Frog Respiratory System

As an amphibian, the frog spends part of the year submerged in water and part of the year active on land, and correspondingly it does not rely on a single respiratory organ the way a fish (with gills alone) or a mammal (with lungs alone) does. Instead, the frog uses three distinct modes of respiration, and which mode dominates at any given time depends directly on where the frog is and how active it is.

Cutaneous respiration is gas exchange occurring directly across the frog's skin. The skin is thin, kept constantly moist by mucous glands, and lies close beneath its surface to a dense network of blood capillaries, all of which make it an efficient respiratory surface in its own right — moist, thin, permeable membranes in close contact with blood are exactly what any gas-exchange surface requires. Cutaneous respiration becomes the frog's dominant means of gas exchange when it is submerged in cold, well-oxygenated water, since cold water holds more dissolved oxygen and the frog's metabolic rate, and therefore its oxygen demand, is correspondingly low in such conditions; the skin alone can meet nearly all of its respiratory needs. This is also why cutaneous respiration sustains the frog through hibernation, when it buries itself in mud at the bottom of a pond and its overall metabolic activity falls sharply.

Buccopharyngeal respiration is a smaller-scale but essentially continuous supplementary mode, occurring across the moist, well-vascularised lining of the buccal (mouth) cavity. The frog ventilates this surface by rhythmically raising and lowering the floor of its mouth while its nostrils and glottis remain open, drawing air in and out over the buccal lining without necessarily filling the lungs; because this movement can continue almost constantly, buccopharyngeal respiration contributes a steady, if modest, supply of additional oxygen alongside whichever of the other two modes is currently dominant. …