• Respiratory system is comprised of the organs that help in the intake and supply of oxygen to the tissues as well as to get rid of excess carbon dioxide.
• Branchial respiration occurs in case of tadpoles or larval stage.
• Branchial respiration takes place by means of external gills.
• Respiration in adult frog occurs through 3 different ways:
  • Cutaneous respiration: It takes place through moist surface of outer skin.
  • Buccal respiration: It takes place through lining of bucco-pharyngeal cavity.
  • Pulmonary respiration: It takes place through lungs.
• In each of the above cases, there are numerous blood capillaries lying close to the epithelium.
• Through these capillaries, the incoming O₂ and outgoing CO₂ diffuse readily.

i. Cutaneous respiration in frog:

• Cutaneous respiration takes place all the time, whether frog is in or out of water.
• When the frog is under water or hibernating, it is the only mode of respiration.
• There is rich supply of blood in the skin and it is permeable to gases.
• Oxygen must first dissolve in a moist surface before it diffuses into blood.
• This is also one of the reasons for frogs to stay near water and keep their skin moist.
• The mucus glands also secrete mucus so that the skin doesn’t dry out of water.
• Movement is not required in cutaneous respiration as skin remains exposed to air or water.

ii. Buccal respiration in frog:

• In buccal respiration on land, the mouth stays permanently closed while the nostrils remain open.
• The floor of the buccal cavity is alternately raised and lowered.
• It allows the air to be drawn into and expelled out of the buccal cavity repeatedly through the open nostrils.
• The glottis remains closed during buccal respiration. It is done so that no air enters or leaves the lungs into buccal cavity.
• The mucus epithelial lining of buccal cavity is rich in blood capillaries which absorbs O₂ in the air and gives out CO₂.

iii. Pulmonary respiration and sound production in frog:

• The breathing that takes place on land in the atmospheric air by the help of lungs is termed as pulmonary respiration.
• In case of frogs, the lungs are poorly developed.
• Thus, the insufficient supply of O₂ obtained through lungs is supplemented by the moist skin and buccal cavity.

Respiratory Organs of frog:

• The main organs for aerial respiration are the two lungs.
• The passage through which the air enters and leaves the lungs is termed as respiratory tract.
• Respiratory tract; It consists of:
  • External nostrils
  • Internal nostrils
  • Nasal chambers
  • Bucco-pharyngeal cavity
  • Glottis
  • Laryngo-tracheal chamber
  • Two bronchi
  • On the floor of pharynx, the median slit-like glottis is present.
  • Glottis opens into larynx. Larynx or laryngo-tracheal chamber is a thin walled chamber.
  • Cartilages (2 arytenoid + 1 cricoid) support the walls of larynx.
  • Its internal lining forms a pair of elastic horizontal bands, the vocal cords.
  • Vocal cords are for sound production thus larynx is also called as voice box.
  • The characteristic croaking is produced because of the vibrations formed when the air from lungs is forced outside.
  • The tension of the cords can be changed by special muscles, hence, changing the pitch of the sound.
  • Only male frogs have vocal sacs so as to amplify the croaking sound.
  • From larynx, bronchus leads to each lung.
  • Bronchus is a very small tube.
• Lungs:
  • Lungs are ovoid in structure.
  • They are thin-walled and highly elastic sacs.
  • They are suspended freely inside the peritoneal body cavity, one on either side of the heart.
  • Peritoneum covers the lung externally.
  • The inner surface of each lung is divided by septa (a network of folds) into many small air sacs or alveoli, leaving a clear large central cavity.
  • The alveoli are lined by thin epithelium.
  • This epithelium is richly supplied with blood capillaries that contains de-oxygenated blood for gaseous exchange.
  • O₂ in the inhaled air diffused to blood whereas CO₂ is released into alveoli.

Mechanism of Respiration in frog:

• Pulmonary respiration occurs in between buccal respiration.
• Here, buccal cavity acts as a force pump.
• The action of two special sets of muscles brings the rhythmic up and down movements of the floor of buccal cavity.
• Sternohyal muscles:
  • These muscles are attached at the lower end to the sternum and at the upper end to the undersurface of cartilaginous hyoid apparatus embedded in the floor of buccal cavity.
• Pterohyal muscles:
  • These muscles are attached below to the upper surface of hyoid apparatus and above to the squamosal bone of skull.
• The whole respiratory mechanism involves two phases:

Inspiration:

• The frog closes its glottis and mouth while drawing air into the lungs.
• In this phase, nostrils remain open.
• Now, the sternohyal muscles contract.
• This contraction allows the hyoid apparatus and the floor of buccal cavity to be lowered.
• The buccal cavity is enlarged and the air is drawn into the buccal cavity through the nostrils.
• Now, the glottis opens and the mentomeckalian bones of lower jaw push upwards the premaxillae bones of upper jaw. This closes the nostrils.
• Then, the pterohyal muscles contracts which raises the hyoid apparatus and the floor of buccal cavity.
• The reduction in volume of buccal cavity forces the compressed air through the opened glottis into the two lungs.
• This process by which lungs are filled with air is called inspiration.

Expiration:

• The glottis closes when lungs are filled with air.
• For some time, air is held in the lungs during which buccal floor is repeatedly raised and lowered to carry on buccal respiration.
• Soon, the glottis opens.
• The air in the lungs are driven out into the buccal cavity by lowering its floor.
• It is aided by the elasticity of lungs and contraction of the body muscles.
• The glottis closes when the buccal floor is raised again.
• Now, the air is expelled through the opened nostrils to outside.
• This process by which the lungs are emptied is called expiration.

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