BIOLOGY - GAS EXCHANGE IN AMPHIBIANS

Greater SA:V ratio than mammals. Moist skin and mouth lining are the main sites of gas exchange when the frog is inactive on land or when it is in water. If it's inactive then the frog's oxygen demands are low and can easily be met by these regions. Frogs tend to stay in damp areas so their skin stays moist, the skin is thin and permeable with an extensive capillary network below it (short diffusion pathway). Amphibians have a lower body temperature so less energy is needed.

Two internal sac-like lungs are the site of gas exchange when the frog is physically active and when it is active these organs provide the extra surface area needed to obtain extra oxygen. The lungs are ventilated by movements of the mouth and nostrils

Very high metabolic rates (to maintain body temperature) and ventilation is required to maintain a steep concentration gradient at the alveoli to enable sufficient gas exchange

Mouth/nose to lungs, movement along a system of air passages (tubes) known as the trachea, bronchi and bronchioles. Gas exchange occurs in the alveoli in the lungs at the end of each bronchiole

External intercostal muscles contract and pull the rib cage up and out, outer pleural membrane is pulled out and this pulls on the surface of the lungs and causes an increase in the volume of the alveoli. This reduces pressure in the pleural cavity and the inner pleural membrane and the inner pleural membrane is pulled outward and alveolar pressure decreases to below atmospheric pressure and air is drawn into the lungs.