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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.
Respiration in Amphibia • In Amphibia ,the two lungs communicates separately with a single laryngotracheal chamber which is supported by small cartilages but there are no well-defined trachea or bronchial tubes. This chamber opens into the hind end of the pharyngeal cavity through a slit-like glottis which is operated by muscles. The walls of the lungs contain numerous elastic fibers and plain muscle fibers are also present which enable the lung to make spontaneous contractions when isolated.
Ventilation of the Frog • When a frog is observed at rest, movements of the throat are visible at frequencies between 80 and 120 per minute . These form the so called *Bucco-pharyngeal movement. During this activity, 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.
*Pulmonary ventilation • It is essentially a buccal force- pump, involving changes in volume of the buccal cavity co-ordinate with the operation of valves in the nostrils and the glottis. The mechanism is illustrated diagrammatically in next figure where Four main phases are recognizable. 1-In the first stage a-The glottis is closed.
b- The nostrils open 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.
d-The effects result in a passive extra lowering of the floor of the mouth.
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 pulmonary ventilation represents a mechanism in which a co-ordinated activity of 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.
The cutaneous respiration • Many amphibians are able to live beneath the water surface for very long periods. Under these conditions , oxygen is mainly obtained through the SKIN which is profusely supplied with blood derived from the pulmo-cutaneous arch. Table 6 shows the important role of cutaneous respiration at different times of the year.1) Most of carb.dioxide loss is through the skin at all times of the year.2)But ,uptake of oxygen by skin is most important during winter months .
It accounts for two-thirds of the total oxygen uptake during the winter but only a quarter in the summer.
Respiration of Reptiles • All reptiles possess lungs, and none passes through an aquatic larval stage with gills, as do many of the amphibians. In snakes, presumably as an adaptation to their long, thin bodies, the left lung is reduced in size or entirely lacking. Although lungs are the primary means of respiration in all reptiles and the only means of respiration in most reptiles, a number of species are also able to utilize other parts of the body for the absorption of oxygen and the elimination of carbon dioxide.
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.
Skin and scales 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. • The lungs have an increased internal surface and they communicate with the pharyngeal cavity by a distinct tracheal tube supported by circular rings of cartilage.Fig.12 shows a different forms of reptilian’s lung.
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.%.
As in amphibians ,three mechanisms for obtaining oxygen have been described: • 1-Bucco-pharyngeal, • 2- Pulmonary ,and • 3- Cutaneous. • The last of these is apparently not so important because the HORNYlayers of the skin not only prevent water loss but also hinder any gaseous exchange .However , there is evidence for the importance of cutaneous respiration in some aquatic reptiles.
Oscillations of the pharyngeal floor occur in many reptiles but their function is doubtful. • Any gaseous exchange which they assist is very slight , as shown by determination of the carbon dioxide content of the expired air, their function appears to be mainly Olfactory.
Pulmonary ventilation • Lungs being filed by the action of a costal pump. • Ribs are universal among reptiles(they are absent in modern Amphibia). • Movement of ribs is produced by the intercostal muscles which expand the cage in the thorax and produce a reduction of the pressure within the lung. • Inspiration is active and a suction pump is the main mechanism ventilating the lung where Expiratorymovements are partly passive but
may be assisted by contraction of transverse abdominal muscles and smooth muscles of the lungs themselves. • As in Fig.15 ventilation in Lizards has THREE –phase sequence of movements. 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 gas contained in the lung is held at a pressure which exceeds atmospheric. • Ventilation of the lungs themselves is certainly diphasic in character , for the glottis is open at the beginning of the initial expiratory phase and closes before the final passive expiration.
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.
Snake Respiratory System Anatomy • Snakes have a small opening just behind the tongue called the glottis, which opens into the trachea, or windpipe. Unlike what mammals have, the reptile glottis is always closed, forming a vertical slit, unless the snake takes a breath. A small piece of cartilage just inside the glottis vibrates when the snake forcefully expels air from its lungs. This produces a snake’s characteristic hiss. Snakes are able to extend their glottis out the side of their mouth while they eat, which allows for respiration while they consume large prey items.
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
