Cholesterol is a ringed molecule that dissolves mostly in fat-like substances but also has parts that can dissolve in water. It is a type of lipid. Lipids are fat-like molecules that include cholesterol, fats, fatty acids and fat-soluble vitamins. Some aspects of cholesterol are necessary for life, while others contribute to poor health.
Cholesterol Functions
Cholesterol is obtained through synthesis in the liver and in the diet. Cholesterol has a number of functions in the body. It is metabolized, or biochemically changed, into steroid hormones, largely in the adrenal gland. It is made into progesterone, which helps support pregnancy and the menstrual cycle. It also forms aldosterone, which controls sodium, potassium and blood pressure, as well as the stress hormone cortisol and the sex steroids testosterone and estrogen. Cholesterol also fits into the cell membrane, which is the protective covering around cells. There it helps protect cells from temperature fluctuations.
Types of Lipoproteins
The word cholesterol is sometimes applied to compounds in the blood known as lipoproteins. A lipoprotein is a combination of lipids, such as cholesterols and fatty acids, and protein. Lipoproteins are classified by their density. Low density lipoprotein, or LDL, is commonly referred to "bad cholesterol." In contrast, high density lipoprotein, or HDL is commonly referred to as "good cholesterol."
Atherosclerosis
High levels of total cholesterol, triglycerides or fats and low density lipoprotein contribute to atherosclerosis. Atherosclerosis is the buildup of plaque on the inside of blood vessel walls. The plaques are deposited largely by LDL. Immune cells infiltrate the plaque, causing inflammation and narrowing of the vessel. The vessel becomes narrow and hard, restricting the flow of blood to organs.
Consequences
Over time, restricted blood flow to the organs from atherosclerosis can cause damage to the organs, resulting in kidney or heart disease. In addition, a plaque can rupture, suddenly occluding a blood vessel. In the heart, this causes a heart attack; in the brain, this causes a stroke. In this manner, LDL is bad because it increases the risk of these conditions. In contrast, HDL removes plaque from the blood vessels and transports it to the liver to be metabolized, hence its role as "good cholesterol."
References
- "Molecular Biology of the Cell"; Bruce Alberts et al.; 4th Ed 2002
- "Physiology"; Linda S.Costanzo; 4th Ed 2008
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci, et al.; 17th Ed 2008
