- The heart has four chambers, two associated with the pulmonary circuit (right atrium and right ventricle) and two with the systemic circuit (left atrium and left ventricle). The left ventricle has a greater workload and is much more massive than the right ventricle, but the two pump equal amounts of blood. AV valves prevent backflow from the ventricles into the atria, and semilunar valves prevent backflow from the aortic and pulmonary trunks into the ventricles.
- The heart rate is normally established by the cells of the SA node, but that rate can be modified by autonomic activity, hormones, and other factors. From the SA node the stimulus is conducted to the AV node, the AV bundle, the bundle branches, and Purkinje fibers before reaching the ventricular muscle cells. The electrical events associated with the heart beat can be monitored in an electrocardiogram (ECG).
- Cardiac output is the amount of blood pumped by the left ventricle each minute. It is adjusted on a moment-to-moment basis by the ANS, and in response to circulating hormones, changes in blood volume, and alterations in venous return. Most healthy people can increase cardiac output by 300-500 percent.
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