Adult frogs breathe through the skin when under water. They have thin skin, which has a massive network of blood vessels and capillaries under it. Oxygen enters the frog’s body through the skin and goes into the blood stream, which carries it to the rest of the body. Once the blood cells have released the oxygen, they absorb carbon dioxide and carry it back to the skin for excretion.
The exchange of gases through the frog’s skin, known as cutaneous respiration, occurs by diffusion. Oxygen and carbon dioxide diffuse down the concentration gradient, from an area of high concentration to an area of low concentration. Cutaneous respiration accounts for more than half of the respiration in adult frogs. It occurs both in and out of water but requires moist skin. The frog’s skin has glands that secrete mucus to keep it moist when the animal is on dry land.
Frog larvae, or tadpoles, breathe through external gills when under water. The gills, like the adult frog’s skin, absorb oxygen from the water and excrete carbon dioxide. However, as tadpoles mature into adults, their bodies absorb the gills and turn them into internal organs. They also develop lungs, which they use to supplement the oxygen absorbed by the skin.