key concept 49.2 Vertebrate Circulatory Systems Evolved from Single to Double Circuits

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-way valves prevent backflow of blood as the heart cycles between contraction and relaxation. Comparisons of the circulatory systems of different vertebrates reveals that as circulatory systems become more complex, the blood that flows to the gas exchange organs (gills or lungs; see Figure 48.3) becomes increasingly separated from the blood that flows to the rest of the body.

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  • The circulatory system in fishes is organized as a single circuit in which blood is pumped by a four-chambered heart through the gills and then to the other organs of the body.

  • The lungfish is a transitional form in the evolution of a single blood-flow circuit into separate pulmonary and systemic circuits in other vertebrates.

  • 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—a single circuit. In birds and mammals, blood is pumped from the heart to the lungs and back to the heart in a pulmonary circuit, and then from the heart to the rest of the body and back to the heart in a systemic circuit. In amphibians and ectothermic reptiles we see various adaptations that incompletely separate the blood flow into pulmonary and systemic circuits.

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-walled vessels where materials are exchanged between blood and the interstitial fluid. Small vessels called venules drain capillary beds. The venules join to form the larger veins that ultimately deliver blood back to the heart.

We can trace the evolutionary history of vertebrate circulatory systems by comparing the circulatory systems of fishes, lungfishes, amphibians, reptiles (including crocodilians), and mammals.