What do reptiles use for gas exchange

The environment in which the animal lives greatly determines how an animal respires. The complexity of the respiratory system correlates with the size of the organism. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops.

In unicellular single-celled organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell. Diffusion is a slow, passive transport process. In order to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake must match the rate of diffusion across the membrane. In other words, if the cell were very large or thick, diffusion would not be able to provide oxygen quickly enough to the inside of the cell.

Therefore, dependence on diffusion as a means of obtaining oxygen and removing carbon dioxide remains feasible only for small organisms or those with highly-flattened bodies, such as flatworms platyhelminthes. Larger organisms have had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages, accompanied by a complex circulatory system to transport oxygen throughout their entire body. For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs.

Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter.

In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist so that gases diffuse quickly via direct diffusion. The flat shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen.

If the flatworm had a cylindrical body, then the cells in the center would not be able to get oxygen. Respiration can occur using a variety of respiratory organs in different animals, including skin, gills, and tracheal systems. There are various methods of gas exchange used by animals.

As seen in mammals, air is taken in from the external environment to the lungs. Other animals, such as earthworms and amphibians, use their skin integument as a respiratory organ. A dense network of capillaries lies just below the skin, facilitating gas exchange between the external environment and the circulatory system. The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell membranes.

Organisms that live in water also need a way to obtain oxygen. Oxygen dissolves in water, but at a lower concentration in comparison to the atmosphere, which has roughly 21 percent oxygen. Fish and many other aquatic organisms have evolved gills to take up the dissolved oxygen from water.

Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in the water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid. Gills are found in mollusks, annelids, and crustaceans. Common carp : This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygen from water.

The folded surfaces of the gills provide a large surface area to ensure that fish obtain sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached.

In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water high concentration to blood low concentration. Similarly, carbon dioxide molecules diffuse from the blood high concentration to water low concentration.

Oxygen transport and gills : As water flows over the gills, oxygen is transferred to blood via the veins. Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct role in oxygen transport. Insects have a highly-specialized type of respiratory system called the tracheal system, which consists of a network of small tubes that carries oxygen to the entire body. The tracheal system, the most direct and efficient respiratory system in active animals, has tubes made of a polymeric material called chitin.

Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the tubular network, allowing oxygen to pass into the body, regulating the diffusion of CO 2 and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the tracheal system with body movements. Insect respiration : Insects perform respiration via a tracheal system, in which openings called spiracles allow oxygen to pass into the body.

Birds and amphibians have different oxygen requirements than mammals, and as a result, different respiratory systems. Amphibians have evolved multiple ways of breathing. Young amphibians, like tadpoles, use gills to breathe, and they do not leave the water. As the tadpole grows, the gills disappear and lungs grow though some amphibians retain gills for life.

These lungs are primitive and are not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing through the lungs is forced.

The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist.

It has vascular tissues to make this gaseous exchange possible. This moist skin interface can be a detriment on land, but works well under water. Birds are different from other vertebrates, with birds having relatively small lungs and nine air sacs that play an important role in respiration.

Snakes smell scents in the air using their forked tongue see Figure below. This helps them locate prey. Some snakes have heat -sensing organs on their head that help them find endothermic prey, such as small mammals and birds.

A snake flicks its tongue in and out to capture scent molecules in the air. Why did amphibians evolve into reptiles? Structure and Function in Reptiles Reptiles are a class of tetrapod vertebrates that produce amniotic eggs. Reptile Respiration The scales of reptiles prevent them from absorbing oxygen through their skin, as amphibians can. Ectothermy in Reptiles Like amphibians, reptiles are ectotherms with a slow metabolic rate.

Other Reptile Structures Like amphibians, most reptiles have a heart with three chambers, although crocodiles and alligators have a four-chambered heart like birds and mammals. Summary Reptiles are a class of ectothermic, tetrapod vertebrates. Reptiles have several adaptations for living on dry land, such as tough keratin scales and efficient lungs for breathing air. Reptiles have a three-chambered heart and relatively well-developed brain. Review Describe reptile scales and the functions they serve.

What is a diaphragm? What does it do? Describe two senses that snakes may use to locate prey. Pretend you are a reptile such as a lizard. As was the case for fish performing ASR, there is evidence that air-breathing reflexes are significantly influenced by higher central processing, in particular a perceived risk of predation. Smith and Kramer 41 reported that exposure of the Florida gar Lepisosteus platyrhincus to a model avian predator resulted in a decrease in air-breathing frequency and an increase in gill ventilation effort.

Herbert and Wells 42 found that fear of predation reduced air-breathing frequency by the blue gourami, Trichogaster trichopterus , an obligate air breather, which compensated by reducing activity, presumably to conserve the O 2 stored in the ABO. Very little is known about the reflex control of breathing in amphibious marine species.

The giant mudskipper, Periopthalmodon schlosseri , lives on tidal mudflats, where it builds a deep burrow, from which it emerges to forage between shallow puddles and the exposed mudflats. It is an obligate air breather, which stores air in a highly vascularized orobranchial cavity, containing much reduced gills. The gills are only ventilated with water when the animal expires the air from its orobranchial cavity, at which point it will submerge the mouth and perform a few cycles of gill ventilation McKenzie DJ, unpublished personal observations.

Aquatic hypoxia has no effect on patterns of gill ventilation or air-breathing in the mudskipper A representative trace of this is shown in Figure 6A. Figure 6C shows the effects of cyanide gas, given as a bolus into the buccal cavity, on heart rate, blood pressure and air-breathing in a giant mudskipper.

There is a typical teleost bradycardia in response to cyanide, followed by vigourous air-breathing responses. Cyanide given as a bolus into the bloodstream, via an indwelling catheter in the ventral aorta, was without effect on air-breathing or heart rate.

Thus, this meagre data set demonstrates that the giant mudskipper possesses chemoreceptors, which elicit bradycardia and air-breathing, but that these apparently monitor oxygen levels in the air held in the mouth, with no sensitivity to blood oxygen levels. Baroreceptors in air-breathing fish.

There is no experimental evidence for baroreceptor responses in air-breathing fish. Most air-breathing fish supply their various air-breathing organs from the systemic circulation. Lungfish and all tetrapods have distinct pulmonary arteries and veins in association with true lungs, having highly permeable surfaces; the lungfish, Protopterus , has a diffusion distance of 0. However, possibly because they retain gills, lungfish have similar, relatively low blood pressures in the respiratory and systemic circuits and, as a consequence, may not have a functional requirement for baroreceptor responses to protect their lungs against oedema resulting from hypertension.

It could of course be argued that control of blood pressure in a relatively low pressure system requires sensitive pressoreception. This remains to be demonstrated. A detailed review by West and Van Vliet 45 considered the roles of peripheral chemoreceptors and baroreceptors in cardiorespiratory control in amphibians, while the factors influencing the progressive transition from water to air-breathing during amphibian metamorphosis were reviewed by Burggren and Infantino Responses to hypoxia in larval bullfrogs were eliminated by ablation of the first gill arches, suggesting that they are the site of the O 2 -sensitive chemoreceptors.

A residual slow response was interpreted as stimulation of a second population of receptors, possibly monitoring the cerebrospinal fluid CSF. The rapid responses to hypoxia are blunted in later stage bullfrog larvae, in which the lungs are developing and the gills degenerating.

In an earlier study of the bimodally breathing bullfrog tadpole, mild aquatic hypoxia was found to increase gill ventilation but more severe hypoxia promoted high frequencies of lung ventilation and a suppression of gill ventilation 1,47 , which was in response both to lung inflation per se and to the resulting increase in PO 2 In the neotenous, gill-bearing axolotl, Ambystoma mexicanum , both gill ventilation and air-breathing were stimulated by cyanide, infused either into the ventilatory water stream or into the blood stream 1.

Cardiac responses were complex with an initial bradycardia, presumably in response to stimulation of peripheral chemoreceptors, followed by a tachycardia at the first air breath, possibly in response to stimulation of lung stretch receptors, a situation comparable to the mammalian response to hypoxia 1. Heart rate in the bullfrog tadpole did not change during aquatic hypoxia, with access to air 1, While their larvae may retain functional oxygen receptors on the gill arches, the carotid labyrinths are the main putative sites for oxygen receptors in adult amphibians.

They are situated at the bifurcation of the internal and external carotid arteries and innervated by branches of the glossopharyngeal nerve, which projects its afferent fibres to the NTS in the brainstem These receptors are functionally similar to the mammalian carotid bodies, as they also respond to hypercapnia and their discharge can be modulated by sympathetic stimulation 1.

More recent studies have also shown that the receptors are sensitive to oxygen partial pressure, rather than content 45 ; a finding consistent with the results of whole animal study of the stimulus modality of the hypoxic ventilatory response in toads 1.

The degree of shunting is likely to be referred to input from peripheral chemoreceptors. In the adult bullfrog, more blood is directed towards the lungs during aquatic hypoxia, while aerial hypoxia elicits an increase in cutaneous perfusion.

The return of blood to the right side of the heart from the cutaneous circulation may specifically serve to improve oxygen supply to the myocardium, which in amphibians is devoid of a coronary circulation 1. A similar response was recorded from the in vitro preparation of the bullfrog brainstem 1,4. This dominant role for central chemoreceptors in the generation of respiratory drive in amphibians appears at metamorphosis. An in vitro preparation of the isolated brainstem from the bullfrog tadpole displayed co-ordinated, rhythmic bursting activity in cranial nerves V, VII and X, which could be characterised as representing fictive gill or lung ventilation.

In early stage larvae, variations in pH of the superfusate were without effect on gill or lung burst frequency. Later stage larvae showed an increasing predominance of neural lung burst activity, which markedly increased in acid pH The onset of episodic breathing patterns during metamorphosis was coincident with developmental changes in the nucleus isthmi in the bullfrog, and it seems possible that this region of the brainstem is involved in integration of central chemoreceptor information Pulmonary stretch receptors PSR constitute another important source of feedback, contributing to the control of breathing in amphibians.

There are three different types of PSR in amphibians responding to 1 the degree of lung inflation, 2 the rate at which lung volume changes or 3 both stimuli 51, These receptors are innervated by afferent fibres in the pulmonary vagi 51,52 , which project to the solitary tract in the brainstem The receptors are mostly slowly adapting and their firing rates decrease when the intrapulmonary CO 2 concentration is increased 47,52, There is evidence for interactions between mechanoreceptor and chemoreceptor reflexes.

In the toad lung, inflation decreased the effect of cyanide injection, while these same effects were increased by hypercapnia Pulmonary afferent fibres play a key role in the termination of lung inflation in the adult and inhibition of buccal oscillation in the pre-metamorphic tadpoles.

The evidence is that pulmonary deafferentation by vagotomy in Xenopus results in an increase in the number of inspirations in a ventilatory period, and overinflation of the lungs 1.

This matter remains unresolved as decerebrate paralysed anurans showed that lung inflation inhibited fictive breathing, as would be predicted from work on mammals, while another study, of a similar preparation, indicated that lung inflation stimulated breathing 1, Amphibians which breathe discontinuously, often in association with periods of submersion, typically display large increases in heart rate and pulmonary blood flow at the onset of bouts of lung ventilation.

However, the contribution of lung stretch receptors to this response is not resolved Whereas artificial lung inflation increased heart rate in anaesthetised toads, in conscious Xenopus laevis , PSR denervation did not abolish the increase in heart rate associated with lung inflation and in lightly anaesthetised animals artificial lung inflation did not affect heart rate, though pulmo-cutaneous blood flow increased 1.

A similar response was demonstrated in Bufo marinus Scattered groups of glomus cells have been identified in the connective tissue surrounding the main and collateral branches of the carotid arteries in lizards.

This area is profusely innervated by the superior laryngeal branch of the vagus nerve and possibly the glossopharyngeal nerve 1. All primary afferent fibres of the glossopharyngeal and the majority of vagal afferent fibres, enter the NTS in the monitor lizard.

Although activity in these putative receptors has not been recorded, denervation of this area abolished the increase in ventilation shown by lizards when hypoxic or hypercapnic blood was injected into the carotid arch 1,4, In turtles, O 2 chemoreceptors are not present at the anatomically defined carotid bifurcation because it is formed by division of the internal carotids, the external carotids having atrophied during development Milsom 26 suggests that due to this process the site homologous to the carotid bifurcation in amphibians and mammals is part of the aortic arch in turtles.

The largest aggregations of chemoreceptive tissue are found in this central cardiovascular area and are innervated by the superior and inferior branches of the vagus nerve arising from the nodose ganglion.

The inferior branch also innervates chemoreceptors located on the pulmonary artery of the turtle 1. These receptor groups have been shown to respond to changes in oxygen level but their roles in establishing resting ventilatory drive or in reflex responses to hypoxia are unknown Peripheral oxygen receptors in reptiles seem to respond to a reduction in oxygen content i.

Similar characteristics have been attributed to arterial chemoreceptors in fish and in birds and mammals Reptiles, in common with some air-breathing fish and amphibians, have pulmonary and systemic circulations that are incompletely separated, so that some systemic venous blood can bypass the lungs to re-enter the systemic circulation, while some arterialized blood can re-enter the pulmonary circulation.

Consequently, arterial blood gas composition is affected by the degree of admixture of oxygenated arterialized blood and oxygen-depleted venous blood, rather than lung gas composition alone as it is in mammals. This presents the intriguing possibility that regulation of these central vascular shunts, with reference to peripheral chemoreceptors, may play an important role in control of arterial blood gas composition in reptiles, independent of ventilatory control 1, The hypercapnic ventilatory response is well developed in reptiles, and changes in Pa, CO 2 rather than Pa, O 2 provide the dominant drive to breathe 21, Central chemical control of ventilation in an ectothermic, air-breathing vertebrate was first demonstrated in the unanaesthetized turtle, Pseudemys scripta elegans Perfusion of the lateral and 4th cerebral ventricles with artificial CSF caused an increase in ventilation to 4 times control, following a calculated pH change of only 0.

Inhalation of gas mixtures enriched with CO 2 stimulates ventilation in crocodilians. It causes ventilation volume to rise, decreases periods of breath hold and increases the number of breaths in each breathing episode. Snakes and lizards may respond to environmental hypercarbia, which stimulates lung receptors, with decreased ventilation but show a marked increase in response to venous CO 2 loading Pulmonary stretch receptors are present in testudines, lizards, snakes, and alligators.

Recordings from the central cut end of the vagus in a range of reptilian species during artificial lung inflation have shown clear evidence for the presence of functional lung stretch receptors Figure 6 These receptors are sensitive to lung volume and thus provide feedback regarding lung filling and emptying, and with that, of tidal volume.

In general, activation of stretch receptors suppresses inspiration and enhances expiration the Breuer-Hering reflex. Snakes and alligators have both rapidly and slowly adapting stretch receptors 1. Consistent with mammals, the sensitivity of the pulmonary stretch receptors in reptiles is depressed and, in some instances, even silenced by CO 2.

This was true in snakes but not in testudines Figure 7. In addition to the stretch receptors, most reptiles possess intrapulmonary chemoreceptors sensitive to CO 2 1, Similar receptors are present in birds but have not been demonstrated in amphibians or mammals. The discharge frequency of the intrapulmonary chemoreceptors IPC is inversely proportional to PCO 2 and the depressing effect of hypercapnia on minute ventilation in both amphibians and reptiles has been attributed to an inhibitory effect of CO 2 on IPC However, when lung CO 2 changes in a physiologically realistic fashion IPC may in fact stimulate, rather than depress, ventilation so that IPC may play an important role in regulating ventilation whenever metabolic rate is increased Figure 1.

Recording of electromyographic EMG activity from intercostal muscles of Uromastyx aegyptius microlepis , together with the pressure generated within a plethysmograph by an aspiratory lung-inflation cycle.

The inflation cycle is triphasic, with active expiration-I leading to active inspiration then a brief, passive, partial expiration-II, followed by a ventilatory pause with the lungs inflated, terminated by the next cycle. Figure 2. Cardiovascular changes associated with discontinuous ventilatory activity in a decerebrated and paralysed toad Bufo marinus.

Ventilatory activity was measured as nervous activity in the fifth cranial nerve, which innervates the respiratory muscles in the buccal cavity. As the toad was unidirectionally ventilated and paralysed, there were no changes in afferent input during bouts of fictive ventilation, and the rise in pulmocutaneous blood flow Qpc during ventilation is likely to be caused by central feed-forward mechanisms.

There were no obvious changes in systemic blood pressure Pb or heart rate f H Modified from Ref. Figure 3. Ventilatory airflow rates in Varanus exanthematicus running on a treadmill. The upper trace is a recording of lateral bending of the trunk whilst running for 30 s; the lower trace is a pneumotachograph recording of ventilatory airflow at the nares, with expiration recorded as deviations above the baseline. The active lizard shows a marked disruption of normal airflow Modified from Ref.

Figure 4. A series of thoracic plus buccal ventilatory cycles recorded from an anaesthetised Uromastyx aegyptius microlepis using whole body plethysmography. This started in the respiratory pause immediately after passive expiration and resulted in additional inflation of the lung signified by an increase in baseline plethysmograph pressure, which is held until the subsequent active expiration. Note the short interval between one respiratory pumping cycle and the next.

EMG, electromyographic activity in sphincter colli and geniohyoideus muscles inserted on the pectoral girdle and from intercostal muscles, with this recording including the ECG Modified from Ref. Figure 5. Open circles represent ASR events in the absence of a model avian predator; solid circles represent events in the presence of the model. The outer lines at the top of the diagrams represent a sheltered area in the aquarium Reproduced from Ref.

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