An adult frog lives on land but goes back into the water during the breeding season.
A frog uses three different respiratory surfaces. These are the skin, buccal cavity and lungs.
Skin
The skin is used both in water and on land. It is quite efficient and accounts for 60% of the oxygen taken in while on land.
Adaptations of a Frog’s Skin for Gaseous Exchange
The skin is a thin epithelium to allow fast diffusion. The skin between the digits in the limbs (i.e. webbed feet) increases the surface area for gaseous exchange. It is richly supplied with blood vessels for transport of respiratory gases.
The skin is kept moist by secretions from mucus glands. This allows for respiratory gases to dissolve. Oxygen dissolved in the film of moisture diffuses across the thin epithelium and into the blood which has a lower concentration of oxygen.
Carbon (IV) oxide diffuses from the blood across the skin to the atmosphere along the concentration gradient.
Buccal (Mouth) Cavity
Gaseous exchange takes place all the time across thin epithelium lining the mouth cavity. Adaptations of Buccal Cavity for Gaseous Exchange It has a thin epithelium lining the walls of the mouth cavity allowing fast diffusion of gases. It is kept moist by secretions from the epithelium for dissolving respiratory gases.
It has a rich supply of blood vessels for efficient transport of respiratory gases. The concentration of oxygen in the air within the mouth cavity is higher than that of the blood inside the blood vessels.
Oxygen, therefore dissolves in the moisture lining the mouth cavity and then diffuses into the blood through the thin epithelium. On the other hand, carbon (IV) oxide diffuses in the opposite direction along a concentration gradient.
Lungs
There is a pair of small lungs used for gaseous exchange.
Adaptation of Lungs
- The lungs are thin walled for fast diffusion of gases.
- Have internal folding to increase surface area for gaseous exchange.
- A rich supply of blood capillaries for efficient transport of gases.
- Moisture lining for gases to dissolve.
Ventilation
Inspiration
During inspiration, the floor of the mouth is lowered and air is drawn in through the nostrils. When the nostrils are closed and the floor of the mouth is raised, air is forced into the lungs. Gaseous exchange occurs in the lungs; oxygen dissolves in the moisture lining of the lung and diffuses into the blood through the thin walls. Carbon (IV) oxide diffuses from blood into the lung lumen.
Expiration
When the nostrils are closed and the floor of mouth is lowered by contraction of its muscles, volume of mouth cavity increases. Abdominal organs press against the lungs and force air out of the lungs into buccal cavity. Nostrils open and floor of the mouth is raised as its muscles relax. Air is forced out through the nostrils.
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