the body's digestive system (stomach, small intestines, liver, pancreas, etc.). Oxygen
comes from the air that is breathed in and absorbed by the lungs. Oxygen in the lungs
and food in the intestines, however, cannot help muscles and other cells unless the
oxygen and food can be delivered to those cells. Delivering food and oxygen to the
cells is the function of blood and of the body's cardiovascular system.
THE CARDIOVASCULAR SYSTEM
The cardiovascular system consists of the heart and the blood vessels. The
cardiovascular system keeps all parts of the body supplied with blood. Blood brings
oxygen and nutrients to the cells and carries away waste products.
a. Heart. The heart (figure 3-1) is the pump that keeps the blood circulating.
Actually, the heart could be described as two pumps, both of which are under the
control of a natural pacemaker called the sinoatrial node. Each side (right half and left
half) of the heart has a receiving chamber for the blood called the atrium and a pumping
chamber called the ventricle. The two halves of the heart are separated by a wall-like
structure called the interventricular septum. The sinoatrial node (SA node), located at
the junction of the superior vena cava and the right atrium, is a small bundle of nerve
tissue that produces an electrical stimulus. This electrical stimulus causes the muscles
of the ventricles to contract.
Figure 3-1. The human heart (front view).