The pressure required to drive blood through blood vessels is generated by the heart. Animal hearts are pumps made of muscle that rhythmically contracts, producing this pressure. Early hearts were as simple as a muscular thickening of a small region of a blood vessel. Such hearts are still found in insects (see Fig. 39.16a). More complex hearts have chambers that expand and fill with deoxygenated blood returning from the animal’s tissues. After filling, the heart muscle contracts, pumping deoxygenated blood to lungs or gills and the newly oxygenated blood onward to the animal’s tissues. The closed circulatory system of cephalopod mollusks has three hearts—two that pump deoxygenated blood through the gills and another that pumps oxygenated blood to the body. In contrast, vertebrates have a single heart with at least two chambers, one for receiving blood and the other for pumping blood to the body during each heartbeat cycle. Hearts also have one-way valves to ensure that blood does not flow backward when the heart contracts.

A theme runs through the evolution of vertebrate circulatory systems: the progressive separation of circulation to the gas exchange organ from circulation to the rest of the body. In fish, deoxygenated blood is first pumped to the gills to gain O₂; then the freshly oxygenated blood flows to the rest of the body and returns to the heart. This single circulation path limits the rate of blood flow to metabolically active body tissues. By contrast, birds and mammals evolved a separate pulmonary circulation to the lungs and systemic circulation to the rest of the body. This double circulation was made possible by the evolution of a four-chambered heart. This organization has two advantages: It increases the supply of oxygenated blood to active tissues, and it increases the uptake of O₂ at the gas exchange surface.

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