Cholesterol, a complex alcohol constituent of all animal fats and oils. It can be activated to form vitamin D. Cholesterol is one of a group of compounds known as sterols and is related to such other sterols as the sex hormones and the hormones of the adrenal cortex.
A close relationship exists among levels of blood cholesterol in the body, those of other fats or lipids, and the development of atherosclerosis(the disease in which cholesterol, accumulating in the wall of arteries, forms bulky plaques that inhibit the flow of blood until a clot eventually forms, obstructing an artery and causing a heart attack or a stroke. In this disorder, plaques containing cholesterol are deposited on the walls of arteries, particularly those of small and medium size, reducing their inside diameter and the flow of blood. Clotting of blood, such as may occur in the coronary arteries to cause a heart attack, is most likely to develop at places where arterial walls are roughened by such plaques.
Although many foods, particularly dairy products and meat fat, contain cholesterol, the body also synthesizes this sterol from cholesterol-free substances. Nevertheless, investigation indicates that a cholesterol-rich diet causes abnormally high levels of cholesterol and the related fats and lipids in the blood. Evidence strongly indicates that people with such high levels are more likely to develop atherosclerosis and heart attacks than those with lower levels. Also significant is the fact that scientists have identified two forms of cholesterol-carrying proteins in the blood, called high-density and low-density lipoproteins. The low-density form is thought to promote atherosclerosis, whereas the high-density component may retard it.
Persons who have abnormally high levels of cholesterol—especially low-density cholesterol—on a congenital basis can reduce their risk of heart attack by lowering their blood cholesterol. This is done by following a diet low in cholesterol and saturated fats, getting sufficient exercise, and using certain drugs. Cholesterol and its derivatives are secreted through the oil glands of the skin to act as a lubricant and protective covering for the hair and skin. Lanolin, grease extracted from raw sheep wool and composed largely of cholesterol esters, has a variety of commercial uses in lubricants, leather preservatives, ointments, and cosmetics.
In addition, total serum cholesterol comprises all of the cholesterol found in the various lipoproteins. Cholesterol is the major component of LDLs and a minority component of VLDLs and HDLs. Since LDL has consistently been associated with risk of atherosclerosis and since LDL is difficult to measure, serum total cholesterol has been used for many years as a substitute. There is general agreement that a strong correlation exists between considerably elevated serum cholesterol levels and an increased tendency for atherosclerosis.
However, cholesterol is synthesized in the body by the liver, because 500-1000 produced by the liver and 600-1000gm/dl secreted by the body glands in the intestine. There are also specialized proteins, called LDL receptors, that project from the surface of animal cells. The receptors bind LDL particles and extract them from the fluid that bathes the cells. The LDL is taken into the cells and broken down, yielding its cholesterol to serve each cell's needs. In supplying cells with cholesterol the receptors perform a second physiological function, which is critical to the development of atherosclerosis: they remove LDL from the bloodstream.The number of receptors displayed on the surface of cells varies with the cells' demand for cholesterol. When the need is low, excess cholesterol accumulates; cells make fewer receptors and take up LDL at a reduced rate. This protects cells against excess cholesterol, but at a high price: the reduction in the number of receptors decreases the rate at which LDL is removed from the circulation, the blood level of LDL rises and atherosclerosis is accelerated. Cholesterol can only be excreted from the body by way of the liver via these receptors.
On the other hand, cholesterol can cause the heart diseases. These heart diseases can be accelerated by the accumulation of the cholesterol in the blood stream because of the decreased number of the LDL receptors. Thus, the level of cholesterol will be high in the blood stream. After that, the plaque is formed (deposits called plaque that form on the insides of arteries and partially restrict the flow of blood). Plaque deposits are associated with high concentrations of cholesterol in the blood. Blood flow is often further reduced by the formation of blood clots, which are most likely to form where the artery walls have been roughened by plaque. These blood clots can also break free and travel through the circulatory system until they become lodged somewhere else and reduce blood flow there. Reduction in blood flow can cause organ damage. When brain arteries become blocked and brain function is impaired, the result is a stroke. A heart attack occurs when a coronary artery becomes blocked and heart muscle is destroyed.The total cholesterol to HDL cholesterol ratio is a number that is helpful in predicting atherosclerosis, because HDL acts as a protective from high cholesterol in taking cholesterol from extra hepatic tissues to the liver for excretion. The number is obtained by dividing total cholesterol by HDL cholesterol. (High ratios indicate higher risks of heart attacks, low ratios indicate lower risk).
High total cholesterol and low HDL cholesterol increases the ratio, and is undesirable. Conversely, high HDL cholesterol and low total cholesterol lowers the ratio, and is desirable. An average ratio would be about 4.5 ideally. Thus the best ratio would be 2 or 3 or less than 4. Another ratio is LDL/HDL. The LDL/HDL ratio is actually a more pure ratio than total cholesterol/HDL. Because LDL is a measure of bad cholesterol and HDL is a measure of good cholesterol, whereas the total cholesterol is the sum of HDL, LDL, and the VLDL. Even though total cholesterol/HDL ratio is not as accurate or pure as the LDL/HDL ratio, the former is more commonly obtained because the total cholesterol is easier and cheaper to obtain than the LDL cholesterol level.
Furthermore, hypercholesterolemia is caused by the increase of level of the cholesterol in the blood stream. It has two types, one is homozygous in which it affects 1:1,000,000 of the population, while the other one that is heterozygous in which it affects 1:500 of the population. The first type affects those people who have 20 years old and it is due to the absence of LDL receptors. The blood cholesterol level in this case is around 800-1000mg/dl and 20-26mmol/L. However, the second type of hypercholesterolemia affects people who have (20-50) years old. The blood cholesterol levels approximately are 300-600mg/dl and 8.5-15mmol/L.