Gastrointestinal physiology is the branch of

human physiology The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the viability of the human body. It comprises a head, ...

that addresses the physical function of the gastrointestinal (GI) tract. The function of the GI tract is to process ingested food by mechanical and chemical means, extract nutrients and excrete waste products. The GI tract is composed of the alimentary canal, that runs from the mouth to the anus, as well as the associated glands, chemicals, hormones, and enzymes that assist in digestion. The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation. The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.

Motility

The gastrointestinal tract generates

motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

using smooth muscle subunits linked by

gap junctions Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulate ...

. These subunits fire spontaneously in either a tonic or a phasic fashion. Tonic contractions are those contractions that are maintained from several minutes up to hours at a time. These occur in the sphincters of the tract, as well as in the anterior stomach. The other type of contractions, called phasic contractions, consist of brief periods of both relaxation and contraction, occurring in the posterior stomach and the small intestine, and are carried out by the

muscularis externa The muscular layer (muscular coat, muscular fibers, muscularis propria, muscularis externa) is a region of muscle in many organs in the vertebrate body, adjacent to the submucosa. It is responsible for gut movement such as peristalsis. The Latin, ...

. Motility may be overactive (hypermotility), leading to diarrhea or vomiting, or underactive (hypomotility), leading to constipation or vomiting; either may cause abdominal pain.

Stimulation

The stimulation for these contractions likely originates in modified smooth muscle cells called

interstitial cells of Cajal Interstitial cells of Cajal (ICC) are interstitial cells found in the gastrointestinal tract. There are different types of ICC with different functions. ICC and another type of interstitial cell, known as platelet-derived growth factor recepto ...

. These cells cause spontaneous cycles of

slow wave potential A slow-wave potential is a rhythmic electrophysiological event in the gastrointestinal tract. The normal conduction of slow waves is one of the key regulators of gastrointestinal motility. Slow waves are generated and propagated by a class of pac ...

s that can cause

action potentials An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ca ...

in smooth muscle cells. They are associated with the contractile smooth muscle via gap junctions. These slow wave potentials must reach a threshold level for the action potential to occur, whereupon Ca²⁺ channels on the smooth muscle open and an action potential occurs. As the contraction is graded based upon how much Ca²⁺ enters the cell, the longer the duration of slow wave, the more action potentials occur. This, in turn, results in greater contraction force from the smooth muscle. Both amplitude and duration of the slow waves can be modified based upon the presence of

neurotransmitters A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotr ...

hormones A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required fo ...

or other

paracrine signaling Paracrine signaling is a form of cell signaling, a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse ove ...

. The number of slow wave potentials per minute varies based upon the location in the digestive tract. This number ranges from 3 waves/min in the stomach to 12 waves/min in the intestines.

Contraction patterns

The patterns of GI contraction as a whole can be divided into two distinct patterns,

peristalsis Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which ...

and segmentation. Occurring between meals, the

migrating motor complex Migrating motor complex, also known as migrating myoelectric complex, migratory motor complex, migratory myoelectric complex and MMC, is a pattern of electrical activity observed in the gastrointestinal tract in a regular cycle during fasting. MMC w ...

is a series of peristaltic wave cycles in distinct phases starting with relaxation, followed by an increasing level of activity to a peak level of peristaltic activity lasting for 5–15 minutes. This cycle repeats every 1.5–2 hours but is interrupted by food ingestion. The role of this process is likely to clean excess bacteria and food from the digestive system.

Peristalsis

Peristalsis is one of the patterns that occur during and shortly after a meal. The contractions occur in wave patterns traveling down short lengths of the GI tract from one section to the next. The contractions occur directly behind the bolus of food that is in the system, forcing it toward the anus into the next relaxed section of smooth muscle. This relaxed section then contracts, generating smooth forward movement of the bolus at between 2–25 cm per second. This contraction pattern depends upon hormones, paracrine signals, and the

autonomic nervous system The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies internal organs, smooth muscle and glands. The autonomic nervous system is a control system t ...

for proper regulation.

Segmentation

Segmentation also occurs during and shortly after a meal within short lengths in segmented or random patterns along the intestine. This process is carried out by the longitudinal muscles relaxing while circular muscles contract at alternating sections thereby mixing the food. This mixing allows food and digestive enzymes to maintain a uniform composition, as well as to ensure contact with the

epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...

for proper absorption.

Secretion

Every day, seven liters of fluid are secreted by the digestive system. This fluid is composed of four primary components: ions, digestive enzymes, mucus, and bile. About half of these fluids are secreted by the salivary glands, pancreas, and liver, which compose the accessory organs and glands of the digestive system. The rest of the fluid is secreted by the GI epithelial cells.

Ions

The largest component of secreted fluids is ions and water, which are first secreted and then reabsorbed along the tract. The ions secreted primarily consist of H⁺, K⁺, Cl⁻, HCO₃⁻ and Na⁺. Water follows the movement of these ions. The GI tract accomplishes this ion pumping using a system of proteins that are capable of

active transport In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellul ...

facilitated diffusion Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is the process of spontaneous passive transport (as opposed to active transport) of molecules or ions across a biological membrane via specific transmembra ...

and open channel ion movement. The arrangement of these proteins on the

apical Apical means "pertaining to an apex". It may refer to: *Apical ancestor, refers to the last common ancestor of an entire group, such as a species (biology) or a clan (anthropology) *Apical (anatomy), an anatomical term of location for features loc ...

and

basolateral The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (the ...

sides of the epithelium determines the net movement of ions and water in the tract. H⁺ and Cl⁻ are secreted by the

parietal cells Parietal cells (also known as oxyntic cells) are epithelial cells in the stomach that secrete hydrochloric acid (HCl) and intrinsic factor. These cells are located in the gastric glands found in the lining of the fundus and body regions of the ...

into the lumen of the stomach creating acidic conditions with a low pH of 1. H⁺ is pumped into the stomach by exchanging it with K⁺. This process also requires ATP as a source of energy; however, Cl⁻ then follows the positive charge in the H⁺ through an open apical channel protein. HCO₃⁻ secretion occurs to neutralize the acid secretions that make their way into the

duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine ...

of the small intestine. Most of the HCO₃⁻ comes from pancreatic

acinar cells Centroacinar cells are spindle-shaped cells in the exocrine pancreas. They represent an extension of the intercalated duct into each pancreatic acinus. These cells are commonly known as duct cells, and secrete an aqueous bicarbonate solution unde ...

in the form of NaHCO₃ in an aqueous solution. This is the result of the high concentration of both HCO₃⁻ and Na⁺ present in the duct creating an

osmotic gradient Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...

to which the water follows.

Digestive enzymes

The second vital secretion of the GI tract is that of digestive enzymes that are secreted in the mouth, stomach and intestines. Some of these enzymes are secreted by accessory digestive organs, while others are secreted by the epithelial cells of the stomach and intestine. While some of these enzymes remain embedded in the wall of the GI tract, others are secreted in an inactive

proenzyme In biochemistry, a zymogen (), also called a proenzyme (), is an inactive precursor of an enzyme. A zymogen requires a biochemical change (such as a hydrolysis reaction revealing the active site, or changing the configuration to reveal the active ...

form. When these proenzymes reach the lumen of the tract, a factor specific to a particular proenzyme will activate it. A prime example of this is

pepsin Pepsin is an endopeptidase that breaks down proteins into smaller peptides. It is produced in the gastric chief cells of the stomach lining and is one of the main digestive enzymes in the digestive systems of humans and many other animals, ...

, which is secreted in the stomach by

chief cells In human anatomy, there are three types of chief cells, the gastric chief cell, the parathyroid chief cell, and the type 1 chief cells found in the carotid body. Cell types The gastric chief cell (also known as a zymogenic cell or peptic cell) is a ...

. Pepsin in its secreted form is inactive (

pepsinogen Pepsin is an endopeptidase that breaks down proteins into smaller peptides. It is produced in the gastric chief cells of the stomach lining and is one of the main digestive enzymes in the digestive systems of humans and many other animals, ...

). However, once it reaches the gastric lumen it becomes activated into

by the high H+ concentration, becoming an enzyme vital to digestion. The release of the enzymes is regulated by neural, hormonal, or paracrine signals. However, in general, parasympathetic stimulation increases secretion of all digestive enzymes.

Mucus

Mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...

is released in the stomach and intestine, and serves to lubricate and protect the inner mucosa of the tract. It is composed of a specific family of

glycoproteins Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glyco ...

termed

mucins Mucins () are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most animals. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most ...

and is generally very viscous. Mucus is made by two types of specialized cells termed mucus cells in the stomach and

goblet cells Goblet cells are simple columnar epithelial cells that secrete gel-forming mucins, like mucin 5AC. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their sec ...

in the intestines. Signals for increased mucus release include parasympathetic innervations, immune system response and enteric nervous system messengers.

Bile

Bile is secreted into the duodenum of the small intestine via the

common bile duct The common bile duct, sometimes abbreviated as CBD, is a duct in the gastrointestinal tract of organisms that have a gallbladder. It is formed by the confluence of the common hepatic duct and cystic duct and terminates by uniting with pancreati ...

. It is produced in liver cells and stored in the gall bladder until release during a meal. Bile is formed of three elements:

bile salts Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Diverse bile acids are synthesized in the liver. Bile acids are conjugated with taurine or glycine residues to give anions called bile salts. Primary b ...

bilirubin Bilirubin (BR) (Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from t ...

and cholesterol. Bilirubin is a waste product of the breakdown of hemoglobin. The cholesterol present is secreted with the feces. The bile salt component is an active non-enzymatic substance that facilitates fat absorption by helping it to form an emulsion with water due to its

amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphiprot ...

nature. These salts are formed in the

hepatocytes A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass. These cells are involved in: * Protein synthesis * Protein storage * Transformation of carbohydrates * Synthesis of cholesterol, ...

from bile acids combined with an

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

. Other compounds such as the waste products of drug degradation are also present in the bile.

Regulation

The digestive system has a complex system of motility and secretion regulation which is vital for proper function. This task is accomplished via a system of long reflexes from the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all par ...

(CNS), short reflexes from the

enteric nervous system The enteric nervous system (ENS) or intrinsic nervous system is one of the main divisions of the autonomic nervous system (ANS) and consists of a mesh-like system of neurons that governs the function of the gastrointestinal tract. It is capable of ...

(ENS) and reflexes from GI

peptides Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A p ...

working in harmony with each other.

Long reflexes

Long reflexes to the digestive system involve a sensory neuron sending information to the brain, which integrates the signal and then sends messages to the digestive system. While in some situations, the sensory information comes from the GI tract itself; in others, information is received from sources other than the GI tract. When the latter situation occurs, these reflexes are called feedforward reflexes. This type of reflex includes reactions to food or danger triggering effects in the GI tract. Emotional responses can also trigger GI response such as the butterflies in the stomach feeling when nervous. The feedforward and emotional reflexes of the GI tract are considered

cephalic A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may not ...

reflexes.

Short reflexes

Control of the digestive system is also maintained by ENS, which can be thought of as a digestive brain that can help to regulate motility, secretion and growth. Sensory information from the digestive system can be received, integrated and acted upon by the enteric system alone. When this occurs, the reflex is called a short reflex. Although this may be the case in several situations, the ENS can also work in conjunction with the CNS; vagal afferents from the viscera are received by the medulla, efferents are affected by the

vagus nerve The vagus nerve, also known as the tenth cranial nerve, cranial nerve X, or simply CN X, is a cranial nerve that interfaces with the parasympathetic control of the heart, lungs, and digestive tract. It comprises two nerves—the left and right v ...

. When this occurs, the reflex is called

vagovagal reflex Vagovagal reflex refers to gastrointestinal tract reflex circuits where afferent and efferent fibers of the vagus nerve coordinate responses to gut stimuli via the dorsal vagal complex in the brain. The vagovagal reflex controls contraction of ...

. The

myenteric plexus The myenteric plexus (or Auerbach's plexus) provides motor innervation to both layers of the muscular layer of the gut, having both parasympathetic and sympathetic input (although present ganglion cell bodies belong to parasympathetic innervation ...

and

submucosal plexus The submucosal plexus (Meissner's plexus, plexus of the submucosa, plexus submucosus) lies in the submucosa of the intestinal wall. The nerves of this plexus are derived from the myenteric plexus which itself is derived from the plexuses of para ...

are both located in the gut wall and receive sensory signals from the lumen of the gut or the CNS.

Gastrointestinal peptides

''For further information see

Gastrointestinal hormone The gastrointestinal hormones (or gut hormones) constitute a group of hormones secreted by enteroendocrine cells in the stomach, pancreas, and small intestine that control various functions of the digestive organs. Later studies showed that most of ...

'' GI peptides are signal molecules that are released into the blood by the GI cells themselves. They act on a variety of tissues including the brain, digestive accessory organs, and the GI tract. The effects range from excitatory or inhibitory effects on motility and secretion to feelings of satiety or hunger when acting on the brain. These hormones fall into three major categories, the

gastrin Gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the pyloric antrum of the stomach, duodenum, and the pancreas. Gastrin ...

and

secretin Secretin is a hormone that regulates water homeostasis throughout the body and influences the environment of the duodenum by regulating secretions in the stomach, pancreas, and liver. It is a peptide hormone produced in the S cells of the duoden ...

families, with the third composed of all the other hormones unlike those in the other two families. Further information on the GI peptides is summarized in the table below.

Digestion

* carbohydrates (

monosaccharide Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. They are usually colorless, water-solu ...

disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosaccharides, disaccharides are simple sugars soluble in water. Three common examples are sucrose, la ...

) *

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

lipids Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...

Splanchnic circulation

Superior mesenteric artery In human anatomy, the superior mesenteric artery (SMA) is an artery which arises from the anterior surface of the abdominal aorta, just inferior to the origin of the celiac trunk, and supplies blood to the intestine from the lower part of the d ...

Inferior mesenteric artery In human anatomy, the inferior mesenteric artery, often abbreviated as IMA, is the third main branch of the abdominal aorta and arises at the level of L3, supplying the large intestine from the distal transverse colon to the upper part of the an ...

External links

Notes
at

University of Bristol The University of Bristol is a Red brick university, red brick Russell Group research university in Bristol, England. It received its royal charter in 1909, although it can trace its roots to a Society of Merchant Venturers, Merchant Venturers' sc ...

Notes and references

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