The heart is a four-chambered pump and is the driving force for moving blood through the circulatory system. The four chambers can be broken down into two upper chambers or atria, and two lower chambers, known as ventricles. Check valves between the chambers ensure that the blood moves in only one direction and enables the pressure in the aorta, for example, to be much higher than the pressure in the lungs, without blood flowing backwards from the aorta toward the lungs.
The four chambers of the heart including the right atrium, right ventricle, left atrium, and left ventricle are shown in Fig. 2.3. Blood enters the right atrium from the vena cava. From the right atrium, blood is pumped into the right ventricle. From the right ventricle, blood is pumped downstream through the pulmonary artery, to the lungs where it is enriched with oxygen and gives up carbon dioxide.
On the left side of the heart, oxygen enriched blood enters the left atrium from the pulmonary vein. When the left atrium contracts it pumps blood into the left ventricle. When the left ventricle contracts it
pumps blood to a relatively high pressure and ejects the blood from the left ventricle into the aorta.
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