Question

Oxygen flux across the skin of a frog is nearly constant throughout the year, while gas exchange across the lungs increases markedly during summer. In both circumstances, oxygen partial pressures in the blood and the atmosphere are steady. Explain why pulmonary exchange should vary while cutaneous exchange should not.

Answer 1

Gas exchange in animals is ultimately diffusion based, generally occurring across dedicated respiratory organs. In many aquatic amphibians, however, multiple modes of gas exchange exist, allowing for the partitioning of O2 uptake and CO2 excretion between respiratory organs with different efficiencies. For example, due to the physical properties of O2 being vastly different between air and water phases, the lung and skin play disproportionately important roles in O2 uptake. Many aquatic frogs are renowned for their cutaneous gas exchange capacity, where often the majority of CO2 is excreted across the skin. Furthermore, the roles of these gas exchange organs change with the animal's behavior. Under diving conditions, most of the frog's gas exchange needs must be met by the skin. In this article, we describe an interactive undergraduate laboratory that allows a class of students to share equipment while assessing pulmonary and cutaneous respiration in frogs provided with an air/water choice and under enforced dive conditions. Concepts explored in this laboratory exercise include animal energetics, diving reflex, pulmonary and cutaneous gas exchange processes, diffusion-based gas flux, and O2 debt.

the objectives of this laboratory are to compare the relative importance of two respiratory surfaces (i.e., the lungs and skin) in O2 uptake, to evaluate whether cutaneous respiration is sufficient to fuel metabolism or whether an “O2 debt” is incurred during enforced diving, and whether such a debt is repaid by preferential use of aerial or aquatic gas exchange. In this laboratory, students establish, by simple observation, the natural diving pattern of the frog. In addition, using closed-system respirometry, they measure the frog's O2 uptake from air (i.e., via the lungs) and from water (i.e., via the skin) during normal diving sequences, during enforced submergence, and during recovery. These measurements are then used to assess the normal proportion of total gas exchange attributed to cutaneous gas exchange, whether cutaneous respiration can be upregulated during a dive, and whether the postdive O2 debt is preferentially resolved by pulmonary gas exchange.