Cholesterol is classified as a steroid. Its structure is made up of four rings fused together. The entire ring system (the steroid ring), is known by the complicated name of cyclopentanoperhydrophenanthrene. There are many steroids in the animal system. Cholesterol is the most important steroid, present in our system. Other examples of steroids include the bile acids, the sex hormones, and some other important hormones.
We have seen that cholesterol and other fatty substances are carried in the blood, by proteins called the lipoproteins. The lipoproteins are usually classified, based on density. The lightest lipoprotein is the chylomicron. The heaviest is the HDL(high density lipoprotein). In between, we have the LDL (low density lipoprotein) and VLDL (very low density lipoprotein). The major lipoproteins concerned with the movement of cholesterol in the blood, are the LDL and the HDL.
Cholesterol present in the HDL is called the good cholesterol. It is beneficial for the human beings to have ahigh level of this form of cholesterol.
Cholesterol, carried in the form of the LDL, is called the bad cholesterol. Individuals with low LDL levels are deemed to be healthy.
Analogy of the dry cleaning system
When we take our clothes to the dry cleaners, they become nice, fresh, and clean. They subsequently return to us, only to get dirty.
Similar is the situation with the LDL and the HDL.
Tissues ——→ Liver
Liver ——→ Tissues
The liver is the chemical factory of the body. It is concerned with the biotransformation of the molecules, both the endogenous and the exogenous varieties. Like many other molecules, cholesterol is also transformed in the liver. For example, cholesterol can be converted into the bile acids and the bile salts in the liver. Thus, the HDL which transports cholesterol to the liver is considered to be the good cholesterol. The LDL which does the reverse, is considered to be bad. A high level of LDL is considered to be a major risk factor for the development of the coronary heart disease.