Consider a pump that no human engineer could ever duplicate perfectly​—the heart. This tiny pump pulsates, on the average, seventy times a minute, forty million times a year and passes about 7,000 quarts of fluid a day, nearly 200,000 tons in the average lifetime.

Traffic problems of the body’s “roads” for circulation are ingeniously controlled to perfection. The blood, for instance, can go in only one direction. This one-way movement is achieved by ideally engineered valves built into our veins, known in engineering terms as “check valves.” The blood is pumped by the heart through the arteries under great pressure to the capillaries. Then, returning to the heart from the lower extremities under very little pressure, the blood is forced upward through the veins by the exertion of leg and stomach muscles. The tightening and contracting of these muscles force the blood through the veins. In addition to the muscle action, flow in the direction of the heart is maintained by means of the “check valves.”