Five major classes of lipoprotein complexes transport the hydrophobic fats through the aqueous environment of the bloodstream (Table 18.1 and Figure 18.7). Free lipids are all but undetectable in the blood. The protein components of the lipoprotein complexes (Table 18.2) are called apo(lipo)proteins. A summary of the functions of the various lipoprotein complexes is as follows:

Chylomicrons - Chylomicrons emulsify dietary lipids and carry them from the lymph system through the blood stream to tissues. Chylomicrons carry dietary cholesterol and have the lowest density of the lipoprotein complexes. Digestion of fats (in capillaries) from chylomicrons produces chylomicron remnants, which are taken up by the liver.

Very Low-Density Lipoprotein (VLDL) - VLDLs transport triacylglycerols (and other lipids to a lesser extent) that are synthesized in the liver to target tissues via the bloodstream. Fats in VLDLs are largely hydrolyzed in capillaries of peripheral tissues by lipoprotein lipase.

Intermediate-Density Lipoprotein (IDL) - IDLs are remnants of the VLDLs remaining after the fats have been digested in the capillaries are called IDLs (Figure 18.7). IDLs, like the chylomicron remnants, are taken up by the liver via a specific receptor.

Low-Density Lipoprotein (LDL) - LDLs are produced by the liver and are the principal form in which cholesterol is transported to tissues (Figures 18.7, 18.10). More than 50% of the weight of the LDL particle is composed of cholesterol esters and free cholesterol. LDLs are absorbed by target cells in a process called receptor-mediated endocytosis. This occurs when the LDL interacts with a specific LDL receptor. Components of LDLs, such as unsaturated fatty acids, cholesterol, and amino acid residues of the apoprotein, are readily oxidized (forming oxidized LDLs). Oxidized LDLs are taken up by white blood cells via their scavenger receptor. Cholesterol-engorged white blood cells are called foam cells, which are primary constituents of atherosclerotic plaques. As a result, LDLs are often referred to as "bad cholesterol."

High-Density Lipoprotein (HDL) - carries excess cholesterol from tissue to the liver for metabolism or excretion (Figure 18.7) . Also referred to as "good cholesterol" because it removes cholesterol from the blood and returns it to the liver.