The Circulatory System

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The Circulatory System

The circulatory system in anatomy and physiology is the course taken by the blood through the arteries, capillaries, and veins and back to the heart. In humans and the higher vertebrates, the heart is made up of four chambers the right and left auricles, or atria, and the right and left ventricles. The right side of the heart pumps oxygen-poor blood from the cells of the body back to the lungs for new oxygen; the left side of the heart receives blood rich in oxygen from the lungs and pumps it through the arteries to the various parts of the body. Circulation begins early in fetal life. It is estimated that a given portion of the blood completes its course of circulation in approximately 30 seconds. Pulmonary
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This action is sudden and occurs almost simultaneously in both auricles. The mass of blood in the veins makes it impossible for any blood to flow backward. The force of blood flowing into the ventricles is not powerful enough to open the semilunar valves, but it distends the ventricles, which are still in a condition of relaxation. The tricuspid and mitral valves open with the blood current and close readily at the beginning of ventricular contraction.

The ventricular systole immediately follows the auricular systole. The ventricular contraction is slower, but far more forcible then the ventricular chambers are virtually emptied at each systole. The apex of the heart is thrown forward and upward with a slight rotary motion then this impulse, called the apex beat, can be detected between the fifth and sixth ribs. The heart is entirely at rest for a short time after the ventricular systole occurs. The entire cycle can be divided into three periods then in the first, the auricles contract and in the second, the ventricles contract; in the third, both the auricles and the ventricles remain at rest. In humans, with a normal heart rate of approximately 72 heartbeats per minute, the cardiac cycle has a duration of about 0.8 second. Auricular systole requires about 0.1 second; ventricular systole occupies
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