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Cholesterol is a waxy, fat-like substance that is found in all cells of the body. In addition to being a structural component of cell membranes, cholesterol also plays important roles in making hormones, vitamin D, and bile acids that aid in the digestion of foods.
Cholesterol itself is not harmful, but too much cholesterol in the blood, or high blood cholesterol, can be dangerous. Heart attack and stroke, two of the most common cardiovascular diseases, originate from high blood cholesterol, or more specifically, high concentration of cholesterol carried by low-density lipoproteins (LDLs).
Blood is watery, and cholesterol is fatty. Just like oil and water, the two do not mix. To travel in the bloodstream, cholesterol is carried in small packages called lipoproteins. There are two kinds of lipoproteins that carry cholesterol throughout the body.
Why LDL is Bad
LDL plays an essential role in the pathogenesis of atherosclerosis, which can cause many complications, including heart attack and stroke. Diseases caused by atherosclerosis are the leading cause of illness and death in the United States. The initial step leading to atherosclerosis is the trap of LDL particles in the tunica intima (or simply called intima), which is the innermost layer of the artery wall. The intima contains endothelium, basement membrane and underlying connective tissue. The endothelium is in direct contact with the blood flow and the basement membrane separates epithelium from the underlying connective tissue.
The size of LDL particles varies from 18 to 25 nm in diameter. They can penetrate the endothelium efficiently. The LDL particle contains mainly the apolipoprotein B (ApoB), which can bind to proteoglycans in the complex matrix beneath the endothelium. The trapped LDL particles could be oxidized and eventually form a plague. The rupture of plague can induce the formation of thrombus (blood clot) and block blood flow. This will result in stroke or heart attack, depending on whether the blockage is in the brain or the heart.
Why HDL is Good
The size of HDL particles varies from 5 to 17 nm in diameter. They can penetrate the endothelium even more efficiently than LDL particles. However, they are less likely to be trapped in the intima, because the HDL particle contains mainly apolipoprotein A-I (ApoA-I) which does not bind to proteoglycans. Only a small amount of HDL particles may be trapped because they contain apolipoprotein E (ApoE) which can bind with proteoglycans.
Generally, LDL carries cholesterol from the liver to the rest of the body, while HDL carries cholesterol from the blood back to the liver. Not only can HDL remove the cholesterol in the blood, it also possesses anti-oxidant and anti-inflammatory properties to prevent atherosclerosis (reference).