Heartbeat To be an efficient pump, the heart must beat in an orderly and coordinated way. Two networks of muscle fibers coordinate the heart's pumping action—one in the atria and one in the ventricles. When a single muscle fiber in either network is stimulated, the entire network contracts.

1. Atria Contract Each contraction begins in a small group of cardiac muscle fibers—the sinoatrial node (SA node)—located in the right atrium. The SA node “sets the pace” for the heart, so it is also called the pacemaker. When the SA node fires, an electrical impulse spreads through the entire network of muscle fibers in the atria and the atria contract.

2. Ventricles Contract The impulse from the SA node is then picked up by another group of muscle fibers called the atrioventricular node (AV node). Here the impulse is delayed for a fraction of a second while the atria contract and pump blood into the ventricles. Then the AV node produces impulses that spread through the ventricles and cause the ventricles to contract, pumping blood out of the heart. This two-step pattern of contraction—first the atria and then the ventricles—makes the heart an efficient pump.

Control of Heart Rate Your heart rate varies depending on your body's need to take in oxygen and release carbon dioxide. During vigorous exercise, for example, your heart rate could increase to about 200 beats per minute. Heartbeat is not directly controlled by the nervous system, but the autonomic nervous system does influence the activity of the SA node. Neurotransmitters released by the sympathetic nervous system increase heart rate. Those released by the parasympathetic nervous system decrease heart rate.

FIGURE 33–4 Heartbeat The SA node generates an impulse that spreads through the atria, causing the muscle fibers to contract and pump blood to the ventricles. The AV node picks up the signal and, after a slight delay, sends an impulse through the ventricles, causing them to contract.

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