The Circulation

The central organ of the human circulatory system, the heart, must be among the most remarkable creations of nature. In the longest-living individuals, it works continuously for a hundred or more years, executing something like 4,000 million working strokes and moving 350,000 cubic meters of blood, enough to make a small lake. In individuals who die of heart disease, it is not, as a rule, the heart itself that fails but some auxiliary mechanism, like one of its arteries, or the pacemaker. If an artery, supplying a small part of the heart, is blocked, the tissues receiving oxygen and nutrients from that vessel die. If the area involved is not so large as to endanger the entire heart, the damage is gradually repaired by the immune system. The dead cells are removed but cannot be replaced; the gap they leave is filled with scar tissue. While the repair is carried out, the heart continues to work.

Among other remarkable properties of the heart is a virtual immunity from cancer and a good resistance to inflammatory diseases. When the body is at rest, the heart contracts approximately once every second. Its contraction – the systole – lasts about one-third of a second; its relaxation period – the diastole – two-thirds of a second. During hard physical exercise, the heart rate increases about three times.

The arterial system is like a many-branched tree. Its trunk, the aorta, is about three centimeters in diameter at its origin. The branches become progressively smaller and end in a network of capillaries of microscopic size. On the return side, blood is collected by small venules, which join to form veins and end in two large venous trunks. The entire length of the system is more than enough to encircle the earth.