Blood lipids

Blood lipids (or blood fats) are lipids in the blood, either free or bound to other molecules. 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. Examples of these lipids include cholesterol and triglycerides. The concentration of blood lipids depends on intake and excretion from the intestine, and uptake and secretion from cells. Hyperlipidemia is the presence of elevated or abnormal levels of lipids and/or lipoproteins in the blood, and is a major risk factor for cardiovascular disease.

Short- and medium chain fatty acids are absorbed directly into the blood via intestine capillaries and 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 a membrane composed of phospholipids and proteins, forming a large transporter particle called chylomicron. The chylomicron enters a lymphatic capillary, then it is transported into the bloodstream at the left subclavian vein (having bypassed the liver).

In any case, the concentration of blood fatty acids increase temporarily after a meal.

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 and muscle cells. This uptake is stimulated by insulin from the pancreas. As a result, the blood concentration of fatty acid stabilizes again after a meal.

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.

In addition, when a 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 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^{[clarification needed]}.

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.