RESPIRATION IN TOAD
The young toad called tadpole uses gills for gaseous exchange. At the tadpole stage, the animal develops external gills through which direct diffusion of gases takes place between the animal (tadpole) and the respiratory medium (water). Oxygen dissolved in water can easily diffuse into the body and carbon dioxide can also easily diffuse out of the body by simple diffusion. This is made possible because the ratio of body surface to volume is very large.
In the next tadpole stage, the external gills disappear and internal gills develop under an operculum or spout. In this stage, the tadpole breaths in a manner similar to that of fish.
The adult toad uses three different respiratory organs or systems for gaseous exchange. The organs are the mouth, skin and lungs.
Mouth or Buccal Respiration
The toad is capable of respiring through the mouth when on land. The buccal cavity (mouth) is able to serve as an efficient respiratory organ because:
- It is very large, i.e ., it has a large surface area.
- It has a thin mucus membrane for easy diffusion.
- It is well supplied with blood capillaries.
For toad to take in air, the mouth is closed, the nostrils are opened and the floor of the buccal cavity is lowered. This action creates a low air pressure within the buccal cavity and this makes air to be drawn in from outside through the nostrils. After this, the nostrils and glottis are closed. Gaseous exchange then takes place in the buccal cavity between the blood in the capillaries and the inhaled air. To remove the carbon dioxide from the body, the toad will raise the floor of the buccal cavity, thereby increasing the air pressure inside the cavity. By so doing, the nostrils are opened and air containing carbon dioxide is forced out to the environment.
Skin or Cutanous Respiration
The adult toad is capable of carrying out respiration using the skin both on land and in water. The skin serves as an efficient respiratory organ because it has a large surface area and a thin covering. The skin is also moist because of continuous secretions from the mucus gland. It is also well vascularised, i.e ., it is well supplied with blood capillaries and as a result, oxygen present in the water or on land can diffuse directly through the entire body covering into the blood of the capillaries from where it is transported to cells and tissues within the body. Carbon dioxide from the respiratory cells also diffuse out from the blood into the capillaries through the skin of the body.
Lungs or Pulmonary Respiration
The adult toad carries out gaseous exchange with its lungs when on land. The mechanism of lung respiration is similar to buccal respiration. As the toad is about drawing in air, the mouth is closed while the nostrils are opened and the floor of the buccal cavity is lowered. This action creates a low air pressure within the buccal cavity and this makes air containing oxygen to be drawn in from outside through the nostrils. After this, the nostrils are closed while the glottis is opened and the air is then forced into the lungs through the glottis, larynx and finally to the lungs. Oxygen then diffuses into the blood of the capillaries of the aveoli while carbon dioxide diffuses out from the blood capillaries into the lungs where it is sent out. The air containing carbon dioxide is drawn out of the lungs into the glottis by the lowering of the buccal cavity while the nostrils are closed. After this, the floor of the mouth is raised several times while air is pumped out of the lungs and finally forced out through the nostrils.