The whole circuit
The two loops make one circuit, and the blood passes through the heart twice on each round: the right side sends it to the lungs, the left side to the body. Arteries carry blood away from the heart and veins bring it back, whatever its oxygen. The pulmonary arteries carry dark blood, the pulmonary veins bright.
The main arteries and veins of the body from the frontWikimedia Commons, Mariana Ruiz Villarreal (LadyofHats), Public domain (Circulatory System en.svg)
At rest a red cell completes the round in about a minute. At any moment the veins hold about two thirds of the blood, the arteries about an eighth, and the arterioles and capillaries less than a tenth. The heart and the vessels of the lungs hold the rest.
Wide and slack, the veins act as a reservoir. When the body needs more output, their walls tighten and send part of their store to the heart.
Sources
- Arthur Guyton and John Hall, Textbook of Medical Physiology, 13th ed. (2016), ch. 14 and 15.
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