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    cardiovascular system: human


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    NARRATOR: The heart and the blood vessels together constitute the cardiovascular system. Blood circulates within this network in two separate loops: the systemic bloodstream and the pulmonary bloodstream.

    The pulmonary bloodstream is fed by the heart's right ventricle, which sends the blood to the lungs. The blood is loaded with oxygen, which turns it bright red, and then sent back to the heart via the pulmonary veins. The blood is then expelled by the left ventricle and enters the systemic bloodstream. This circuit irrigates the entire body and supplies different tissues with oxygen. When the blood returns to the heart--less than a minute later--it is loaded with carbon dioxide and is darker in color. These two cardiovascular circuits form a vast network composed of an infinite number of blood vessels with a total length of almost 150,000 kilometers.

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