Atherosclerosis

Atherosclerosis is a form of arteriosclerosis, which involves the build-up of plaque. 60-70% of build-ups cause an MI to occur, but most fatal heart attacks result from a 40% blockage.

Plaque formed on vessel walls crack or break, causing the fibrous cap (scab) to break as well. The contents of the plaque are released into the bloodstream, causing a clot to develop. The clot moves through smaller blood vessels and ends up blocking the bloodstream. This clot is called a thrombus.

Thrombolytic drugs can dissolve clots if administered within 2-6 hours, but most people don't want to go to the Emergency Room.

Atherosclerosis is the accumulation of lipids and other substances in the innermost lining of the blood vessel wall, the endothelial cells. These lipids are not an isolated thing- everyone has some. They start in childhood but manifest in adulthood, becoming visible to the naked eye by age 3.

The "Injury Theory" suggests that an unknown injury occurs at the endothelial cells. A monocyte, white blood cell, moves to the area and can be trapped under the lining due to a disconnect in the endothelial cell. Possibly an inflammatory response occurs, which attracts more monocytes to the area.

The monocytes become macrophages, which swallow other things including lipids and LDL. The macrophages then become foam cells, which are a component of the fatty streak. As the accumulation develops under the blood vessel wall, smooth muscle surrounds the blood vessel and allows vasocontriction and vasodilation. Macrophages release a signal for reinforcement and smooth muscle cells and LDL come to the site, as well as clotting factors that initiate a clot.

The accumulation bulges out into the blood vessel and reduces flow. A fibrous cap forms that covers it. It may crack or break open, then form a clot in a smaller blood vessel.