Cholesterol is a fat-like substance that your body uses for a variety of purposes, including cell-wall maintenance and vitamin D and hormone production. While some of your cholesterol comes from the foods you eat, your body also secretes the substance in your liver and transports it through your bloodstream with the help of carrier particles called lipoproteins.
Cholesterol Secretion Basics
Your liver produces cholesterol in cells called hepatocytes, according to the University of Washington. The rate of secretion in any given cell is regulated in a negative feedback loop by an enzyme called HMG-CoA reductase, which decreases its activity as cholesterol levels build. In addition to cholesterol, your liver secretes lipoproteins. Because of its fatty composition, cholesterol will not dissolve in your bloodstream, and you need lipoproteins to carry it to various points throughout your body, according to the National Heart, Lung and Blood Institute.
Lipoproteins
Lipoproteins contain specialized proteins called apolipoproteins and waxy substances called phospholipids, according to the University of Washington. Specific types of lipoprotein get their names from their relative content of these two materials. Low-density lipoprotein, or LDL, contains relatively high amounts of phospholipids and relatively low amounts of apolipoproteins. High-density lipoprotein, or HDL, contains relatively low amounts of phospholipids and relatively high amounts of apolipoproteins. Another type of lipoprotein, called very low-density lipoprotein, or VLDL, has an even lower apolipoprotein content than LDL.
LDL Secretion
Your liver secretes cholesterol into your bloodstream within particles of LDL, the University of Washington says. Once in your blood, cholesterol is absorbed into your cells with the help of specialized LDL receptors that sit on the cells' outer surfaces. Under normal circumstances, your body produces enough cholesterol to meet your cells' needs, according to the American Academy of Family Physicians. If you take in too much cholesterol from dietary sources, your blood will contain more of the substance than your cells can use. This excessive cholesterol can build up in your arteries and create dangerous clogs as well as an artery-hardening condition called atherosclerosis.
HDL Secretion
Your body secretes HDL in both your liver and small intestine, the University of Washington says. When it enters your bloodstream, this type of lipoprotein seeks out excess cholesterol particles, binds to them and carries them to your liver. Once inside your liver, HDL breaks apart and leaves its cholesterol content behind. Your liver then takes this cholesterol and purges it from your body in a digestive juice called bile or in a related substance called bile salt.
Considerations
Some individuals have genetic malfunctions in their cells' LDL receptors, according to the University of Washington. Without proper activity in these receptors, your cells cannot readily absorb the cholesterol secreted by your liver. As a result, cholesterol stays in your bloodstream and builds up in a manner similar to excess dietary cholesterol. Malfunctions in your LDL receptors can also override the feedback loop that normally controls cholesterol secretion in your liver's hepatocytes. Consult your doctor for more information on internal cholesterol secretion and its relationship to dietary cholesterol.
