Wednesday, February 15, 2012

What is a Heart Attack?

This is a layperson's term for a sudden blockage of a coronary artery. This blockage, which doctors call a coronary artery occlusion, may be fatal, but most patients survive it. Death can occur when the occlusion leads to an abnormal heartbeat (severe arrhythmia) or death of heart muscle (extensive myocardial infarction). In both of these situations, the heart can no longer pump blood adequately to supply the brain and other organs of the body. Almost all heart attacks occur in people who have coronary coronary atherosclerosis. So, this photo essay will review the structure (anatomy) of the normal coronary artery, the structural abnormalities (pathology) of the coronary artery in atherosclerosis, and the effect of these abnormalities on the heart.

What are the structures and functions of a normal coronary artery?

The coronary arteries carry blood to the heart to supply oxygen and necessary nutrients. As seen in Figure 1, the wall of a coronary artery has 3 distinct layers: the inner (intima), middle (media), and outer (adventitia) layers. The wall of the artery surrounds the lumen of the artery, which is the channel through which blood flows

Figure 1, smooth muscle is red, and connective (supporting) tissue is black (elastic) or blue (collagen).

The intima is best seen in the close-up view in Figure 1. It is composed of a layer of so-called endothelial cells that covers the artery's inner (lumenal) surface, connective (supporting) tissue (collagen and elastin), and a layer of compact elastic tissue called the internal elastic lamina (IEL). In the past, the intima was thought to be simply a passive layer whose major purpose was to serve as a barrier. Now, however, we know that the endothelial cells actually keep track of the pressure, flow, and "health" of the artery. Moreover, endothelial cells secrete chemicals that can adjust the function of the artery (e.g., vasodilator chemicals to widen and vasoconstrictors to narrow it) and growth of the artery wall (e.g., growth factors).

The media (M) is a layer made up primarily of smooth muscle cells (SMCs). The muscle can contract and relax to control the blood pressure and flow in the artery. Elastic tissue and collagen in the media, along with elastic tissue in the IEL, increase the elasticity and strength of the wall of the artery, as the artery contracts and relaxes. The adventitia is a layer of connective tissue and cells (e.g., SMCs) that produce this connective tissue. The adventitia contains potent factors, including one called tissue thromboplastin, that promote blood clotting. The clots are useful when the artery becomes injured because they can limit excessive bleeding from the injured artery.