Arteries are essentially high-pressure pipes that carry blood pumped by the heart out to all of the tissues of the body, such as muscle, skin, bone, liver, kidneys, etc. The arterial system resembles a tree with a large main trunk - the aorta - and progressively smaller arteries branching out from the aorta. The thick-walled aorta, somewhat less than an inch in diameter, emerges from the top of the heart, gives off branches that supply the head and arms and then curves downward, running just in front of the spine, to supply the trunk and internal organs, and eventually splits into two large arteries that supply the legs. After many stages of branching, the tiniest arteries become capillaries - the smallest blood vessels. In the capillaries, oxygen and nutrients in the blood are delivered to tissues, and waste products are picked up. Capillaries then join together to form tiny veins, which join together many times again to form larger veins. Veins are thin-walled, low-pressure blood vessels, which bring blood from the tissues back to the heart.
At the very beginning of the aorta, just beyond the heart valve that separates the pumping chamber of the heart from the aorta, the left and right coronary arteries branch off to supply the heart muscle itself. The left main coronary artery splits almost immediately into two branches, called the left anterior descending coronary artery and the left circumflex coronary artery. Coronary artery disease due to atherosclerosis is often described as 1-, 2-, or 3-vessel disease, depending on whether major blockages are found in various combinations of the right, left anterior descending, and left circumflex coronary arteries and their branches. The most important coronary artery is the left anterior descending artery, which supplies the front side of the heart as well as the muscular septum that separates the right and left pumping chambers. Almost all Americans over 50 years of age have some cholesterol deposition and thickening of the inner arterial wall near the beginning of the left anterior descending coronary artery. This thickened, cholesterol-loaded area is an atherosclerotic plaque, sometimes called the "widowmaker" plaque.
If we look at a cross-section (a thin, perpendicular slice) of a normal artery under the microscope, 3 tissue layers will be observed in the arterial wall. Most prominent is the middle layer, called the media, which consists of tightly packed smooth muscle cells, fibrous tissue proteins such as collagen and elastin, and gel-forming proteoglycans. The inner tissue layer is the intima, which is a much looser structure with fewer cells, less elastin, and considerably more open spaces between tissue components. The outer tissue layer, called the adventitia, is also a relatively loose tissue consisting mostly of bundles of collagen along with a few connective tissue cells.
Atherosclerosis is a disease of the arterial intima - so it's worth giving a little more attention to the intima, which is the innermost layer of the arterial wall. The intima, like the other arterial tissue layers, is a type of connective tissue. Connective tissues give form and structure to the body, and keep the organs in place. Examples of connective tissues are the deeper layers of skin, the fascia that separate muscle layers (think of "gristle" in meat), bones, and tendons. Much of the strength in connective tissues comes from fibrous tissue proteins - elastin and various kinds of collagen - which are located between the cells in the tissue and are laid down by the cells. In the arterial intima, scattered smooth muscle cells manufacture the collagen, elastin, and proteoglycans that form the bulk of intimal tissue. When injured, connective tissues form scars, and some features of atherosclerosis are very similar to scar formation.
At the boundary between the arterial intima and flowing blood are found the lining cells of the artery, called endothelial cells. These cells are very long and wide, but have almost no thickness at all (often less than one thousandth of a millimeter), and they are arranged like paving stones on the inner surface of the arterial wall. There are no gaps between the endothelial cells. They appear to be carefully designed to keep blood cells and blood proteins from coming into contact with the underlying connective tissue of the intima. Historically, endothelial cells were considered to act only as a blood-tissue barrier in blood vessels, but in recent years we have learned that endothelial cells are important for determining arterial tone and size and for causing inflammation. The key role of endothelial cells in atherosclerosis will be described later.
John R. Guyton, MD