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.
Source: Brainly.in
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.
