Blood lipids (or blood fats) are

lipids Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and others. The fu ...

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 is com ...

, either free or bound to other

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s. They are mostly transported in a phospholipid capsule, and the type of protein embedded in this outer shell determines the fate of the particle and its influence on

metabolism Metabolism (, from ''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 cellular processes; the co ...

. Examples of these lipids include

cholesterol Cholesterol is the principal sterol of all higher animals, distributed in body Tissue (biology), tissues, especially the brain and spinal cord, and in Animal fat, animal fats and oils. Cholesterol is biosynthesis, biosynthesized by all anima ...

and

triglyceride A triglyceride (from '' tri-'' and '' glyceride''; also TG, triacylglycerol, TAG, or triacylglyceride) is an ester derived from glycerol and three fatty acids. Triglycerides are the main constituents of body fat in humans and other vertebrates ...

s. The concentration of blood lipids depends on intake and

excretion Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, Kidney (vertebrates), kidneys, and skin. This is in contrast with secretion, where the substa ...

from the

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...

, and uptake and

secretion Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical mec ...

from

cells Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a d ...

Hyperlipidemia Hyperlipidemia is abnormally high levels of any or all lipids (e.g. fats, triglycerides, cholesterol, phospholipids) or lipoproteins in the blood. citing: and The term ''hyperlipidemia'' refers to the laboratory finding itself and is also use ...

is the presence of elevated or abnormal levels of

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

s and/or

lipoprotein A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, sur ...

s in the

, and is a major risk factor for

cardiovascular disease Cardiovascular disease (CVD) is any disease involving the heart or blood vessels. CVDs constitute a class of diseases that includes: coronary artery diseases (e.g. angina, heart attack), heart failure, hypertensive heart disease, rheumati ...

Fatty acids

Intestine intake

Short- and medium chain fatty acids are absorbed directly into the blood vi
intestine capillaries
and travel through the

portal vein The portal vein or hepatic portal vein (HPV) is a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver. This blood contains nutrients and toxins extracted from digested contents. Approxima ...

Long-chain fatty acids In chemistry, in particular 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, ...

, on the other hand, are too large to be directly released into the tiny intestine capillaries. Instead they are coated with a membrane composed of

phospholipids Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typi ...

and

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s, forming a large transporter particle called

chylomicron Chylomicrons (from the Greek χυλός, chylos, meaning ''juice'' (of plants or animals), and micron, meaning ''small''), also known as ultra low-density lipoproteins (ULDL), are lipoprotein particles that consist of triglycerides (85–92%), ...

. The chylomicron enters a

lymphatic Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the origi ...

capillary, then it is transported into the bloodstream at the left

subclavian vein The subclavian vein is a paired large vein, one on either side of the body, that is responsible for draining blood from the upper extremities, allowing this blood to return to the heart. The left subclavian vein plays a key role in the absorption ...

(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 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, bile ...

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. ...

and

muscle cells A muscle cell, also known as a myocyte, is a mature contractile cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal, smooth, and cardiac (cardiomyocytes). A skeletal muscle cell is long and threadli ...

. This uptake is stimulated by

insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the insulin (''INS)'' gene. It is the main Anabolism, anabolic hormone of the body. It regulates the metabol ...

from the

pancreas The pancreas (plural pancreases, or pancreata) is an Organ (anatomy), organ of the Digestion, digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a ...

. 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 lipoprotein Very-low-density lipoprotein (VLDL), density relative to extracellular water, is a type of lipoprotein made by the liver. VLDL is one of the five major groups of lipoproteins (chylomicrons, VLDL, intermediate-density lipoprotein, low-density li ...

s (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

to release stored fatty acids into the blood as

free fatty acids In chemistry, in particular 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 ...

, 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 Fatty acid metabolism consists of various metabolic processes involving or closely related to fatty acids, a family of molecules classified within the lipid macronutrient category. These processes can mainly be divided into (1) catabolic processe ...

Cholesterol

The fate of cholesterol in the blood is highly determined by its constitution of

lipoproteins A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid (also known as fat) molecules in water, as in blood plasma or other extracellular fluids. They consist of a triglyceride and cholesterol center, sur ...

, 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 {{Primary sources, article, date=May 2011The National Cholesterol Education Program is a program managed by the National Heart, Lung and Blood Institute, a division of the National Institutes of Health. Its goal is to reduce increased cardiovascula ...

, 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 Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits, berries and vegetables. It is also a generic prescription medication and in some countries is sold as a non-prescription di ...

intake.

Intestine intake

lipid digestion Digestion is the breakdown of large insoluble food compounds into small water-soluble components so that they can be absorbed into the blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into th ...

, cholesterol is packed into

chylomicrons Chylomicrons (from the Greek χυλός, chylos, meaning ''juice'' (of plants or animals), and micron, meaning ''small''), also known as ultra low-density lipoproteins (ULDL), are lipoprotein particles that consist of triglycerides (85–92%), ...

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 ...

, which are delivered to the

and

lymph Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the ori ...

. The chylomicrons are ultimately taken up by liver

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, bile ...

via interaction between

apolipoprotein E Apolipoprotein E (Apo-E) is a protein involved in the metabolism of fats in the body of mammals. A subtype is implicated in Alzheimer's disease and cardiovascular diseases. It is encoded in humans by the gene ''APOE''. Apo-E belongs to a family ...

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 ...

lipoprotein receptor-related protein Lipoprotein receptor-related proteins, low density lipoprotein receptor-related proteins (HGNC) or prolow-density lipoprotein receptor-related protein (UniProt), abbreviated LRP, are a group of proteins. They include: *LRP1 *LRP1B *LRP2 (megalin) ...

In lipoproteins

Cholesterol is minimally soluble in

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

; it cannot dissolve and travel in the water-based bloodstream. Instead, it is transported in the bloodstream by

s that are water-soluble and carry cholesterol and

s internally. The

apolipoprotein Apolipoproteins are proteins that bind lipids (oil-soluble substances such as fats, cholesterol and fat soluble vitamins) to form lipoproteins. They transport lipids in blood, cerebrospinal fluid and lymph. The lipid components of lipoprotei ...

s 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 The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...

mucosa A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It ...

to the

liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...

, are called

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

s (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 density ...

(LDL) particles and non-esterified fatty acids, which can affect other body cells. In healthy individuals, most of the LDL particles are ''large and buoyant'' (less dense, also known as lb-LDL) and they are cardiovascularly neutral: they have no negative and no positive effect on cardiovascular health. In contrast, large numbers of ''small and dense'' LDL (sd-LDL) particles are strongly associated with the presence of

atheroma An atheroma, or atheromatous plaque, is an abnormal accumulation of material in the tunica intima, inner layer of an arterial wall. The material consists of mostly macrophage, macrophage cells, or debris, containing lipids, calcium and a variabl ...

tous disease within the arteries. For this reason, total LDL is referred to as "bad cholesterol," although only a fraction of it is actually bad. Standard chemistry panels typically include total triglyceride, LDL and HDL levels in the blood. Measuring the concentration of sd-LDL is expensive. However, since it is produced from VLDL, it can be inferred indirectly by estimating VLDL levels in the blood. That estimate is typically obtained by measuring triglyceride levels after at least eight hours of fasting, when chylomicrons have been totally removed from the blood by the liver. In the absence of chylomicrons, triglyceride levels have a much larger correlation with risk of

s than total LDL levels.

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 Enterohepatic circulation is the circulation of biliary acids, bilirubin, drugs or other substances from the liver to the bile, followed by entry into the small intestine, absorption by the enterocyte and transport back to the liver. Enterohepa ...

Cell uptake

Cholesterol circulates in the blood in

s and these are taken into the cell by

-mediated

endocytosis Endocytosis is a cellular process in which Chemical substance, substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a Vesicle (biology and chem ...

clathrin Clathrin is a protein that plays a role in the formation of coated vesicles. Clathrin was first isolated by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When the triskel ...

- coated pits, and then hydrolysed in lysosomes.

Cell secretion

In response to low blood cholesterol, different cells of the body, mainly in the

and

intestines The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. ...

, start to synthesize cholesterol from

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidation, o ...

by the enzyme

HMG-CoA reductase HMG-CoA reductase (3-hydroxy-3-methyl-glutaryl-coenzyme A reductase, official symbol HMGCR) is the rate-limiting enzyme (NADH-dependent, ; NADPH-dependent, ) of the mevalonate pathway, the metabolic pathway that produces cholesterol and other ...

. This is then released into the blood.

Related medical conditions

Hyperlipidemia

Hyperlipidemia is the presence of elevated or abnormal levels of

s and/or

s in the

. Lipid and

abnormalities are extremely common in the general population, and are regarded as a highly modifiable risk factor for

. In addition, some forms may predispose to

acute pancreatitis Acute pancreatitis (AP) is a sudden inflammation of the pancreas. Causes include a gallstone impacted in the common bile duct or the pancreatic duct, heavy alcohol use, systemic disease, trauma, elevated calcium levels, hypertriglyceridemia (w ...

. One of the most clinically relevant lipid substances is

, especially on

atherosclerosis Atherosclerosis is a pattern of the disease arteriosclerosis, characterized by development of abnormalities called lesions in walls of arteries. This is a chronic inflammatory disease involving many different cell types and is driven by eleva ...

and

. The presence of high levels of cholesterol in the blood is called

hypercholesterolemia Hypercholesterolemia, also called high cholesterol, is the presence of high levels of cholesterol in the blood. It is a form of hyperlipidemia (high levels of lipids in the blood), hyperlipoproteinemia (high levels of lipoproteins in the blood), ...

Hyperlipoproteinemia Hyperlipidemia is abnormally high levels of any or all lipids (e.g. fats, triglycerides, cholesterol, phospholipids) or lipoproteins in the blood. citing: and The term ''hyperlipidemia'' refers to the laboratory finding itself and is also use ...

is elevated levels of

Hypertriglyceridemia

Hypercholesterolemia

Hypercholesterolemia is the presence of high levels of

in the blood. It is not a

disease A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...

but a

metabolic Metabolism (, from ''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 cellular processes; the ...

derangement that can be secondary to many diseases and can contribute to many forms of disease, most notably

Familial hypercholesterolemia Familial hypercholesterolemia (FH) is a genetic disorder characterized by high cholesterol levels, specifically very high levels of low-density lipoprotein cholesterol (LDL cholesterol), in the blood and early cardiovascular diseases. The mos ...

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 chromosome abnormality. Although polygenic disorders ...

that can occur in families, where sufferers cannot properly metabolise cholesterol.

Hypocholesterolemia

Abnormally low levels of cholesterol are called ''

hypocholesterolemia Hypocholesterolemia is the presence of abnormally low (''hypo-'') levels of cholesterol in the blood (''-emia''). A defect in the body's production of cholesterol can lead to adverse consequences as well. Cholesterol is an essential component of ma ...

''.

References

{{DEFAULTSORT:Blood Lipids Steroids Sterols Lipid disorders Lipids

Fatty acids

Intestine intake

Cell uptake

Cell secretion

Cholesterol

Intestine intake

In lipoproteins

Intestine excretion

Cell uptake

Cell secretion

Related medical conditions

Hyperlipidemia

Hypertriglyceridemia

Hypercholesterolemia

Hypocholesterolemia

See also

References