- Blood lipids
Blood lipids (or blood fats) are
lipids in theblood , either free or bound to other molecules. They are mostly transported in aprotein capsuleFact|date=September 2008, and the density of the lipids and type of protein determines the fate of the particle and its influence onmetabolism . The concentration of blood lipids depends on intake andexcretion from theintestine , and uptake andsecretion from cells. Blood lipids are mainlyfatty acids andcholesterol . Hyperlipidemia is the presence of elevated or abnormal levels oflipid s and/orlipoprotein s in theblood , and is a major risk factor forcardiovascular disease .Fatty acids
Blood fatty acids are in different forms in different stages in the circulation. They are taken in through the intestine in
chylomicrons , but also exist invery low density lipoprotein s (VLDL) after processing in the liver. In addition, when released fromadipocytes , it fatty acids exist in the blood asfree fatty acids Intestine intake
Short- and
medium chain fatty acids are absorbed directly into the blood via intestine capillaries and travel through theportal 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 withcholesterol and protein (protein coat oflipoproteins ) into a compound called achylomicron . The chylomicron enters alymphatic capillary and enters into the bloodstream first at the leftsubclavian 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 uptake of fatty acids in different cells of the body, mainly
liver cells ,adipocytes andmuscle cells . This uptake is stimulated byinsulin from thepancreas . 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 s (VLDL) and again secreted into the bloodMolecular cell biology. Lodish, Harvey F. 5. ed. : - New York : W. H. Freeman and Co., 2003. Page 321. b ill. ISBN 0-7167-4366-3 ] .In addition, when long time has passed since the last meal, the concentration of fatty acids in the blood decreases, which triggers
adipocytes to release stored fatty acids into the blood asfree 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 Clarifyme|date=September 2008.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. [cite journal |author= |title=Report of the National Cholesterol Education Program Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults. The Expert Panel |journal=Arch. Intern. Med. |volume=148 |issue=1 |pages=36–69 |year=1988 |pmid=3422148 |doi=]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 cite journal |author=Ockene IS, Chiriboga DE, Stanek EJ 3rd, Harmatz MG, Nicolosi R, Saperia G, Well AD, Freedson P, Merriam PA, Reed G, Ma Y, Matthews CE, Hebert JR. |title=Seasonal variation in serum cholesterol levels: treatment implications and possible mechanisms. |journal=Arch Intern Med |volume=164 |pages=863 – 70 |year=2004 |pmid=15111372 |doi=10.1001/archinte.164.8.863] .
Intestine intake
In
lipid digestion , cholesterol is packed intolow-density lipoprotein s in theintestine , which are delivered to the bloodstream.In lipoproteins
Cholesterol is minimally soluble in
water ; it cannot dissolve and travel in the water-based bloodstream. Instead, it is transported in the bloodstream bylipoprotein s - protein "molecular-suitcases" that are water-soluble and carry cholesterol andtriglyceride s internally. Theapolipoprotein 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
mucosa to the liver, are calledchylomicron s. They carry mostly fats in the form oftriglyceride s and cholesterol. In theliver , chylomicron particles release triglycerides and some cholesterol. The liver converts unburned food metabolites intovery low density lipoprotein s (VLDL) and secretes them into plasma where they are converted tolow-density lipoprotein (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 ofatheroma tous disease within the arteries. For this reason, LDL is referred to as "bad cholesterol".High-density lipoprotein (HDL) particles transport cholesterol back to the liver for excretion, but vary considerably in their effectiveness for doing thisFact|date=September 2008. Having large numbers of large HDL particles correlates with better health outcomes, and hence it is commonly called "good cholesterol". In contrast, having small amounts of large HDL particles is independently associated withatheroma tous disease progression within the arteries.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 s and these are taken into the cell byLDL receptor -mediatedendocytosis inclathrin -coated pits , and then hydrolysed in lysosomes.Cell secretion
In response to low blood cholesterol, different cells of the body, mainly in the
liver andintestines , start to synthesize cholesterol fromacetyl-CoA by the enzymeHMG-CoA reductase . This is then released into the blood.Related medical conditions
Hyperlipidemia
Hyperlipidemia is the presence of elevated or abnormal levels of
lipid s and/orlipoprotein s in theblood .Lipid and
lipoprotein abnormalities are extremely common in the general population, and are regarded as a highly modifiable risk factor forcardiovascular disease . In addition, some forms may predispose toacute pancreatitis . One of the most clinically relevant lipid substances ischolesterol , especially onatherosclerosis andcardiovascular disease . The presence of high levels of cholesterol in the blood is calledhypercholesterolemia cite journal |author=Durrington P |title=Dyslipidaemia |journal=Lancet |volume=362 |issue=9385 |pages=717–31 |year=2003 |pmid=12957096 |doi=10.1016/S0140-6736(03)14234-1] .Hyperlipoproteinemia is elevated levels oflipoproteins .Hypercholesterolemia
Hypercholesterolemia is the presence of high levels of
cholesterol in the blood cite journal |author=Durrington P |title=Dyslipidaemia |journal=Lancet |volume=362 |issue=9385 |pages=717–31 |year=2003 |pmid=12957096 |doi=10.1016/S0140-6736(03)14234-1] . It is not adisease but a metabolic derangement that can be secondary to many diseases and can contribute to many forms of disease, most notablycardiovascular disease . It is closely related to the terms.Familial hypercholesterolemia is a raregenetic disorder 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 andcerebral hemorrhage it is unclear whether the low cholesterol levels are a cause for these conditions or anepiphenomenon [http://www.americanheart.org/presenter.jhtml?identifier=1208] .ee also
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Lipid hypothesis References
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