ll animals breathe, that is to say they take in oxygen and give out carbon dioxide. This gaseous exchange is called respiration. Respiration can be defined as process by which living beings obtain oxygen to oxidize their reserve digested food in order to release energy. The organs which help in intake or supply of oxygen to the tissues and remove excess Co2 form respiratory system.
Breathing in frogs are of three types:
1. Cutaneous respiration (Respiration through skin)
2. Respiration through lining of bucco-pharyngeal cavity
3. Pulmonary respiration (Respiration through lungs)
1) Cutaneous respiration:
It is an important mode of respiration in frog which occurs all the time whether the frog is on land or in water. It is the only mode of respiration when the frog in dormancy period, i.e during hibernation and aestivation. The skin of frog has good supply of blood and this is provided by cutaneous artery. Moreover the skin is very thin and do not possesses any structure that may prevent the diffusion of the dissolved oxygen from water into blood. To carry on cutaneous respiration frog keeps its skin moist by taking occasional plunges into nearby water. The skin is also kept moist by mucus secreted by mucous glands of skin.
Mechanism: Oxygen first dissolves in the film of water over the skin and then passes through the epidermal cells and capillary walls into the blood. On the other hand carbon dioxide passes out from blood into the water.
2) Respiration through lining of bucco-pharyngeal cavity:
In buccal respiration on land, the mouth remains permanently shut while the nostrils remain open. the floor of buccal cavity is alternately raised and lowered so that air is drawn into and expelled out of the buccal cavity through the opened nostrils. During buccal respiration the glottis remains closed so that no air enters or leaves the lungs. As the mucous lining of buccal cavity is richly supplied with blood capillaries, O2 diffuses into the blood and CO2 is given out.
3) Pulmonary respiration:
Breathing on land in the atmospheric air with the help of lungs is called pulmonary respiration. The chief organ for pulmonary respiration is a pair of lungs. The organs of respiration include two lungs. The passage through which air passes into lungs forms respiratory tract.
a. Respiratory tract:
The structures concerned with respiration in lungs include:
• external nares
• olfactory chambers(nasal chambers)
• internal nares
• buccal cavity
• pharynx
• laryngo-tracheal chamber (larynx)
• lungs
The median slit like opening on the floor of pharynx known as glottis, leads down into thin-walled chamber, larynx or laryngo-tracheal chamber. The walls of larynx are supported internally by a pair of Artyenoid cartilages and a very irregular ring-like Cricoid cartilage. The internal lining of larynx forms a pair of elastic horizontal bands the vocal cords which produce sound. There being no neck in frog, trachea or wind pipe is absent.
Between the vocal cords is a narrow slit through which the air passes to and from the lungs.
b. Lungs:
The lungs are a pair of thin-walled, more or less translucent sacs, which possess remarkable power of distension. They are situated in the anterior part of the body cavity, one on each side of heart and somewhat covered by liver.
Externally each lung is covered by peritoneum but internally the surface of each lung is divided by a series of irregular septa into number of small air sacs called alveoli. The respiratory surface of lungs is considerably increased due to alveoli. The walls of alveoli are richly supplied with arteries, veins and blood capillaries. The inner surface of each of these septa is made up of extremely thin and flattened cells whereas the cells on the outer edges of septa are cylindrical and ciliated.
Mechanism of Respiration:
Passage of in and out of air into lungs is known as ventilation of lungs. It is affected by oscillatory or lowering and raising of bucco-pharyngeal cavity. This oscillatory movement of bucco-pharyngeal cavity is brought about by two sets of muscles:
Sterno-hyoid (sternohyal) muscles- which is attached anteriorly to the lower-surfaces of hyoid bone and posteriorly to sternum bone and
Petro-hyoid (petrohyal) muscles- attached anteriorly to squamosal bone and posteriorly to upper-surfaces of hyoid bone.
The respiratory mechanism involves two processes: Inspiration and Expiration
Inspiration: It involves following steps:
• glottis closes and nostrils opens
• sternohyal muscles contracts
• buccal cavity enlarges
• air enters into the cavity through opened nostrils
• glottis opens
• mento-meckelian bones of lower jaws push upwards the upper jaw so that nostrils are closed
• petrohyal muscles contracts
• floor of buccal cavity raises and the cavity reduces
• reduction of volume of buccal cavity forces the compressed air into lungs through glottis
Expiration: The lungs are filled with air. Glottis remains closed and the air is held in lungs for sometime.
The buccal floor raises and lowers for buccal respiration. Expiration involves following steps:
• glottis opens
• air in lungs driven out into buccal cavity by lowering of floor
• buccal floor raises
• glottis closed
• air expelled out through nostrils to outside
Physiology of Respiration:
In vertebrates, the respiratory process occurs in the following three steps:
1) External Respiration
2) Transport of respiratory gases
3) Internal respiration
1) External Respiration:
External respiration or breathing is a simple process involving exchange of oxygen and carbon-dioxide between the environment and the respiratory surfaces. During this process, oxygen is acquired and carbon dioxide is expelled.
2) Transport of respiratory gases:
Blood is the chief component for oxygen transportation. It carries oxygen from respiratory surfaces to tissues and Co2 from tissues to the respiratory surfaces.
• Transport of O2:
The greater part of O2 diffused into blood forms loose chemical combination with respiratory pigment present in blood, the haemoglobin. This combination is known as oxy-haemoglobin.
Hb + O2 ———->Hb4O8 (oxyhaemoglobin)
Blood transports oxyhaemoglobin to tissues where oxygen gets separated. This separated free O2 passes through the tissue-fluid into the tissue cells where oxidation of reserved digested food occurs to release energy.
• Transport of Co2:
Co2 dissolves freely into tissue-fluid surrounding the cells and from there passes into plasma within the blood capillaries. Blood carries CO 2 from tissues to the respiratory surfaces in different forms:
As Carbonic acid- CO2 combines with the water of plasma to form carbonic acid
CO2 + H2o————–> H2 CO3 (Carbonic acid)
As Carboxyhaemoglobin: Carbon dioxide combines with haemoglobin to form carboxyhaemoglobin.
CO2 + Hb ——> Carboxyhaemoglobin
As bicarbonates of sodium and potassium- Carbon dioxide combines with potassium and sodium present in blood to give the carbonates of potassium and sodium respectively.
CO2 + H2O + K——> KHCO3
CO2 + H2O + Na——–> NaHCO3
In lungs there is an enzyme called Carbonic anhydrase which separates carbon dioxide from all the above mentioned forms. Thus liberated free CO2 in lungs cavity is expelled out during expiration.
3) Internal respiration:
This is also called tissue respiration. In this phase, reserved digested food in the tissue cells is oxidized to release energy. This is often called biochemical oxidation which is accomplished by several biochemical reactions leading to release of energy and formation of carbon dioxide and water.
C6H12O6 + 6O2 6CO2 + 6H2O + Energy
Sound production:
The larynx or the sound producing organ, which is box-like structure, lies just below the pharyngeal cavity and at the beginning of entrance into the lungs. It opens on one side into the pharynx through glottis. The inner lining of larynx is thrown into lastic folds or vocal cords. When the frog croaks, air from lungs is forced through the larynx between the stretched vocal cords. As a result the vocal cords vibrate producing sound. The so produced sound is amplified by the vocal sacs in male frogs.
Questions:
i) Give an account on cutaneous respiration in frog. (3 marks question) ii) Describe the physiology of respiration. (8 marks question) iii) Describe the process of inspiration and expiration