Cutaneous respiration is a form of respiration in which gas exchange occurs across the skin rather than the lungs or gills. They occur in many organisms like insects, amphibians, fish, sea snakes, turtles and few mammals too. The skin of such organisms is very special that they are used for breathing. In order to breathe through the skin, it has to be moist for oxygen and carbon dioxide to pass through.
Certain amphibians have a lot of folds in their skin in order to increase the rate of respiration. Frogs are known to drink water and breathe through their skin. They have three ways of breathing – through their skin, lungs and lining of the mouth. While at rest, breathing via the mouth becomes predominate form of breathing.
While the amphibian is completely submerged, respiration takes place through their skin. The skin has membrane pores through which gases diffuse between the blood vessels and their surroundings. Although the cutaneous respiration is predominant, it is only during the colder season that it is able to sustain the life of the frogs. Cutaneous respiration requires constant moisture. When the frog is out of the water, mucus glands in the skin keep the skin moist, which helps absorb dissolved oxygen from the air.
Tadpoles breathe with gills while mature frogs and toads have lungs. Still they get most of their air they need through their moist skin. Oxygen easily passes through the porous membranes to the blood filled capillaries beneath. Desert frogs tend to have a dry skin and so they breathe through their large lungs.