriness, is one of the five basic tastes The gustatory system or sense of taste is the sensory system The sensory nervous system is a part of the nervous system In biology Biology is the nat ...

umami
. The detection of saltiness and sourness enables the control of salt and acid balance. The detection of bitterness warns of poisons—many of a plant's defences are of poisonous compounds that are bitter. Sweetness guides to those foods that will supply energy; the initial breakdown of the energy-giving carbohydrates by salivary amylase creates the taste of sweetness since simple sugars are the first result. The taste of umami is thought to signal protein-rich food. Sour tastes are acidic which is often found in bad food. The brain has to decide very quickly whether the food should be eaten or not. It was the findings in 1991, describing the first
olfactory The sense of smell, or olfaction, is the special sense In medicine Medicine is the Art (skill), art, science, and Praxis (process) , practice of caring for a patient and managing the diagnosis, prognosis, Preventive medicine, prevention, the ...
receptors that helped to prompt the research into taste. The olfactory receptors are located on cell surfaces in the
nose A nose is a protuberance in vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, l ...
which bind to chemicals enabling the detection of smells. It is assumed that signals from taste receptors work together with those from the nose, to form an idea of complex food flavours.


Teeth

Teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. ...
are complex structures made of materials specific to them. They are made of a bone-like material called
dentin Dentin () (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American English ...
, which is covered by the hardest tissue in the body— enamel. Teeth have different shapes to deal with different aspects of
mastication Chewing or mastication is the process by which food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such a ...
employed in tearing and chewing pieces of food into smaller and smaller pieces. This results in a much larger surface area for the action of digestive enzymes. The teeth are named after their particular roles in the process of mastication—
incisors Incisors (from Latin ''incidere'', "to cut") are the front teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, ...
are used for cutting or biting off pieces of food;
canines Canine may refer to: Zoology * dog-like mammals (i.e. members of the canid Canidae (; from Latin, ''canis'', "dog") is a biological family of dog-like carnivora Carnivora is an order of placental mammals that have specialized in primaril ...
, are used for tearing,
premolars The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...
and
molars The molars or molar teeth are large, flat teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium ...
are used for chewing and grinding. Mastication of the food with the help of saliva and mucus results in the formation of a soft bolus which can then be
swallowed Swallowing, sometimes called deglutition in scientific contexts, is the process in the human or animal body that allows for a substance to pass from the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of b ...
to make its way down the to the stomach. The digestive enzymes in saliva also help in keeping the teeth clean by breaking down any lodged food particles.


Epiglottis

The
epiglottis The epiglottis is a leaf-shaped flap in the throat In vertebrate anatomy, the throat is the front part of the neck, internally positioned in front of the vertebra, vertebrae. It contains the Human pharynx, pharynx and larynx. An important secti ...

epiglottis
is a flap of
elastic cartilage Elastic cartilage or yellow cartilage is a type of cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue, rubber-like padding that covers and protects the ends of long bone A bone is a Stiffness, rigid tissue (an ...
attached to the entrance of the
larynx The larynx (), commonly called the voice box, is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist ...

larynx
. It is covered with a mucous membrane and there are taste buds on its lingual surface which faces into the mouth. Its laryngeal surface faces into the larynx. The epiglottis functions to guard the entrance of the
glottis The glottis is the opening between the vocal folds In humans, vocal cords, also known as vocal folds or voice reeds, are folds of tissue in the throat that are key in creating sounds through vocalization. The size of vocal cords affects the ...
, the opening between the
vocal folds In humans, vocal cords, also known as vocal folds or voice reeds, are folds of tissue in the throat that are key in creating sounds through vocalization. The size of vocal cords affects the pitch of voice. Open when breathing and vibrating for ...

vocal folds
. It is normally pointed upward during breathing with its underside functioning as part of the pharynx, but during swallowing, the epiglottis folds down to a more horizontal position, with its upper side functioning as part of the pharynx. In this manner it prevents food from going into the trachea and instead directs it to the esophagus, which is behind. During swallowing, the backward motion of the tongue forces the epiglottis over the glottis' opening to prevent any food that is being swallowed from entering the larynx which leads to the lungs; the larynx is also pulled upwards to assist this process. Stimulation of the larynx by ingested matter produces a strong
cough reflex The cough reflex has both sensory ( afferent) and motor ( efferent) components, the former mainly via the vagus nerve The vagus nerve, historically cited as the pneumogastric nerve, is the tenth cranial nerve Cranial nerves are the nerve A ...
in order to protect the lungs.


Pharynx

The
pharynx The pharynx (plural: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea – the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, thou ...

pharynx
is a part of the
conducting zone The respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network Network and networking may refer to: ...
of the
respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network Network and networking may refer to: Arts, entertainment, and media * ''Network'' (1976 film), a 1976 Ame ...

respiratory system
and also a part of the digestive system. It is the part of the throat immediately behind the
nasal cavity The nasal cavity is a large, air-filled space above and behind the nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for Respiration (physiology), respiration alongside the mouth. Beh ...

nasal cavity
at the back of the mouth and above the esophagus and
larynx The larynx (), commonly called the voice box, is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist ...

larynx
. The pharynx is made up of three parts. The lower two parts—the
oropharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity The nasal cavity is a large, air-filled space above and behind the nose A nose is a protuberance in vertebrate Vertebrates () comprise all speci ...
and the
laryngopharynx The pharynx (plural: pharynges) is the part of the throat In vertebrate anatomy, the throat is the front part of the neck, positioned in front of the vertebra. It contains the Human pharynx, pharynx and larynx. An important section of it is the ...
are involved in the digestive system. The laryngopharynx connects to the esophagus and it serves as a passageway for both air and food. Air enters the larynx anteriorly but anything swallowed has priority and the passage of air is temporarily blocked. The pharynx is innervated by the pharyngeal plexus of the vagus nerve. Muscles in the pharynx push the food into the esophagus. The pharynx joins the esophagus at the oesophageal inlet which is located behind the
cricoid cartilage The cricoid cartilage , or simply cricoid (from the Greek ''krikoeides'' meaning "ring-shaped") or cricoid ring, is the only complete ring of cartilage around the Vertebrate trachea, trachea. It forms the back part of the larynx, voice box and func ...
.


Esophagus

The
esophagus The esophagus (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United State ...

esophagus
, commonly known as the foodpipe or gullet, consists of a muscular tube through which food passes from the pharynx to the stomach. The esophagus is continuous with the laryngopharynx. It passes through the posterior
mediastinum The mediastinum (from ) is the central compartment of the thoracic cavity 250px, The picture displays the Mediastinum on sagittal plane, Thoracic diaphragm">sagittal_plane.html" ;"title="Mediastinum on sagittal plane">Mediastinum on sagittal p ...

mediastinum
in the
thorax The thorax or chest is a part of the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Bioc ...

thorax
and enters the
stomach The stomach is a muscular, in the of humans and many other animals, including several s. The stomach has a dilated structure and functions as a vital organ. In the digestive system the stomach is involved in the second phase of digestion, ...

stomach
through a hole in the
thoracic diaphragm The thoracic diaphragm, or simply the diaphragm ( grc, διάφραγμα, diáphragma, partition), is a sheet of internal skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscu ...
—the
esophageal hiatus In human anatomy, the esophageal hiatus is an opening in the diaphragm through which the esophagus The esophagus (American English) or oesophagus (British English; American and British English spelling differences#ae and oe, see spelling dif ...
, at the level of the tenth
thoracic vertebra In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebra (anatomy), vertebrae and they are intermediate in size be ...

thoracic vertebra
(T10). Its length averages 25 cm, varying with an individual's height. It is divided into cervical,
thoracic The thorax or chest is a part of the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανι ...
and
abdominal The abdomen (colloquially called the belly, tummy, midriff or stomach) is the part of the body between the thorax The thorax or chest is a part of the anatomy of humans, mammals, other tetrapod animals located between the neck and the abdomen. I ...

abdominal
parts. The pharynx joins the esophagus at the esophageal inlet which is behind the
cricoid cartilage The cricoid cartilage , or simply cricoid (from the Greek ''krikoeides'' meaning "ring-shaped") or cricoid ring, is the only complete ring of cartilage around the Vertebrate trachea, trachea. It forms the back part of the larynx, voice box and func ...
. At rest the esophagus is closed at both ends, by the upper and lower esophageal sphincters. The opening of the upper sphincter is triggered by the swallowing reflex so that food is allowed through. The sphincter also serves to prevent back flow from the esophagus into the pharynx. The esophagus has a mucous membrane and the epithelium which has a protective function is continuously replaced due to the volume of food that passes inside the esophagus. During swallowing, food passes from the mouth through the pharynx into the esophagus. The epiglottis folds down to a more horizontal position to direct the food into the esophagus, and away from the trachea. Once in the esophagus, the bolus travels down to the stomach via rhythmic contraction and relaxation of muscles known as
peristalsis Peristalsis is a radially symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take ...

peristalsis
. The lower esophageal sphincter is a muscular sphincter surrounding the lower part of the esophagus. The Stomach#Sections, gastroesophageal junction between the esophagus and the stomach is controlled by the lower esophageal sphincter, which remains constricted at all times other than during swallowing and vomiting to prevent the contents of the stomach from entering the esophagus. As the esophagus does not have the same protection from acid as the stomach, any failure of this sphincter can lead to heartburn.


Diaphragm

The Thoracic diaphragm, diaphragm is an important part of the body's digestive system. The muscular diaphragm separates the thoracic cavity from the abdominal cavity where most of the digestive organs are located. The Suspensory muscle of the duodenum, suspensory muscle attaches the ascending duodenum to the diaphragm. This muscle is thought to be of help in the digestive system in that its attachment offers a wider angle to the duodenojejunal flexure for the easier passage of digesting material. The diaphragm also attaches to, and anchors the liver at its bare area of the liver, bare area. The esophagus enters the abdomen through a esophageal hiatus, hole in the diaphragm at the level of thoracic vertebrae, T10.


Stomach

The
stomach The stomach is a muscular, in the of humans and many other animals, including several s. The stomach has a dilated structure and functions as a vital organ. In the digestive system the stomach is involved in the second phase of digestion, ...

stomach
is a major organ of the gastrointestinal tract and digestive system. It is a consistently J-shaped organ joined to the esophagus at its upper end and to the duodenum at its lower end.
Gastric acid Gastric acid, gastric juice, or stomach acid, is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of for ...
(informally ''gastric juice''), produced in the stomach plays a vital role in the digestive process, and mainly contains hydrochloric acid and sodium chloride. A peptide hormone, gastrin, produced by G cells in the
gastric glands The gastric glands are located in different regions of the stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure a ...
, stimulates the production of gastric juice which activates the
digestive enzyme Digestive may refer to: Biology *Digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of c ...
s. Pepsinogen is a precursor enzyme (zymogen) produced by the gastric chief cells, and gastric acid activates this to the enzyme pepsin which begins the digestion of proteins. As these two chemicals would damage the stomach wall, mucus is secreted by innumerable gastric glands in the stomach, to provide a slimy protective layer against the damaging effects of the chemicals on the inner layers of the stomach. At the same time that protein is being digested, mechanical churning occurs through the action of
peristalsis Peristalsis is a radially symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take ...

peristalsis
, waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes. Gastric lipase secreted by the chief cells in the gastric glands, fundic glands in the gastric mucosa of the stomach, is an acidic lipase, in contrast with the alkaline pancreatic lipase. This breaks down fats to some degree though is not as efficient as the pancreatic lipase. The pylorus, the lowest section of the stomach which attaches to the
duodenum The duodenum is the first section of the small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from foo ...

duodenum
via the pyloric canal, contains countless glands which secrete digestive enzymes including gastrin. After an hour or two, a thick semi-liquid called
chyme Chyme or chymus (; from Ancient Greek, Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenumglycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
called
intrinsic factor Intrinsic factor (IF), also known as gastric intrinsic factor (GIF), is a glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within or ...
which is essential for the absorption of vitamin B12. Vitamin B12 (cobalamin), is carried to, and through the stomach, bound to a glycoprotein secreted by the salivary glands - transcobalamin, transcobalamin I also called haptocorrin, which protects the acid-sensitive vitamin from the acidic stomach contents. Once in the more neutral duodenum, pancreatic enzymes break down the protective glycoprotein. The freed vitamin B12 then binds to intrinsic factor which is then absorbed by the
enterocyte Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biol ...
s in the ileum. The stomach is a distensible organ and can normally expand to hold about one litre of food. This expansion is enabled by a series of gastric folds in the inner walls of the stomach. The stomach of a newborn baby will only be able to expand to retain about 30 ml.


Spleen

The spleen is the largest lymphoid organ in the body but has other functions. It breaks down both red and white blood cells that are ''spent''. This is why it is sometimes known as the 'graveyard of red blood cells'. A product of this ''digestion'' is the pigment bilirubin, which is sent to the
liver The liver is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's t ...

liver
and secreted in the bile. Another product is iron, which is used in the formation of new blood cells in the bone marrow. Medicine treats the spleen solely as belonging to the
lymphatic system The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the circulatory system and the immune system. It is made up of a large network of lymph, lymphatic vessels, lymph nodes, lymphatic or lymphoid organs, an ...

lymphatic system
, though it is acknowledged that the full range of its important functions is not yet understood.


Liver

The Human liver, liver is the second largest organ (after the skin) and is an accessory digestive gland which plays a role in the body's metabolism. The liver has many functions some of which are important to digestion. The liver can detoxify various metabolites; synthesise proteins and produce biochemicals needed for digestion. It regulates the storage of glycogen which it can form from glucose (glycogenesis). The liver can also synthesise glucose from certain amino acids. Its digestive functions are largely involved with the breaking down of
carbohydrate A carbohydrate () is a biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a ...
s. It also maintains protein metabolism in its synthesis and degradation. In
lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
metabolism it synthesises cholesterol. Fats are also produced in the process of lipogenesis. The liver synthesises the bulk of lipoproteins. The liver is located in the upper right quadrant of the abdomen and below the diaphragm to which it is attached at one part, the bare area of the liver. This is to the right of the stomach and it overlies the
gall bladder In vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotrop ...
. The liver synthesises bile acids and lecithin to promote the digestion of fat.


Bile

Bile produced by the liver is made up of water (97%), bile salts, mucus and pigment (biology), pigments, 1% fats and inorganic salts. Bilirubin is its major pigment. Bile acts partly as a surfactant which lowers the surface tension between either two liquids or a solid and a liquid and helps to emulsify the fats in the
chyme Chyme or chymus (; from Ancient Greek, Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenumlipase A lipase (, ) is any enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into different molecules known as . Almost all ...
to work on. Lipase digests the triglycerides which are broken down into two fatty acids and a monoglyceride. These are then absorbed by intestinal villi, villi on the intestinal wall. If fats are not absorbed in this way in the small intestine problems can arise later in the large intestine which is not equipped to absorb fats. Bile also helps in the absorption of vitamin K from the diet. Bile is collected and delivered through the common hepatic duct. This duct joins with the cystic duct to connect in a common bile duct with the gallbladder. Bile is stored in the gallbladder for release when food is discharged into the duodenum and also after a few hours.


Gallbladder

The
gallbladder In vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotro ...

gallbladder
is a hollow part of the biliary tract that sits just beneath the liver, with the gallbladder body resting in a small depression. It is a small organ where the bile produced by the liver is stored, before being released into the small intestine. Bile flows from the liver through the bile ducts and into the gall bladder for storage. The bile is released in response to cholecystokinin (CCK) a peptide hormone released from the duodenum. The production of CCK (by endocrine cells of the duodenum) is stimulated by the presence of fat in the duodenum. It is divided into three sections, a fundus, body and neck. The neck tapers and connects to the biliary tract via the cystic duct, which then joins the common hepatic duct to form the common bile duct. At this junction is a mucosal fold called ''Hartmann's pouch'', where gallstones commonly get stuck. The muscular layer of the body is of smooth muscle tissue that helps the gallbladder contract, so that it can discharge its bile into the bile duct. The gallbladder needs to store bile in a natural, semi-liquid form at all times. Hydrogen ions secreted from the inner lining of the gallbladder keep the bile acidic enough to prevent hardening. To dilute the bile, water and electrolytes from the digestion system are added. Also, salts attach themselves to cholesterol molecules in the bile to keep them from crystallization, crystallising. If there is too much cholesterol or bilirubin in the bile, or if the gallbladder doesn't empty properly the systems can fail. This is how gallstones form when a small piece of calcium gets coated with either cholesterol or bilirubin and the bile crystallises and forms a gallstone. The main purpose of the gallbladder is to store and release bile, or ''gall''. Bile is released into the small intestine in order to help in the digestion of fats by breaking down larger molecules into smaller ones. After the fat is absorbed, the bile is also absorbed and transported back to the liver for reuse.


Pancreas

The
pancreas The pancreas is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ ...

pancreas
is a major organ functioning as an accessory digestive gland in the digestive system. It is both an endocrine gland and an
exocrine gland Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( ...
. The endocrine part secretes insulin when the blood sugar becomes high; insulin moves glucose from the blood into the muscles and other tissues for use as energy. The endocrine part releases glucagon when the blood sugar is low; glucagon allows stored sugar to be broken down into glucose by the liver in order to re-balance the sugar levels. The pancreas produces and releases important digestive enzymes in the pancreatic juice that it delivers to the duodenum. The pancreas lies below and at the back of the stomach. It connects to the duodenum via the pancreatic duct which it joins near to the bile duct's connection where both the bile and pancreatic juice can act on the
chyme Chyme or chymus (; from Ancient Greek, Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenumpancreatic duct cells contain bicarbonate ions which are alkaline and help with the bile to neutralise the acidic chyme that is churned out by the stomach. The pancreas is also the main source of enzymes for the digestion of
fat In nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided ...

fat
s and proteins. Some of these are released in response to the production of cholecystokinin, CKK in the duodenum. (The enzymes that digest polysaccharides, by contrast, are primarily produced by the walls of the intestines.) The cells are filled with secretory granules containing the precursor digestive enzymes. The major proteases, the pancreatic enzymes which work on proteins, are trypsinogen and chymotrypsinogen. Elastase is also produced. Smaller amounts of lipase and amylase are secreted. The pancreas also secretes phospholipase A2, lysophospholipase, and cholesterol esterase. The precursor zymogens, are inactive variants of the enzymes; which avoids the onset of pancreatitis caused by autodegradation. Once released in the intestine, the enzyme enteropeptidase present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin; further cleavage results in chymotripsin.


Lower gastrointestinal tract

The lower gastrointestinal tract (GI), includes the
small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...

small intestine
and all of the
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human dig ...

large intestine
. The intestine is also called the bowel or the gut. The lower GI starts at the pyloric sphincter of the stomach and finishes at the anus. The small intestine is subdivided into the
duodenum The duodenum is the first section of the small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from foo ...

duodenum
, the jejunum and the ileum. The cecum marks the division between the small and large intestine. The large intestine includes the rectum and anal canal.


Small intestine

Partially digested food starts to arrive in the
small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...

small intestine
as semi-liquid
chyme Chyme or chymus (; from Ancient Greek, Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenumduodenum The duodenum is the first section of the small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from foo ...

duodenum
by the addition of bile from the gall bladder combined with the bicarbonate secretions from the pancreatic duct and also from secretions of bicarbonate-rich mucus from duodenal glands known as Brunner's glands. The chyme arrives in the intestines having been released from the stomach through the opening of the pyloric sphincter. The resulting alkaline fluid mix neutralises the gastric acid which would damage the lining of the intestine. The mucus component lubricates the walls of the intestine. When the digested food particles are reduced enough in size and composition, they can be absorbed by the intestinal wall and carried to the bloodstream. The first receptacle for this chyme is the duodenal bulb. From here it passes into the first of the three sections of the small intestine, the duodenum. (The next section is the jejunum and the third is the ileum). The duodenum is the first and shortest section of the small intestine. It is a hollow, jointed C-shaped tube connecting the stomach to the jejunum. It starts at the duodenal bulb and ends at the suspensory muscle of duodenum. The attachment of the suspensory muscle to the diaphragm is thought to help the passage of food by making a wider angle at its attachment. Most food digestion takes place in the small intestine. Segmentation contractions act to mix and move the chyme more slowly in the small intestine allowing more time for absorption (and these continue in the large intestine). In the duodenum, pancreatic lipase is secreted together with a co-enzyme, colipase to further digest the fat content of the chyme. From this breakdown, smaller particles of emulsified fats called chylomicrons are produced. There are also digestive cells called
enterocyte Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells Epithelium () is one of the four basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biol ...
s lining the intestines (the majority being in the small intestine). They are unusual cells in that they have intestinal villus, villi on their surface which in turn have innumerable microvilli on their surface. All these villi make for a greater surface area, not only for the absorption of chyme but also for its further digestion by large numbers of digestive enzymes present on the microvilli. The chylomicrons are small enough to pass through the enterocyte villi and into their lymph capillaries called lacteals. A milky fluid called
chyle Chyle (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximat ...
, consisting mainly of the emulsified fats of the chylomicrons, results from the absorbed mix with the lymph in the lacteals. Chyle is then transported through the
lymphatic system The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the circulatory system and the immune system. It is made up of a large network of lymph, lymphatic vessels, lymph nodes, lymphatic or lymphoid organs, an ...

lymphatic system
to the rest of the body. The suspensory muscle marks the end of the duodenum and the division between the upper gastrointestinal tract and the lower GI tract. The digestive tract continues as the jejunum which continues as the ileum. The jejunum, the midsection of the small intestine contains circular folds, flaps of doubled mucosal membrane which partially encircle and sometimes completely encircle the lumen (anatomy), lumen of the intestine. These folds together with villi serve to increase the surface area of the jejunum enabling an increased absorption of digested sugars, amino acids and fatty acids into the bloodstream. The circular folds also slow the passage of food giving more time for nutrients to be absorbed. The last part of the small intestine is the ileum. This also contains villi and vitamin B12; bile acids and any residue nutrients are absorbed here. When the chyme is exhausted of its nutrients the remaining waste material changes into the semi-solids called Human feces, feces, which pass to the large intestine, where bacteria in the gut flora further break down residual proteins and starches. Transit time through the small intestine is an average of 4 hours. Half of the food residues of a meal have emptied from the small intestine by an average of 5.4 hours after ingestion. Emptying of the small intestine is complete after an average of 8.6 hours.


Cecum

The cecum is a pouch marking the division between the small intestine and the large intestine. It lies below the ileocecal valve in the Quadrants and regions of abdomen, lower right quadrant of the abdomen. The cecum receives chyme from the last part of the small intestine, the ileum, and connects to the ascending colon of the large intestine. At this junction there is a sphincter or valve, the ileocecal valve which slows the passage of chyme from the ileum, allowing further digestion. It is also the site of the vermiform appendix, appendix attachment.


Large intestine

In the
large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human dig ...

large intestine
, the passage of the digesting food in the Large intestine#Structure, colon is a lot slower, taking from 30 to 40 hours until it is removed by
defecation frame, Human anatomy of the ''anorecturm'' (anus and rectum">anus.html" ;"title="Human anatomy of the ''anorecturm'' (anus">Human anatomy of the ''anorecturm'' (anus and rectum) Defecation (or defaecation) is the final act of digestion, by whi ...
. The colon mainly serves as a site for the fermentation of digestible matter by the gut flora. The time taken varies considerably between individuals. The remaining semi-solid waste is termed feces and is removed by the coordinated contractions of the intestinal walls, termed
peristalsis Peristalsis is a radially symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take ...

peristalsis
, which propels the excreta forward to reach the
rectum The rectum is the final straight portion of the large intestine The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the re ...

rectum
and exit via defecation from the anus. The wall has an outer layer of longitudinal muscles, the taeniae coli, and an inner layer of circular muscles. The circular muscle keeps the material moving forward and also prevents any back flow of waste. Also of help in the action of peristalsis is the basal electrical rhythm that determines the frequency of contractions. The taeniae coli can be seen and are responsible for the bulges (haustrum (anatomy), haustra) present in the colon. Most parts of the GI tract are covered with serous membranes and have a mesentery. Other more muscular parts are lined with adventitia.


Blood supply

The digestive system is supplied by the celiac artery. The celiac artery is the first major branch from the abdominal aorta, and is the only major artery that nourishes the digestive organs. There are three main divisions – the left gastric artery, the common hepatic artery and the splenic artery. The celiac artery supplies the liver, stomach, spleen and the upper 1/3 of the duodenum (to the sphincter of Oddi) and the pancreas with oxygenated blood. Most of the blood is returned to the liver via the portal venous system for further processing and detoxification before returning to the circulatory system, systemic circulation via the hepatic veins. The next branch from the abdominal aorta is the superior mesenteric artery, which supplies the regions of the digestive tract derived from the midgut, which includes the distal 2/3 of the duodenum, jejunum, ileum, cecum, appendix, ascending colon, and the proximal 2/3 of the transverse colon. The final branch which is important for the digestive system is the inferior mesenteric artery, which supplies the regions of the digestive tract derived from the hindgut, which includes the distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum, and the anus above the pectinate line. Blood flow to the digestive tract reaches its maximum 20–40 minutes after a meal and lasts for 1.5–2 hours.


Nerve supply

The enteric nervous system consists of some one hundred million neurons that are embedded in the peritoneum, the lining of the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
extending from the esophagus to the anus. These neurons are collected into two plexuses - the myenteric plexus, myenteric (or Auerbach's) plexus that lies between the longitudinal and the smooth muscle layers, and the submucosal plexus, submucosal (or Meissner's) plexus that lies between the circular smooth muscle layer and the mucosa. Parasympathetic nervous system, Parasympathetic innervation to the ascending colon is supplied by the vagus nerve. Sympathetic nervous system, Sympathetic innervation is supplied by the splanchnic nerves that join the celiac ganglia. Most of the digestive tract is innervated by the two large celiac ganglia, with the upper part of each ganglion joined by the Thoracic splanchnic nerves, greater splanchnic nerve and the lower parts joined by the Thoracic splanchnic nerves, lesser splanchnic nerve. It is from these ganglia that many of the gastric plexuses arise.


Development

Early in human embryogenesis, embryonic development, the embryo has three germ layers and abuts a yolk sac. During the second week of development, the embryo grows and begins to surround and envelop portions of this sac. The enveloped portions form the basis for the adult gastrointestinal tract. Sections of this foregut begin to differentiate into the organs of the gastrointestinal tract, such as the
esophagus The esophagus (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United State ...

esophagus
,
stomach The stomach is a muscular, in the of humans and many other animals, including several s. The stomach has a dilated structure and functions as a vital organ. In the digestive system the stomach is involved in the second phase of digestion, ...

stomach
, and intestines. During the fourth week of development, the stomach rotates. The stomach, originally lying in the midline of the embryo, rotates so that its body is on the left. This rotation also affects the part of the gastrointestinal tube immediately below the stomach, which will go on to become the
duodenum The duodenum is the first section of the small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from foo ...

duodenum
. By the end of the fourth week, the developing duodenum begins to spout a small outpouching on its right side, the hepatic diverticulum, which will go on to become the hepatobiliary system, biliary tree. Just below this is a second outpouching, known as the ''cystic diverticulum'', that will eventually develop into the gallbladder.


Clinical significance

Each part of the digestive system is subject to a wide range of disorders many of which can be congenital disorder, congenital. Oral and maxillofacial pathology, Mouth diseases can also be caused by pathogenic bacteria, viruses, fungus, fungi and as a side effect of some medications. Mouth diseases include tongue diseases and salivary gland pathology, salivary gland diseases. A common gums, gum disease in the mouth is gingivitis which is caused by bacteria in dental plaque, plaque. The most common viral infection of the mouth is herpetic gingivostomatitis, gingivostomatitis caused by herpes simplex. A common mycosis, fungal infection is oral candidiasis, candidiasis commonly known as ''thrush'' which affects the
mucous membrane A mucous membrane or mucosa is a biological membrane, membrane that lines various cavities in the body and covers the surface of internal organs. It consists of one or more layers of Epithelium, epithelial cells overlying a layer of loose connect ...
s of the mouth. There are a number of esophageal diseases such as the development of Schatzki rings that can restrict the passageway, causing difficulties in swallowing. They can also completely block the esophagus. Stomach diseases are often chronic conditions and include gastroparesis, gastritis, and peptic ulcers. A number of problems including malnutrition and anemia can arise from malabsorption, the abnormal absorption of nutrients in the GI tract. Malabsorption can have many causes ranging from
infection An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host A host is a person responsible for guests at an event or for providing hospitality during it. Host may ...

infection
, to enzyme deficiencies such as exocrine pancreatic insufficiency. It can also arise as a result of other gastrointestinal diseases such as coeliac disease. Coeliac disease is an autoimmune disorder of the small intestine. This can cause avitaminosis, vitamin deficiencies due to the improper absorption of nutrients in the small intestine. The small intestine can also be bowel obstruction, obstructed by a volvulus, a loop of intestine that becomes twisted enclosing its attached mesentery. This can cause mesenteric ischemia if severe enough. A common disorder of the bowel is diverticulitis. Diverticula are small pouches that can form inside the bowel wall, which can become inflamed to give diverticulitis. This disease can have complications if an inflamed diverticulum bursts and infection sets in. Any infection can spread further to the lining of the abdomen (peritoneum) and cause potentially fatal peritonitis. Crohn's disease is a common chronic inflammatory bowel disease (IBD), which can affect any part of the GI tract, but it mostly starts in the terminal ileum. Ulcerative colitis an ulcerative form of colitis, is the other major inflammatory bowel disease which is restricted to the colon and rectum. Both of these IBDs can give an increased risk of the development of colorectal cancer. Ulcerative colitis is the most common of the IBDs Irritable bowel syndrome (IBS) is the most common of the functional gastrointestinal disorders. These are idiopathic disorders that the Rome process has helped to define. Giardiasis is a disease of the small intestine caused by a protist parasite ''Giardia lamblia''. This does not spread but remains confined to the lumen of the small intestine. It can often be asymptomatic, but as often can be indicated by a variety of symptoms. Giardiasis is the most common pathogenic parasitic disease, parasitic infection in humans. There are diagnostic tools mostly involving the ingestion of barium sulphate to investigate disorders of the GI tract. These are known as upper gastrointestinal series that enable medical imaging, imaging of the pharynx, larynx, oesophagus, stomach and small intestine and lower gastrointestinal series for imaging of the colon.


In pregnancy

Gestation can predispose for certain digestive disorders. Gestational diabetes can develop in the mother as a result of pregnancy and while this often presents with few symptoms it can lead to pre-eclampsia.


History

In the early 11th century the Islamic medical philosopher Avicenna wrote extensively on many subjects including medicine. Forty of these treatises on medicine survive, and in the most famous one titled the ''Canon of Medicine'' he discusses "rising gas". Avicenna believed that digestive system dysfunction was responsible for the overproduction of gas in the gastrointestinal tract. He suggested lifestyle changes and a compound of herbal drugs for its treatment. In 1497 Alessandro Benedetti viewed the stomach as an unclean organ separated off by the diaphragm. This view of the stomach and intestines as being base organs was generally held until the mid-17th century. In the Renaissance of the 16th century Leonardo da Vinci produced some early drawings of the stomach and intestines. He thought that the digestive system aided the respiratory system. Andreas Vesalius provided some early anatomical drawings of the abdominal organs in the 16th century. In the middle of the 17th century a Flemish physician Jan Baptist van Helmont offered the first Jan Baptist van Helmont#Observations on digestion, chemical account of digestion which was later described as being very close to the later conceptualised enzyme. In 1653 William Harvey described the intestines in terms of their length, their blood supply, the mesenteries, and fat (adenylyl cyclase). In 1823 William Prout discovered hydrochloric acid in the gastric juice. In 1895 Ivan Pavlov described its secretion as being stimulated by a neurologic reflex with the vagus nerve having a crucial role. Black in the 19th century suggested an association of histamine with this secretion. In 1916 Popielski described histamine as a gastric secretagogue of hydrochloric acid. William Beaumont was an army surgeon who in 1825 was able to observe digestion as it took place in the stomach.William Beaumont Papers (1812-1959)
''National Library of Medicine''
This was made possible by experiments on a man with a stomach wound that did not fully heal leaving an opening into the stomach. The churning motion of the stomach was described among other findings. In the 19th century it was accepted that chemical processes were involved in the process of digestion. Gastrointestinal physiology, Physiological research into secretion and the gastrointestinal tract was pursued with experiments undertaken by Claude Bernard, Rudol[ph Heidenhain and Ivan Pavlov. The rest of the 20th century was dominated by research into enzymes. The first to be discovered was secretin by Ernest Starling in 1902, with ensuing results from John Edkins in 1905 who first suggested gastrin with its structure being determined in 1964. Andre Latarjet and Lester Dragstedt found a role for acetylcholine in the digestive system. In 1972 H₂-receptor antagonist, H2 receptor agonists were described by J. Black, that block the action of histamine and decrease the production of hydrochloric acid. In 1980 proton pump inhibitors were described by Sachs. In 1983 the role of ''Helicobacter pylori'' in the formation of ulcers was described by Barry Marshall, and Robin Warren. Art historians have often noted that banqueters on iconographic records of ancient Mediterranean societies almost always appear to be lying down on their left sides. One possible explanation could lie in the anatomy of the stomach and in the digestive mechanism. When lying on the left, the food has room to expand because the curvature of the stomach is enhanced in that position.


See also

*Ulcerative colitis *Abdominal internal oblique muscle


References

{{Authority control Organ systems Digestive system, Metabolism