Blood lipids (or blood fats) are
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 in ...
in the
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
, either free or bound to other
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
s. They are mostly transported in a
protein
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, res ...
capsule, and the density of the lipids and type of protein determines the fate of the particle and its influence on
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run ...
. The concentration of blood lipids depends on intake and
excretion from the
intestine, and uptake and
secretion from
cells
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
.
Hyperlipidemia is the presence of elevated or abnormal levels of
lipid
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 in ...
s and/or
lipoproteins in the
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
, and is a major risk factor for
cardiovascular disease.
Fatty acids
Intestine intake
Short- and
medium chain fatty acids
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...
are absorbed directly into the blood via
intestine capillariesand travel through the
portal vein.
Long-chain fatty acids, on the other hand, are too large to be directly released into the tiny intestine capillaries. Instead they are coated with
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...
and protein (protein coat of
lipoproteins) into a compound called a
chylomicron
Chylomicrons (from the Greek χυλός, chylos, meaning ''juice'' (of plants or animals), and micron, meaning ''small particle''), also known as ultra low-density lipoproteins (ULDL), are lipoprotein particles that consist of triglycerides (8 ...
. The chylomicron enters a
lymphatic capillary and enters into the bloodstream first at the left
subclavian vein (having bypassed the liver).
In any case, the concentration of blood fatty acids increase temporarily after a meal.
Cell uptake
After a meal, when the blood concentration of fatty acids rises, there is an increase in uptake of fatty acids in different cells of the body, mainly
liver cells,
adipocytes
Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. I ...
and
muscle cells. This uptake is stimulated by
insulin from the
pancreas
The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an ...
. As a result, the blood concentration of fatty acid stabilizes again after a meal.
Cell secretion
After a meal, some of the fatty acids taken up by the liver is converted into
very low density lipoproteins (VLDL) and again secreted into the blood.
[Molecular cell biology. Lodish, Harvey F. 5. ed. : - New York : W. H. Freeman and Co., 2003. Page 321. b ill. ]
In addition, when a long time has passed since the last meal, the concentration of fatty acids in the blood decreases, which triggers
adipocytes
Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. I ...
to release stored fatty acids into the blood as
free fatty acids, in order to supply e.g. muscle cells with energy.
In any case, also the fatty acids secreted from cells are anew taken up by other cells in the body, until entering
fatty acid metabolism.
Cholesterol
The fate of cholesterol in the blood is highly determined by its constitution of
lipoproteins, where some types favour transport towards body tissues and others towards the liver for excretion into the intestines.
The 1987 report of
National Cholesterol Education Program, Adult Treatment Panels suggest the total
blood cholesterol level should be: <200 mg/dl normal blood cholesterol, 200–239 mg/dl borderline-high, >240 mg/dl high cholesterol.
The average amount of ''blood cholesterol'' varies with age, typically rising gradually until one is about 60 years old. There appear to be seasonal variations in cholesterol levels in humans, more, on average, in winter.
These seasonal variations seem to be inversely linked to
vitamin C intake.
Intestine intake
In
lipid digestion, cholesterol is packed into
Chylomicrons
Chylomicrons (from the Greek χυλός, chylos, meaning ''juice'' (of plants or animals), and micron, meaning ''small particle''), also known as ultra low-density lipoproteins (ULDL), are lipoprotein particles that consist of triglycerides (85 ...
in the
small intestine, which are delivered to the
Portal vein and
Lymph. The chylomicrons are ultimately taken up by liver
hepatocytes via interaction between
apolipoproteinE and the
LDL receptor
The low-density lipoprotein receptor (LDL-R) is a mosaic protein of 839 amino acids (after removal of 21-amino acid signal peptide) that mediates the endocytosis of cholesterol-rich low-density lipoprotein (LDL). It is a cell-surface receptor t ...
or
Lipoprotein receptor-related proteins.
In lipoproteins
Cholesterol is minimally soluble in
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
; it cannot dissolve and travel in the water-based bloodstream. Instead, it is transported in the bloodstream by
lipoproteins that are water-soluble and carry cholesterol and
triglyceride
A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids (from ''tri-'' and ''glyceride'').
Triglycerides are the main constituents of body fat in humans and other vertebrates, as ...
s internally. The
apolipoproteins forming the surface of the given lipoprotein particle determine from what cells cholesterol will be removed and to where it will be supplied.
The largest lipoproteins, which primarily transport fats from the
intestinal mucosa to the
liver
The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
, are called
chylomicron
Chylomicrons (from the Greek χυλός, chylos, meaning ''juice'' (of plants or animals), and micron, meaning ''small particle''), also known as ultra low-density lipoproteins (ULDL), are lipoprotein particles that consist of triglycerides (8 ...
s. They carry mostly fats in the form of triglycerides. In the liver, chylomicron particles release triglycerides and some cholesterol. The liver converts unburned food metabolites into
very low density lipoproteins (VLDL) and secretes them into plasma where they are converted to intermediate-density lipoproteins(IDL), which thereafter are converted to
low-density lipoprotein
Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons (aka ULDL by the overall densi ...
(LDL) particles and non-esterified fatty acids, which can affect other body cells. In healthy individuals, the relatively few LDL particles are large. In contrast, large numbers of small dense LDL (sdLDL) particles are strongly associated with the presence of
atheromatous disease within the arteries. For this reason, LDL is referred to as "bad cholesterol".
Intestine excretion
After being transported to the liver by HDL, cholesterol is delivered to the intestines via bile production. However, 92-97% is reabsorbed in the intestines and recycled via
enterohepatic circulation.
Cell uptake
Cholesterol circulates in the blood in
low-density lipoprotein
Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons (aka ULDL by the overall densi ...
s and these are taken into the cell by
LDL receptor
The low-density lipoprotein receptor (LDL-R) is a mosaic protein of 839 amino acids (after removal of 21-amino acid signal peptide) that mediates the endocytosis of cholesterol-rich low-density lipoprotein (LDL). It is a cell-surface receptor t ...
-mediated
endocytosis in
clathrin-
coated pits, and then hydrolysed in lysosomes.
Cell secretion
In response to low blood cholesterol, different cells of the body, mainly in the
liver
The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
and
intestines, start to synthesize cholesterol from
acetyl-CoA by the enzyme
HMG-CoA reductase. This is then released into the blood.
Related medical conditions
Hyperlipidemia
Hyperlipidemia is the presence of elevated or abnormal levels of
lipid
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 in ...
s and/or
lipoproteins in the
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
.
Lipid and
lipoprotein abnormalities are extremely common in the general population, and are regarded as a highly modifiable risk factor for
cardiovascular disease. In addition, some forms may predispose to
acute pancreatitis. One of the most clinically relevant lipid substances is
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...
, especially on
atherosclerosis and
cardiovascular disease. The presence of high levels of cholesterol in the blood is called
hypercholesterolemia.
Hyperlipoproteinemia
Hyperlipidemia is abnormally elevated levels of any or all lipids (fats, cholesterol, or triglycerides) or lipoproteins in the blood. citing: and The term ''hyperlipidemia'' refers to the laboratory finding itself and is also used as an umbre ...
is elevated levels of
lipoproteins.
Hypertriglyceridemia
Hypercholesterolemia
Hypercholesterolemia is the presence of high levels of
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...
in the blood.
It is not a
disease but a
metabolic derangement that can be secondary to many diseases and can contribute to many forms of disease, most notably
cardiovascular disease.
Familial hypercholesterolemia is a rare
genetic disorder
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
that can occur in families, where sufferers cannot properly metabolise cholesterol.
Hypocholesterolemia
Abnormally low levels of cholesterol are termed ''
hypocholesterolemia''. Research into the causes of this state is relatively limited, and while some studies suggest a link with
depression,
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
and
cerebral hemorrhage it is unclear whether the low cholesterol levels are a cause for these conditions or an
epiphenomenonbr>
See also
*
Lipid hypothesis
The lipid hypothesis (also known as the cholesterol hypothesis) is a medical theory postulating a link between blood cholesterol levels and the occurrence of cardiovascular disease. A summary from 1976 described it as: "measures used to lower t ...
References
{{DEFAULTSORT:Blood Lipids
Steroids
Sterols
Lipid disorders
Lipids