BACKGROUND KNOWLEDGE

Atherosclerosis

Essential Knowledge

Atherosclerosis is an arterial disease which involves the formation of atheroma. Atheroma is a deposition of fat in large and medium sized arteries and the cause of atheroma is not known. The disease starts early but progresses in silent, so that the signs and symptoms only occur in later life. This is mainly because the risk factors and the incidence increase with age. The significance of this disease is unusual in females before the menopause.

The endothelium of the blood vessels (the inner cell layer, which is in contact with the blood) continuously produces factors which inhibit and stimulate blood coagulation. The factors are produced in equilibrium so that effect of blood coagulation is limited. In atherosclerosis, this balance is disturbed so that blood coagulation is promoted.

The next process in atherosclerosis is the injury to the endothelial wall. Low density lipoproteins (LDLs) start to accumulate at the site of injury and deposit behind the endothelial cell wall layer. LDLs are the carriers for fat and cholesterol. As more and more LDLs accumulate, an atherosclerotic plauqe forms which narrows the luman of the blood vessels, disturbing the blood flow. Repeated rupture, thrombosis (thrombosis forms a thrombus, which is similar to a blood clot) and repair enlarges the plaque.

Every time the plaque ruptures, a thrombus may form and consequentially break off the plaque. As soon as it travels in the blood circulation, it is called an embolus. This embolus can become trapped in an artery which has got a smaller lumen than the diameter of the embolus, so that the blood supply behind the emboli is blocked off. Highly active tissues don't get their nutrients and die in a few minutes or even seconds - this is called an infarct. If the infarct happens in the heart, it is called a myocardial infarction (myo = muscle, cardial = heart), therefore the heart muscle dies and cardiac function might be impaired.

Cholesterol and fat play a key role in the formation of atherosclerosis. Both of them are delivered from the liver to the tissues in LDLs. Fats are needed for energy production, while cholesterol is needed in tiny amounts for

• Synthesis of the cell membrane
• Vitamin D production in the skin
• Sex hormones Formation of bile acids

There are several types of lipoproteins in which cholesterol is carried, but all you need to know for your course are two major types:

Low Density Lipoproteins

These lipoproteins transport cholesterol and triglycerides away from the liver towards the tissues. Some cholesterol may leak out and is then used for the synthesis of the cell membrane, but if cholesterol concentrations are too high, excess cholesterol is released and accumulates on the arterial endothelium.

High Density Lipoproteins

They carry cholesterol and triglycerides from the tissues back to the liver. These are the "good" lipoproteins as they also pick up cholesterol from the arteries to transport them back to the liver.

Cholesterol is then broken down and used to produce bile acid or stored.