Definition for High-density lipoprotein
From Biology Forums Dictionary
High-density lipoproteins (HDL) form a class of lipoproteins, varying somewhat in their size and contents, that carry cholesterol from the body's tissues to the liver.
Because HDL can remove cholesterol from atheroma within arteries, and transport it back to the liver for excretion, they are seen as "good" lipoproteins. When measuring cholesterol, any contained in HDL particles serves as protection to the body's cardiovascular health. (In contrast to "bad" low-density liprotein cholesterol.)
HDL are the smallest of the lipoproteins. They are the densest because they contain the highest proportion of protein. The liver synthesizes these lipoproteins as empty flattened spherical protein particles. They are capable of picking up cholesterol, carried internally, from cells they interact with. They increase in size as they circulate through the bloodstream, if they internal more cholesterol molecules. Thus, it is the concentration of large HDL particles which more accurately reflects protective action, as opposed to the concentration of total HDL particles. This ratio of large HDL to total HDL particles varies widely and is only measured by more sophisticated lipoprotein assays using either electrophoresis.