Respiration of tetrapods.
With few exception the respiratory organs of tetrapods are paired hollow sacs or lungs . They are absent in some Amphibian , notably Salamanders living in torrential streams ,and reduced in forms which have external gills as adults. The lungs develop as a ventral outgrowths of the pharynx and it is generally accepted that they are homologous with the swim bladder and lungs of fishes.
All lungs have a thin epithelial layer abundantly supplied with blood but their internal complexity varies a great deal. In many amphibian it is little more than a hollow sac , as in lung fishes, there being few infoldings of the internal wall to produce an increase in surface area for gaseous exchange.
1-The nostrils are partly open
2-The glottis closed
The result is that air contained in the bucco-pharynx is constantly renewed . Gaseous exchange is thought to occur in the vascularised mucous membranes and would assist in respiration.
1-In the first stage
a-The glottis is closed.
c- The floor of the mouth is lowered by the action of the sternohyoideus muscle.
d-Lastly ,air enters the bucco-pharynx because of the reduction in pressure.
2- In the next (2nd)stage.
a-The nostrils closed.
b-The glottis is open.
c-Air is forced from the lungs into the bucco- pharynx as the flanks contract.
The mixed air now contained in the bucco-pharyngeal cavity is forced into the lungs(3rd stage) through the open glottis and by contraction of the petrohyoideus muscles which raise the hyoid plate . This latter activity may be repeated several times until finally stage four , with the lung filled, the glottis closes and the extra air is forced out through the open nostrils.
the nostril valves , glottis and hyoid apparatus is demonstrated . After periods of intense activity , frogs may show continuous pulmonary ventilation but at rest it is more usual to observe bucco-pharyngeal movements interrupted periodically by the pulmonary movements.
This way of ventilation gives a maximum use of oxygen and water loss is kept to a minimum.
In aquatic turtles, for example, the tissues (mucous membranes) lining the insides of the mouth are capable of extracting oxygen from the water; some snakes, family Acrochordidae, and sea snakes, family Hydrophiidae, as well as the soft-shelled turtle, Trionyx, can use their skin for respiration when submerged.
Part of the ability of the amphibians' descendants, the reptiles, to invade dry-land environments was the development of a dry skin that served as a barrier to moisture and greatly reduced the loss of body water. These scales are not the same as (that is, not homologous to) the scales of fishes, which are bony, are formed in the dermis, and lie beneath the epidermis.
In lizards and Snakes the scales do not increase in size as the animal grows; consequently, the old scales must be periodically shed and replaced by a new set of somewhat larger scales. Shedding may also occur when the outer layer becomes worn or when much food is consumed, as well as for causes not yet fully understood. In the shedding, or molting, process, also called ecdysis, the older upper layer of the epidermis with its attached scales loosens and breaks away from a newer layer that has developed beneath it.
In turtles and crocodilians the large epidermal scales, or scutes, are not molted but are retained and are enlarged and thickened by additional layers of keratin from beneath; the uppermost layers of the scutes, however, may be lost through wear(to wasted by use or time) or other factors.
Reptiles are bitter adapted to a land habitat than amphibians and this is associated with more efficient ventilation.
Many reptiles have secondarily acquired an aquatic habit which in some instances is associated with a decrease in the respiratory area.
In aquatic turtles ,for example , the lung volume is about 3-6 ccs./100g.where in their terrestrial relatives it may be 21 vols.%.
may be assisted by contraction of transverse abdominal muscles and smooth muscles of the lungs themselves.
1- initial expiratory phase (E1 ).
2-Large inspiratory phase (I ).
3-Finally a brief expiratory movement (E2)is passive and results from movements of the abdominal viscera . A pause ensues before the next respiratory act.
During the pause the lung is inflated and the glottis closed so that gaseous exchange continues.
The glottis and nostrils are guarded by sphincters in most lizards . The nostril valves play little part in normal ventilation of the lungs but their importance in aquatic forms is much greater . Thus in some turtles which are wholly marine , it has been observed that the nostrils may be filled with highly vascular tissue which plugs them and prevents the entry of water.
The trachea is a long, straw like structure supported by cartilaginous rings. These rings are incomplete in that the snake looks more like a C than an O. A thin membrane completes the open part of the C. This configuration is also seen in lizards, but the function of the incomplete rings remains unknown. The trachea usually terminates just in front of the heart, and at this point it splits into the two primary bronchi, airways that direct air into either the left or right lung.
In most snakes the short left bronchus terminates in a vestigial, or rudimentary, left lung. The size and functional capacity of this lung varies depending on the species.It can be complete in some of the water snakes where it is used for hydrostatic purposes. The right bronchus terminates in the functional right lung.
Snakes breathe principally by contracting muscles between their ribs. Unlike mammals, they lack a diaphragm, the large smooth muscle responsible for inspiration and expiration between the chest and abdomen. Inspiration is an active process (muscles contract), whereas expiration is passive (muscles relax).
In the majority of species, only one lungis functional. This lung contains a vascularized anterior portion and a posterior portion that does not function in gas exchange . This 'saccular lung' is used for hydrostatic purposes to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species.
The portion of a snake’s lung nearest its head has a respiratory function; this is where oxygen exchange occurs. The lung portion nearest the tail, regardless of the lung’s size, is more of an air sac. The inside of these sac portions look more like the inside of a balloon than a lung. There is no exchange of respiratory gases