The heart is a pump that uses muscular contraction to generate pressure, valves to direct the flow of blood, and a conduction system to coordinate its action. The heart of normal adult mammals has four chambers and four valves. The thickness of the walls of the chambers of the heart reflect the pressures developed within the chambers. The walls of the atria are the thinnest and the left ventricular wall is the thickest. Although we may think of the heart as a single pump it is really two pumps connected in series. Blood enters each pump in through a vein and exits each pump through an artery. One-way valves only allow blood to flow in one direction and maintain the pressure gradient between the arterial and venous circulation. Atrial function is notrequired to support life as demonstrated by patients that survive atrial fibrillation. The atria provide an extra kick to blood entering the ventricles, which improves their performance. Atrial kick is most important in patients with thickened, stiff ventricles.
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