Vertebrates have a closed circulatory system and a heart with two or more chambers. When a heart chamber contracts, it squeezes the blood, putting it under pressure. Blood then flows out of the heart and into vessels, where pressure is lower. Resistance to flow in the vessels dissipates the pressure imparted to the blood by the heart. One-
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The circulatory system in fishes is organized as a single circuit in which blood is pumped by a four-
The lungfish is a transitional form in the evolution of a single blood-
Ectothermic reptiles have the ability to control blood flow to the pulmonary circuit, enabling them to conserve energy when they are not breathing.
Separation of pulmonary and systemic circulations maximizes oxygen transport to body cells and gas exchange in respiratory tissues, and allows the two circuits to operate at different pressures.
In fishes, the phylogenetically oldest vertebrates, blood is pumped from the heart to the gills and then to the tissues of the body and back to the heart—
Both pulmonary and systemic circuits begin with vessels called arteries that carry blood away from the heart. Arteries branch into smaller arterioles that feed blood into capillary beds. Capillaries are tiny, thin-
We can trace the evolutionary history of vertebrate circulatory systems by comparing the circulatory systems of fishes, lungfishes, amphibians, reptiles (including crocodilians), and mammals.