9 - Skin Breathing in Amphibians

Summary

Skin breathing, or cutaneous, gas exchange is an important route of respiration in many aquatic or semiaquatic vertebrates, and is particularly well developed in the amphibians. The skin of amphibians contains a unique vasculature that facilitates oxygen (O₂) uptake and carbon dioxide (CO₂) excretion. Cutaneous gas exchange can fulfill routinely 0% to 100% of O₂ uptake and 20% to 100% of CO₂ excretion (1). Amphibians pay a price for this: They require a relatively thin epidermis and, as a result, suffer from high rates of water loss. Thus, amphibians are, for the most part, tied to an aquatic or semiaquatic life. Furthermore, unlike lungs or gills, the skin lacks a dedicated ventilatory pump and, as such, has been thought to be a poorly regulated respiratory organ, with little scope for change. Research over the past couple of decades has revealed that amphibians may exhibit partial control over the cutaneous vasculature, and that such control is under both neural and possibly hormonal control. For the purposes of this chapter, most information will be taken from literature on ranid frogs (order Anura), because the majority of research has been done on this group.

AMPHIBIAN CUTANEOUS CIRCULATION

Most amphibians possess a double circulation – the pulmonary and systemic – consisting of a right and left atrium and an undivided ventricle. A similar blood flow distribution pattern exists in reptiles and certain air-breathing fishes (2) (see Chapter 5). Primarily deoxygenated (deoxygenated systemic blood plus oxygenated blood from the skin are mixed) blood returns to the heart via the right atrium, whereas oxygenated blood from the lung only returns to the heart via the left atrium.