Closed Circulatory systems

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Many animals, including all vertebrates, have a closed circulatory system in which the blood is enclosed within tubes. This generates higher blood pressure as the blood is forced along fairly narrow channels instead of flowing into large cavities. This means the blood travels faster and so the blood system is more efficient at delivering substances around the body: the blood leaves the heart under pressure and flows along arteries and then arterioles (small arteries) to capillaries. There are extremely large numbers of capillaries. These come into close contact with most of the cells in the body, where substances are exchanged between blood and cells. After passing along the capillaries, the blood returns to the heart by means of venules (small veins) and then veins. Valves ensure that blood flows in only one direction. 

Animals with closed circulatory systems are generally larger in size, and

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