How is the circulatory system of an amphibian characterized?

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Amphibians have a very unusual circulatory system, which is sort of intermediate between the two-chamber system of fish and the four-chamber system of birds and mammals. It is called a pulmocutaneous circulatory system.

The heart has two atria and one ventricle. The left atrium of the heart receives oxygenated blood from the lungs. The right atrium of the heart receives deoxygenated blood from the body. The ventricle pumps oxygenated blood from the left atrium into the body, but it also pumps deoxygenated blood from the right atrium to the pulmocutaneous artery, a structure that only amphibians have. The pulmocutaneous artery branches into the pulmonary artery, which goes to the lungs, and the cutaneous artery, which feeds directly into the skin. Amphibians have very thin skin, which can actually absorb  oxygen directly without breathing; the cutaneous artery brings blood up to the skin to receive oxygen in this way. (Even we mammals absorb a small amount of oxygen through our skin, but it's largely negligible.)

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