As animals moved onto land, the transition from breathing water to breathing air had important consequences for the organization of vertebrate circulatory systems. Land vertebrates evolved hearts that separated the circulation of deoxygenated blood pumped to their gas exchange organs from circulation of oxygenated blood delivered to their body tissues. Gas exchange became more efficient and O₂ delivery increased. Metabolic rates rose, and animals became capable of greater activity.

Reflecting their aquatic and terrestrial lifestyle, amphibians evolved a variety of ways to breathe. Some retain gills, while others use lungs or their skin, or some combination of these, to breathe. Amphibian hearts evolved an additional atrium, becoming divided into two separate atria and a single ventricle (Fig. 39.21). This arrangement partially separates the pulmonary and systemic circulations, so freshly oxygenated blood can be pumped under higher pressure to the body than is the case in fishes. However, because amphibians have a single ventricle, oxygenated blood returning to the heart from the gills or lungs mixes with deoxygenated blood returning from the animal’s body before being pumped from the ventricle. Anatomical features of the ventricle help to limit this mixing, but nonetheless mixing still hinders the supply of oxygenated blood to an amphibian’s body.

Most reptiles also have a three-chambered heart, but their single ventricle has internal ridges to improve the separation of deoxygenated and oxygenated blood. As a result, reptiles are able to deliver more O₂ to their tissues than amphibians can. At the same time, the incompletely divided ventricle provides reptiles with versatile circulatory responses during exercise and when diving underwater. For example, when they dive, the blood of turtles and sea snakes bypasses their lungs, which they cannot use underwater. Notably, crocodiles and alligators have evolved four-chambered hearts, similar to those of mammals and birds, which are discussed next.

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