Because cholesterol is a fat-like substance that is not water-soluble, it can't flow freely through arteries and veins. Instead, it quickly forms fatty droplets similar to grease drops in soup. The body's solution is to transport all lipids such as cholesterol and triglycerides in specialized cargo vehicles called lipoproteins. The most well-known lipoproteins are LDL (low-density lipoprotein) and HDL (high-density lipoprotein).
These lipoproteins, also known as "lipoprotein complexes" or "lipoprotein particles," are actually tiny spheres that carry lipid molecules throughout the blood stream. They consist of a water-soluble outer protein layer that sits on a fat-like membrane of phospholipids and an inner storage space that contains cholesterol or a combination of cholesterol and triglycerides.
The following shows what a lipoprotein particle looks like:
Types Of Lipoproteins
Lipoprotein particles are named according to their density for historical reasons. When biochemists first separated the different lipoprotein particles, they used very fast centrifuges which separated the lipoproteins according to density. The lightest lipoproteins ended up floating towards the top while the denser ones sunk to the bottom of the test tubes.
Interestingly enough, the density of lipoproteins does not always correspond to its size or its function:
The smallest lipoproteins -- low-density lipoprotein (LDL) containing the "bad" cholesterol and high-density lipoprotein (HDL) containing the "good" cholesterol -- carry only cholesterol and are the most important.
Intermediate-density lipoproteins (IDL) are of medium size and carry both cholesterol and triglycerides.
Large lipoprotein complexes which carry cholesterol and many triglycerides include very low density lipoproteins (VLDL) and chylomicrons.
When you hear someone state "my LDL-cholesterol (LDL-c) is 170", or my "HDL-cholesterol (HDL-c) is below 40," this really means "The cholesterol content in my low-density lipoprotein particles is 170 mg per decilitre of blood." However, not even doctors discuss cholesterol levels in such confusing terms.
The Function Of Lipoproteins
Let's clarify the concept of lipoprotein particles again: all cholesterol and triglycerides such as fat that are in the blood are packaged into lipoproteins or lipoprotein particles. These are tiny spheres that serve as transport containers for such substances. There are 5 different lipoprotein particles. However, the only ones worth noting are the LDL (low-density lipoprotein) and HDL (high-density lipoprotein) particles because they are important predictors for the development of heart disease. The following is an illustration of some of the lipoprotein particles in the bloodstream:
1. Cells lining the gut package cholesterol and triglycerided into chylomicron particles.
2. Chylomicrons transport cholesterol and fat such as triglycerides absorbed by the small intestine to the liver. On their way to the liver, they release triglycerides into the bloodstream, which is then absorbed by body cells to be used as energy in muscle cells or to be stored for later use in fat cells. The liver finally combines the "chylomicron remnants" that are filled with the leftover cholesterol, and pools this dietary cholesterol with the cholesterol it synthesizes.
3. Very low density lipoproteins (VLDLs) are released from the liver into the bloodstream. They are similar to the chylomicrons originating from the gut in that they carry cholesterol and triglycerides. The latter are gradually released into the bloodstream to be gradually absorbed by body cells. As the VLDLs lose triglycerides, they start to shrink. They also begin to form intermediate-density lipoproteins (IDLs), and finally LDLs which have lost all of their triglycerides.
Low-density lipoproteins then carry cholesterol to the tissues throughout the body. The entire LDL particles are absorbed by the body cells which use the cholesterol as building blocks for their cell membranes. Leftover LDL may return to the liver where it is absorbed for storage and reprocessing.
If too much LDL cholesterol exists in the blood, it is usually deposited in the artery walls as a soft, wax-like plaque. This is the first step towards atherosclerosis or a hardening of the artery walls which can ultimately lead to heart disease.
4. High-density lipoproteins are able to collect cholesterol from body cells for reprocessing or elimination through the bile fluid in the liver. By doing so, they help to eliminate excess cholesterol that may accumulate in the body. More importantly, they can also absorb the cholesterol from artery walls which delays the build-up of plaque. These mechanisms counteract the harmful effects of high LDL-cholesterol and may slow the progression of atherosclerosis. This is why HDL is also called the "good" cholesterol; HDL cholesterol indicates how many beneficial vessel-cleaning HDL particles exist. Therefore, this is the only form of cholesterol you actually want to increase.