This waxy substance is present in blood plasma and all animal tissues. Chemically, cholesterol is one of a group of compounds known as sterols and is related to other sterols such as the sex hormones and the hormones of the adrenal cortex. In its pure state it is a white, crystalline substance that is odorless and tasteless.
Cholesterol is essential to life, a primary component of the membrane that surrounds every cell. It is also the precursor chemical from which the body synthesizes bile acids, steroid hormones, and vitamin D. Cholesterol circulates in the bloodstream and is manufactured by the liver and several other organs.
Insoluble in the blood, cholesterol must be attached to certain protein complexes called lipoproteins in order to be carried through the bloodstream. Low-density lipoproteins (LDLs) transport cholesterol from its site of synthesis in the liver to the various tissues and body cells, where it is separated from the lipoprotein for use by the cell. High-density lipoproteins (HDLs) may transport excess or unused cholesterol from the tissues back to the liver, where it is broken down into bile acids and excreted. Atherosclerotic deposits, which build up in the blood vessels, consist primarily of cholesterol attached to LDLs. HDLs may actually serve to retard or reduce atherosclerotic buildup.