What are lipoproteins and what do they do?
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Lipoproteins are a normal component of the blood and the factor most associated with heart disease risk. But lipoproteins are vital. They function to shuttle lipids, which are water-insoluble substances such as fat, cholesterol and other nutrients throughout the body in circulation. They are basically lipid-laden submarines (See Lipoprotein Figure). Since the cholesterol in the blood is found aboard lipoproteins, total blood cholesterol is the sum of the cholesterol being carried in the different types of lipoproteins. A clinical laboratory is able to determine the quantity of cholesterol in each lipoprotein class (e.g., HDL-cholesterol or LDL-cholesterol). |
Where does LDL come from and what does it do?
The liver packages up cholesterol and fat (triglyceride) into very low density lipoproteins (VLDLs), which are then released into the blood (see LDL Formation Figure). As VLDLs circulate, they unload their fat cargo with most of it going to fat (adipose) tissue and other tissue such as skeletal muscle and the heart. As they lose their fat, VLDLs become LDLs, which are mostly cholesterol (see Lipoprotein Figure). As LDL circulates they drop off cholesterol in tissue throughout the body and are eventually removed from the blood by the liver and other tissue (see LDL Formation Figure).
Where does HDL come from and what does it do?
HDLs are made by the liver and intestines. As HDLs circulate they pick up excessive cholesterol from tissue throughout the body. HDLs then transfer this cholesterol back to LDLs, which are subject to removal from the blood, or HDLs themselves are removed from the blood by the liver. In either case, much of the cholesterol that HDLs accumulate on their journey is returned to the liver. So in essence LDLs are cholesterol delivery vehicles, while HDLs go out and pick up the excess.
What is atherosclerosis?
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Our arteries can be thought of as blood-filled tubes, the walls of which contain distinct layers. Quite simply, atherosclerosis is the process that allows for the potentially problematic buildup of substances within the walls of arteries. This buildup or plaque consists mostly of lipid, protein, and calcium in conjunction with an excessive presence of macrophages and smooth muscle cells. The lipid is mostly cholesterol derived from LDLs while the protein is largely connective tissue proteins, such as collagen. |
Where does atherosclerosis occur?
The innermost layer, or the layer closest to the blood, is called the intima (see Blood Vessel Figure). The middle layer is referred to as the media, as it sits in the middle of the wall. In between the intima and the surging blood is a thin layer of cells, which is covered with a fine layer of connective tissue proteins (e.g., collagen). It is within the intima that atherosclerosis develops. Damage to the cell lining and connective tissue is often referred to as “injury” and that creates the opportunity for atherosclerosis to develop. Furthermore, cells found in the media will participate in the development of atherosclerosis.
Although atherosclerosis can occur in arteries throughout the body, the most common sites are in those arteries supplying the brain and heart. Interestingly, atherosclerosis is much more common at branching points in arteries. This is where blood flow is more turbulent. Hindrance of blood flow within the brain and heart can result in a stroke or heart attack, respectively.
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As the atherosclerotic plaque grows in size, it causes the wall of that artery to protrude further and further into the blood vessel. This in turn decreases the area for blood to flow through (see Atherosclerosis Figure). If the narrowing becomes severe enough, it becomes an occlusion and blood flow is reduced to a critical level. Furthermore, if a blood clot develops in this location or it circulates to and gets lodged in this narrowed area, it will dam up blood flow. This is often how heart attacks occur, making them seem so sudden. Basically heart tissue “downstream” does not receive the oxygen that it needs to survive. |
