Respiratory System of Frog

Respiration is comprised of breathing in and out of respiratory gases and their exchange in the circulation. In frogs, the respiration takes place by two major mechanisms.

• Pulmonary circulation – through lungs
• Cutaneous circulation – through skin

Apart from the above mechanisms, some amount of respiration takes place through the mucosa of the buccal cavity called Buccal respiration. The respiration in larval form of frogs occurs through gills and is called Branchial respiration.

1. Cutaneous Respiration in Frog

Skin is one of the primary respiratory organs of frogs which functions in both air and water. Frog’s skin is composed of thin membranous tissue with extensive capillary network underneath. This arrangement facilitates the diffusion of respiratory gases from the surrounding and the blood vessels.

The skin should be moist for the gases to diffuse into the skin. The skin is inherently rendered moist by the presence of mucus glands. As the mucus secretions alone are not sufficient, the frog needs moist atmosphere or water body to maintain the moistness for the cutaneous respiration to take place

During winter months of hibernation the frogs submerge in water or bury themselves in mud. Pulmonary respiration ceases in hibernation and the basal energy needed for sustenance is provided entirely by cutaneous respiration.

As soon as the skin dries, cutaneous respiration of frog stops and it soon dies. This is the reason for prevalence of frogs near water bodies and moist foliage.

2. Pulmonary Respiration

In pulmonary respiration, frogs breathe in the atmospheric air into the lungs like human beings, but the mechanism of breathing in and breathing out is different. The respiratory system is comprised of the respiratory tract and the lungs.

Respiratory tract

• External nares
• Internal nares
• Buccopharyngeal cavity
• Glottis
• Tracheo bronchial chamber
• Two bronchi

• The external nares or nostrils lie above the snout and are guarded by valves which open and close during respiration. The movement of tip of upper jaw causes closure of the valve and prevents the entry of air.
• It is connected to the internal nares which are present in the anterior portion of the buccal cavity.
• The floor of the buccal cavity is very distensible to facilitate the expansion during inspiration to hold the air. It is under continuous movement. 
• The buccopharyngeal cavity opens into the larynx through glottis, a midline aperture in the pharynx. The larynx is supported by a pair of arytenoid and a cricoid cartilage.
• Internally, the pharyngeal epithelium is thickened to form the vocal cords responsible for the croaking sound. Larynx leads to a small tube called trachea which bifurcates to a pair of bronchi each ending in a balloon like lungs. 

Lungs

• The lungs are poorly developed in frog that necessitates the presence of cutaneous and buccopharyngeal respiration. They are small ovoid organs composed of cluster of thin walled sacs called alveoli.
• The walls of the alveoli are richly supplied with blood vessels which break up to form a fine capillary network. 
• The alveolar epithelium is made of single layer of flattened squamous cells which facilitates diffusion of cases from the sac to capillary network.
• The epithelial layer is supported underneath by smooth muscle cells which cause the contraction of lungs. The outer surface of both lungs are lined by a layer of peritoneum.    
• When the lungs are inflated, the specific gravity of the animal decreases. This favours the floating of the frog in water.

Picture 7: Light microscopic view of lungs of Frog 

Inspiration and expiration are assisted by the muscles causing movement of jaw bones and the laryngeal cartilages. They are

• Sternohyoid muscle – attached to sternum in the lower end and undersurface of hyoid apparatus in the upper end
• Geniohyoid muscle – attached to upper surface of hyoid in the lower end and to the squamosal bone of skull in the upper end

Mechanism of Respiration in Frogs

• The respiratory movements of frog that can be observed easily are the continuous and regular upward and downward movement of floor of mouth.
• The direct filling and deflation of lungs can be observed as sudden contraction followed by the bulging of body wall followed by a brief closure of the external nares.
• When the frog is in a cool and moist environment, the throat movements may continue for quite a long period without any movements of the body or nares.
• The above movements are composed of movements are inspiration and expiration

Inspiration

• For inspiration to begin, a negative pulmonary pressure is needed. Thus inspiration begins with depression of the floor of the buccal cavity caused by lowering of hyoid by sternohyoid muscle.
• It is followed by opening of external nares and breathing in air. As the air reaches the buccal cavity through internal nares, the floor of mouth is raised by upward movement of premaxilla causing the closure of nares and simultaneous opening of glottis.
• This forces the air into the larynx through glottis. From the larynx, the air reaches the lungs where transfer of gases takes place.
• The inspiration takes place for quite some time before expiration begins to maintain air in the lungs in between respirations.

Expiration

• The expiration begins with contraction of smooth muscles of lungs expelling air.
• It is followed by opening of glottis followed by its immediate closure.
• The air that reaches the buccal cavity is pushed out through the nares by upward movement of the floor of the mouth.
• Most of the air that reaches the buccal cavity during expiration is forced back to lungs in the next inspiration.
• When the frog swims in water, it keeps its snout above the water to perform pulmonary respiration

Buccal Respiration in Frog

• Following inspiration, the floor of the mouth undergoes rhythmic upward and downward movement causing entry and exit of air into the buccal cavity.
• During this movement, the glottis remains closed and air doesn’t reach the lungs.
• The diffusion of respiratory gases along the lining of the buccopharyngeal cavity which has an abundant capillary network underneath is called Buccal respiration.
• This supplements the poorly developed pulmonary respiratory apparatus of frogs.

Respiratory Centre of Frog

Respiratory centre is the part of the brain which controls the respiratory mechanisms of frogs. It is located between the lower part of cerebellum and calamus scriptorius in medulla. If this region is removed, the respiration of frogs stops completely.

Respiration of tadpoles

As tadpoles are present in water until they develop, they survive on the oxygen present in water. Tadpoles have gills similar to fishes and breathe through them.

Factors Affecting Respiration in Frog

Seasons: During Summer which is the breeding season, the rate of respiration increases while in winter, frogs go in for hibernation in which only cutaneous respiration prevails.

Light: Refrangible rays of light have different effects on respiration. The rate of respiration is found to be high than in dark

Animation on respiratory system of frog – link

Frequently Asked Questions

REFERENCES

1. https://en.wikipedia.org/wiki/Frog 
2. Tesler, P. (1999). “The amazing adaptable frog”. Exploratorium:: The museum of science, art and human perception.
3. The Anatomy of Frog – Alexander Ecker
4. A laboratory guide to Frog’s Anatomy – Eli C.Minkoff
5. The Biology of Frog – Samuel J Holmes