Like any pump, the heart has valves to keep the blood flowing in the right direction. Proper function of these small flaps of tissue spells the difference between good health and sickness, and often between life and death.

Almost everyone is familiar with the word valve. Very few people, however, really know what a valve is or what it does. Imagine pumping water through a pipe with a farm pump. To keep the water from flowing back toward the pump between strokes, you could place a valve in the pipe leading out of the pump. The simplest kind of valve would consist of two semicircular flaps hinged to open only one way—forward with the flow of water. These flaps would close the pipe completely when they swung shut. When the water flowed forward from the pump, the flaps of the valve would swing open allowing the water to pass. Between strokes the valves would snap shut if any water attempted to flow back toward the pump (Fig. 2-1).

The valves between the atria and ventricles are called the atrioventricular (AV) valves. The AV valve leading into the right ventricle has three flaps and is called the tricuspid valve (a cusp is a valve flap or leaflet).