Respiratory System of Rana Tigrina (Common Indian bull frog)
Respiration in frogs takes place by various methods such as: by external gills called branchial respiration which occurs in tadpole or larval stage, in adults it takes place either through skin or moist surface called as cutaneous respiration or through bucco-pharyngeal cavity called buccal respiration, or through the lungs called as pulmonary respiration.
Cutaneous respiration occurs every time whether the frog is in or out of the water. It is the only mode of respiration when the frog is under water or hibernating. Oxygen dissolves in a moist surface before diffusing into blood. This is the reason why frogs always stay near water to keep their skin moist.
Buccal respiration occurs when frog is in land, during this, the mouth remains permanently closed while the nostrils remains open. The floor of the buccal cavity is raised and lowered alternatively, so that air is drawn into and expelled out of the buccal cavity repeatedly through the open nostrils. During buccal respiration, the glottis remains closed so that no air enters or leaves the lungs into buccal cavity. When the mucous epithelial lining of buccal cavity is richly supplied with blood capillaries, O2 in the air is richly absorbed by blood while CO2 is given out.
Pulmonary Respiration and Sound Production
Breathing on land in atmospheric air with the help of lungs is called pulmonary respiration. Lungs are poorly developed in frogs, therefore the inadequate supply of O2 obtained through lungs is supplemented through moist skin and buccal cavity.
Organs of respiration:
Organs of aerial respiration are the two lungs. The passage through which air enters and leaves the lungs, is called respiratory tract.
- Respiratory tract- It consists of external nostrils, internal nostrils, nasal chambers, bucco-pharyngeal cavity, glottis, laryngo-tracheal chamber and two bronchi. The glottis on the floor of pharynx opens into a small thin-walled chamber called larynx or laryngo-tracheal chamber. Its walls are supported by cartilages (2 arytenoid + 1 cricoid). Its internal lining forms a pair of elastic horizontal bands called as the vocal cords, for sound production. This is why larynx is known as voice box. When air from lungs is forced out between vocal cords, they start vibrating and making characteristic croaking sound. Special muscles can change the tension of the cords and hence the pitch of the sound. Vocal sacs are found only in male frogs. From larynx a very small tube called as bronchus leads to each lung.
- Lungs- Two lungs are present which are thin-walled, ovoid and highly elastic sacs suspended inside the peritoneal body cavity, one on either side of the heart. They are covered externally by peritoneum. The inner surface of each lung is divided by a network of folds into many small air sacs or alveoli, while leaving a large central cavity. The alveoli are lined with thin epithelium richly supplied with blood capillaries which contains deoxygenated blood for gaseous exchange. O2 of the inhaled air diffuses into blood while CO2 is released into alveoli.
Pulmonary respiration takes place in between buccal respiration, in which buccal cavity acts as a force pump. The rhythmic up and down movements of the floor of buccal cavity are brought by action of two special sets of muscles, sternohyal muscles and petrohyal muscles. The whole mechanism involves two steps: inspiration and expiration.
- Inspiration- Frog closes its glottis and mouth while the nostrils remains open, in order to draw the air into the lungs. Now the sternohyal muscles contract so that the hyoid apparatus and floor of buccal cavity are lowered. This enlargement of buccal cavity draws air through the nostrils into buccal cavity. The glottis now opens and nostrils are closed. Now the petrohyal muscles contract raising the hyoid apparatus and the floor of the buccal cavity. This reduction in volume of buccal cavity forces the compressed air through opened glottis into the two lungs, this process of filling of lungs with air is called inspiration.
- Expiration- When the lungs are filled, the glottis closes and air is held in lungs for some time during which buccal floor is repeatedly raised and lowered to carry on buccal respiration. Soon, the glottis is opened and the air in the lungs is driven out into the buccal cavity by lowering its floor and is also aided by the elasticity if lungs and contractions of the body muscles. When the buccal floor is raised again, the glottis closes and the air is expelled through the opened nostrils to outside. This process through which the lungs are emptied is called expiration.
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