In Amphibians like frog and toad, gaseous exchange occurs through the skin, lining of the buccal cavity and lungs. Only gaseous exchange through the buccal cavity and lungs involves ventilation movements.
Buccal gaseous exchange
The buccal cavity in a toad is large and lined by a thin membrane that is richly supplied with blood capillaries. To draw air into the buccal cavity , the mouth is closed, the nostrils are opened , and the floor of the buccal cavity is lowered. This creates a low air pressure in the buccal cavity, causing air from outside to flow through the nostrils. Then, the nostrils and glottis are closed. Gaseous exchange occurs in the buccal cavity between the blood in the capillaries and the inhaled air. To expel air, the floor of the buccal cavity is raised, increasing the air pressure within the cavity. this forces the valves closing the nostrils to open, allowing air from the buccal cavity to flow out. thus the muscular floor of the buccal cavity acts like a pump: it causes air to be sucked in and pumped out alternatively.
Gaseous exchange through the lungs
Under normal circumstances, the toad gets sufficient oxygen for its needs by gaseous exchange through it's skin and buccal cavity. To draw air into the lungs, air is first sucked into the buccal cavity as just described. Then, the nostrils are closed, the glottis is opened, and the floor of the buccal cavity is raised. This increases the air pressure in the buccal cavity, forcing air into the lungs though the open glottis. The lungs become inflated as air fills the alveoli. Gaseous exchange occurs between the inhaled air in the alveoli and the blood in the capillaries lining the alveoli.
To expel air from the lungs into the mouth, the floor of the buccal cavity is lowered to create a region of low pressure there. When air fills the buccal cavity, the glottis closes and the floor of the buccal cavity is raised. This causes the valves of the nostrils to open , allowing air flow out of the body.
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