The organs concerned with the
exchange of gases forms respiratory system. In frog, respiration takes place
through moist skin, lining of bucco-pharyngeal cavity and lungs. However, in
tadpole larva, respiration takes through gills.The respiration through gills is called as branchial respiration.
Types of respiration: In adult frog 3 types of respiration are found;
1.Cutaneous respiration: Respiration through
moist skin is called as cutaneous respiration. It is the main mode of respiration in frog.The skin is richly supplied with
blood vessels and is permeable to gases. Oxygen get dissolved in water on moist surface
of skin then exchange of gases takes place by diffusion. It goes on all the
time whether frog is in water of on land. It is only mode of respiration during
hibernation or aestivation or when Frog is under water.
2. Buccal respiration:- It takes place on land
and during it mouth and glottis remains closed while nostrils remained open.
The floor of buccal cavity is alternately raised and lowered so that air is
drawn into and expelled out of buccal cavity through nostrils. The epithelial
lining of buccal cavity is mosit and highly supplied with blood capillaries and
exchange of gases takes place through it.
3. Pulmonary respiration: -
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Organs of respiration: It consists of
respiratory tract and lungs.
·
Respiratory tract: The passage through which
air enters and leaves lungs, is called as respiratory tract and it consists of
external nares, nasal chambers, internal nares, bucco-pharyngeal cavity,
glottis, laryngo-tracheal chamber and two bronchi. The glottis opens into a
small, thin-walled chamber called as laryngo-tracheal chamber. It's walls
remain supported by 2 arytenoid and 1-cricoid cartilage. Internally it contains
a pair of elastic horizontal bands called as vocal cords. Vibration of vocal
cords produce sound. Thus, larynx or laryngo-tracheal chamber is called as voice
box.
When air from lungs is
forced out between vocal cords, they vibrate producing croaking sound. Vocal
sacs of male frog amplify the croaking sound.
Form
larynx a very small tube, the bronchus, leads to each lung.
Lungs: A pair of lungs, each on
either side of heart in anterior body cavity are present. Each lungs is ovoid,
thin walled and highly elastic. Externally, they remain covered by peritoneum.
Internally, each lung is divided by a network of folds or septa forming a
number of small air sacs or alveoli with a large central cavity. The alveoli
are linked with thin epithelium highly supplied with capillaries. Exchange of
gases takes place through these alveoli.
Mechaism of Pulmonary respiration:
Pulmonary respiration takes in between buccal respiration
and during it, buccal cavity acts as force pump. The up and down movements of
the floor of buccal cavity is brought by the action of two muscles, sternohyal
muscles and petrohyal muscles. The sternohyal muscles are attached at the lower
end to sternum and at the upper end to undersurface of hyoid bone in the floor
of buccal cavity. The petrohyal muscles are attached below to the upper surface
of hyoid bone and above to squamosal bone of skull. The whole process consists
of two steps, Inspiration and expiration.
Inspiration:
The process in which lungs are filled up with air is
called inspiration. During it, glottis and mouth remains closed while nostrils
remain open. The sternohyal muscles contract due to which the floor of buccal
cavity is lowered. Thus, the buccal cavity get enlarged and air comes into
buccal cavity through nostrils. Now the glottis opens and nostrils are closed.
Now petrohyal muscles contract which raises the floor of buccal cavity. This
decreases size of buccal cavity and air passes into lungs trough glottis. This
completes inspiration.
Expiration:
When lungs are filled with air the glottis closes and air
remains in lungs for some time. At this time, the floor of buccal cavity is
raised and lowered to carry buccal respiration. Now glottis opens and floor of
buccal cavity is lowered by the contractin of sternohyal muscles. This
increases size of buccal cavity. The air in lungs come out into buccal cavity.
The elasticity of lungs and contraction of body muscles also help to move air
out of lungs. Now glottis closes and when floor of buccal cavity is raised by
the contraction of petrohyal muscles the air is expelled out through the
nostrils. This process in which lungs are emptied is called expiration.
According to recent findings, When the floor of buccal
cavity is lowered fresh atmospheric air from nostrils and air of lungs from
glottis come into buccal cavity and get mixed with each other. When floor of
buccal cavity is raised, the mixed air of buccal cavity goes into the lungs
through glottis and goes outside through nostrils. This proves inefficiency of
lungs for respiration.