All other terrestrial vertebrates also have a "double-loop" circulatory pattern. This pattern of circulation provides a significant advantage over the "single-loop" circulation of a fish—faster blood flow to the body. In a fish, the blood loses some of its force as it passes through the narrow capillaries of the gills, and blood flow slows as a result. The lungs of an amphibian also contain narrow capillaries that slow blood flow. But after passing through the capillaries of the lung, blood returns to the heart to be pumped a second time before circulating to the body.

figure 40-9

Frogs breathe by creating pressure that forces air into their lungs. O When the floor of the frog's mouth drops, air capacity increases in the frog's mouth, and air rushes in. 0 When the nostril is closed and the mouth floor rises, the air is forced into the lungs of the frog. © The mouth floor lowers and air is forced out of the lungs. O Then the nostril opens and the mouth floor rises again, forcing air out the nostril.

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