Lipoproteins: Complexes of lipid and protein, the way lipids travel in the blood.
Cholesterol, a building block of the outer layer of cells (cell membranes), is transported through the blood in the form of water-soluble carrier molecules known as lipoproteins. The lipoprotein particle is composed of an outer shell of phospholipid, which renders the particle soluble in water; a core of fats called lipid, including cholesterol and a surface apoprotein molecule that allows tissues to recognize and take up the particle. These lipoproteins are characterized by their density: high density lipoprotein (HDL), low density lipoprotein (LDL), very low density lipoprotein (VLDL).
The first stages of cholesterol build up in the blood vessels (atherosclerosis) occur when LDL particles circulating in the blood penetrate through the inner lining of blood vessels and become trapped in the artery wall. Eventual build up of LDL, fat-filled cells, cells of inflammation, and blood clotting can block the normal blood flow in the coronary, carotid, cerebral, or other arteries including those in the legs. This type of blockage causes clinical syndromes such as heart attack and stroke. Blockage of arteries in the legs is call peripheral arterial disease (PAD). PAD is a factor in amputations of toes, feet, or legs.