The Pattern of Cardiac Contraction

Cardiac muscle cells contain a contractile apparatus like that of other striated muscle, being made up of bundles of myofilaments with a microscopic structure like that discussed in Chapter 10. Unlike other striated muscles in the body, the heart muscle is specialized to produce a rhythmic and coordinated contraction in order to drive the blood efficiently through the blood vessels. The heart has a number of tasks to accomplish in order to carry out its role in providing oxygen to the cells of the body. It must receive the oxygen-poor blood returning from the body tissues via the venous circulation and send that blood to the lungs for oxygenation. It must also receive the oxygenated blood from the lungs and send it out through the arterial circulation to the rest of the body. Carrying out these tasks requires precise timing of the contractions of the various heart chambers; otherwise, the flow of oxygenated blood will not occur efficiently or will cease altogether with disastrous consequences. What is the normal timing sequence of the heart contractions underlying the coordinated pumping of the blood?

A schematic diagram of the flow of blood through a human heart during a single contraction cycle is shown in Figure 12-2. Humans, like all other mammals, have a four-chambered heart, consisting of the left and right atria and the left and right ventricles. The two atria can be thought of as the receiving chambers, or "priming" pumps, of the heart, while the two ventricles are the "power" pumps of the circulatory system. The right atrium receives the blood returning from the body through the veins, and the left atrium receives the freshly oxygenated blood from the lungs. During the phase of the heartbeat when the atria are filling with blood, the valves connecting the atria with the ventricles are closed, preventing flow of blood into the ventricles. When the atria have filled with blood, they contract and the increase in pressure opens the valves leading to the ventricles and drives the collected blood into the ventricles. At this point, the muscle of the ventricles is relaxed, and the valves

Figure 12-2 Schematic drawings of the state of the heart valves and the direction of blood flow during two stages in a single heartbeat. (a) The atria are contracting and the ventricles are filling with blood. (b) The valves between the atria and ventricles are closed and the ventricles are contracting, forcing the blood from the right ventricle to the lungs and from the left ventricle to the arteries supplying the rest of the body.

(b)

To lungs

To lungs

To lungs

To arteries

To lungs

To arteries connecting the ventricles to the vessels leaving the heart are closed. When the ventricles have filled with blood, they contract, opening these valves and delivering the power stroke to drive the blood out to the lungs and to the rest of the body, as shown in Figure 12-2b. Thus, during a normal heartbeat the two atria contract together, followed after a delay by the simultaneous contraction of the two ventricles.

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