It also regulates inflammatory responses and interacts with the adaptive immune response. For schools, families, and young students , lire en ligne , p. It must be great enough to overcome 1 the elasticity of the lung and its surface lining; 2 the frictional resistance of the lungs; 3 the elasticity of the thorax or thoraco-abdominal cavity; 4 frictional resistance in the body-wall structures; 5 resistance inherent in the contracting muscles; and 6 the airway resistance. The gills of mollusks have a relatively elaborate blood supply, although respiration also occurs across the mantle, or general epidermis. This efficient system moves air forward through the lungs when the bird inhales and exhales and makes it possible for birds to fly at high altitudes, where the air has a low oxygen content. In some species, such as the fire-bellied toad Bombina spp. There are three orders of living amphibian s:
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Heterochrony refers to the change in the timing and rate of developmental events, and it is a widespread feature in amphibian evolution, particularly in salamanders. During development, a structure can begin to develop sooner predisplacement or later postdisplacement in an organism than it occurred in the ancestral species or parents. Also, a structure may continue to develop beyond the previous embryological sequence hypermorphosis or the developmental sequence can stop earlier than normal progenesis or hypomorphosis.
Each of these heterochronic events can produce a structurally or functionally different organism. In the mole salamander Ambystoma talpoideum , some populations also display hypomorphic development in which the development of several larval traits to the adult condition is delayed.
Since the gonads mature, a population of sexually mature salamanders with a larval morphology is produced. Heterochrony also explains the presence of larval traits in adults of the salamander families Cryptobranchidae hellbenders and Proteidae olms and mud puppies. Heterochrony is not confined to salamanders. The different sized eardrums in the American bullfrog Lithobates catesbeianus are examples of hypermorphism in male bullfrogs. The development of the eardrums in the male extends beyond that of the female.
Many amphibians have a biphasic life cycle involving aquatic eggs and larvae that metamorphose into terrestrial or semiaquatic juveniles and adults. Commonly, they deposit large numbers of eggs in water; clutches of the tiger salamander Ambystoma tigrinum may exceed 5, eggs, and large bullfrogs L.
Eggs of many anuran species laid in warm water require only one or two days to develop, whereas eggs deposited in cold mountain lakes or streams may not hatch for 30 to 40 days. The development of salamander eggs often requires more time, with hatching occurring 20 to days after fertilization.
Adult amphibians consume a wide variety of foods. Earthworms are the main diet of burrowing caecilians, whereas anurans and salamanders feed primarily on insects and other arthropods. Large salamanders and some large anurans eat small vertebrates , including birds and mammals. Most anurans and salamanders locate prey by sight, although some use their sense of smell.
The majority of salamanders and diurnal that is, active during daylight terrestrial anurans are active foragers, but many other anurans employ a sit-and-wait technique. Caecilians locate their underground prey with a chemosensory tentacle and capture their quarry with a powerful bite see chemoreception. Aquatic salamanders lunge at their prey with an open mouth and appear to suck the victim in by expanding their buccal oral cavity.
The terrestrial lunged salamander extends its sticky tongue, which is attached anteriorly to the floor of the mouth, to ensnare a meal. In lungless salamanders , the hyobranchial apparatus is not part of the process of buccal respiration; this apparatus is modified so that it can project the tongue from the mouth. Primitive anurans have feeding mechanisms that resemble those of the typical terrestrial salamanders.
The pipids, which are completely aquatic, are unique among anurans; they lack a tongue and thus must essentially suck food and water into their mouth. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our staff, and its publication is subject to our final approval.
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Page 1 of 2. Next page Form and function. Learn More in these related Britannica articles: There are three orders of living amphibians: Modern amphibians are characterized by the flexibility of their gaseous exchange mechanisms.
Amphibian skin is moistened by mucous secretions and is well supplied with blood vessels. It is used for respiration to varying degrees. When lungs are present, carbon dioxide may pass out of…. Although true viviparity has been described in the African frog Nectophrynoides , most amphibians lay eggs.
Some salamanders, however, retain the eggs within their body and give birth to live young. Courtship displays in frogs are almost entirely vocal, although in salamanders they may involve….
In amphibians a vertebra is formed from the sclerotomic tissues of two somites, the tissue from the posterior part of one somite joining that from the anterior part of the somite behind it. In modern reptiles the vertebrae are completely ossified. Direct evidence for the occurrence of filtration at the glomerulus was first provided by experiments on the amphibian kidney. Although amphibians are formally given the status of terrestrial animals, they are poorly adapted to life on land.
They excrete nitrogen in the form of…. More About Amphibian 34 references found in Britannica articles Assorted References annotated classification In vertebrate: Annotated classification characteristics of chordates In vertebrate: The tetrapods distribution in Africa In Africa: Reptiles and amphibians endangered species In endangered species: Human beings and endangered species impact of amphibian chytridiomycosis In amphibian chytridiomycosis anatomy circulatory system In circulatory system: Amphibians excretory system In excretion: Amphibians integumentary system In integument: Embryology and evolution muscle system In muscle: Tetrapod musculature View More.
Articles from Britannica Encyclopedias for elementary and high school students. Help us improve this article! Contact our editors with your feedback. Introduction General features Size range and diversity of structure Distribution and abundance Economic importance Natural history Reproduction Embryonic stage Larval stage Metamorphosis Heterochrony Life cycle Food and feeding Form and function Common features Structural differences Salamanders Caecilians Anurans Evolution and classification Annotated classification Critical appraisal.
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Internet URLs are the best. Thank You for Your Contribution! There was a problem with your submission. The genus Mesosaurus from the Permian period is counted among the most recognized early reptiles. Most Reptiles are unable to see properly during nighttime as their vision is mainly adapted to the daylight conditions. They have color vision with the visual depth perception being much more advanced than Amphibians and many Mammals.
The vision is reduced in species like the Blind Snake, while some snakes have extra visual or sensory organs that make them sensitive to heat and infrared radiation. The horny epidermis layer makes their skin watertight, allowing these animals to inhabit dry land.
Reptiles have thinner skin compared to mammals and it also lacks the dermal layer present in mammal skin. The exposed skin areas are covered in scutes or scales which may have a bony base, creating their armors.
In turtles, a hard shell made up of fused scutes covers the entire body. Reptiles use their lungs for breathing. The skin of the aquatic turtles is more permeable for allowing them to respire while the cloaca is modified in various species to increase the gas exchange area.
Despite these adaptations, lungs remain a very important part of their respiratory system. The main Reptile groups accomplish lung ventilation in different procedures. Squamates are known to ventilate the lugs mainly by their axial musculature. Certain lizard species are capable of buccal pumping apart from the normal axial breathing. The proto-diaphragm in Tegu lizards separates their pulmonary cavity from visceral cavity, helping with their respiration by allowing greater lung inflation.
The muscular structure of the diaphragm in the Crocodilians species resembles that of various mammals. However, there are some differences in their diaphragmatic setup. They also have two aortas playing a major role in their systemic circulation. The oxygenated and deoxygenated blood may get mixed with each other in their three-chambered heart with the level of mixing depending on the species and the physiological state of the animal.
Their circulatory system is capable of shunting back the deoxygenated blood to the body and the oxygenated blood to the lungs if necessary. Unlike other Reptiles, animals in the crocodilian subgroup have four-chambered hearts. But, their two systemic aortas are only capable of bypassing their pulmonary circulation. On the other hand, the three-chambered hearts in various lizard and snake species can function as the four-chambered ones during contraction.
Majority of these animals have short digestive tracts because their diet mainly consists of meat, which is very simple to digest. Their digestion process is slower than that in mammals due to their inability of mastication and their low metabolism rate while resting.
The energy requirements for their poikilotherm metabolism are very low which allows large animals from this class such as various constrictors and crocodiles to survive for months from one large meal, digesting it slowly.
Herbivorous reptiles are also unable to masticate their food, which slows down the digestive process. Some species are known to swallow pebbles and rocks that help in grinding up plant matters within the stomach, assisting their digestion.
The basic nervous system in the Reptiles is similar to that in the Amphibians. But, Reptiles have slightly larger cerebrum and cerebellum. Most of the important sensory organs are properly developed in these creatures. However, there are certain exceptions such as the absence of external ears in snakes they have the inner and middle ears. Reptiles have twelve cranial nerve pairs. They have to use electrical tuning for expanding the range of their audible frequencies because they have short cochlea.
These animals are believed to be less intelligent compared to mammals and birds because the relative size of their brain and body is much smaller than that of the latter. However, the brain development can be more complex in some larger Reptiles. Modern species also have pineal glands in their brains. Most of these animals are tetrapods, meaning they have four legs. Snakes are examples of legless Reptiles. Their skeletal system is similar to other tetrapods with a spinal column supporting their bodies.
Their excretory system consists of two small kidneys. The diapsid species excrete uric acid as the principal nitrogenous waste product. But, turtles excrete mainly urea. Some of these species use their colons for reabsorbing water, while some are able to absorb the water stored in their bladders. Certain Reptiles excrete the excess salts in their bodies through the lingual and nasal salt glands.
Reptiles have certain characteristic features that help in distinguishing them from Amphibians, Mammals and Aves:. They are capable of adapting to almost all kinds of habitats and environmental conditions, except for extremely cold regions.
These animals can inhabit dry deserts, forests, grasslands, wet meadows, shrub lands and even marine habitats. Reptiles are capable of adapting to extremely high temperatures because they are cold blooded. Various snakes including the Rattle Snakes and King Snakes as well as different lizards like the Gila Monsters live in desert habitats. Grassland is another common type of habitat for various snakes and lizards e.
Garter Snakes, Fox Snakes. The vegetation in this habitat attracts many insects and rodents, making it easier for the Reptiles to catch prey. Swamps and large water bodies are inhabited by different Reptiles such as crocodiles, alligators, certain turtles and snakes. Animals like the Saltwater Crocodile and Marine Iguana inhabit seaside, travelling in and out of ocean as necessary. Some species, such as the Sea Snakes and Sea Turtles, live in the ocean. They leave the waters only during the breeding season for laying eggs.
These animals typically practice sexual reproduction with some specific species using asexual reproduction. Majority of these animals are amniotes, laying eggs covered with calcareous or leathery shells. The eggs are generally laid in underground burrows dug by the females. The viviparity and ovoviviparity modes of reproduction are used by many species such as all boas and many vipers. However, the level of viviparity may vary with some species retaining their eggs until shortly before hatching while others nourish the eggs for supplementing the yolks.
The lungs expand and contract during the breathing cycle, drawing air in and out of the lungs. Not all the air in the lungs can be expelled during maximally forced exhalation.
This is the residual volume of about 1. Volumes that include the residual volume i. Their measurement requires special techniques. The rates at which air is breathed in or out, either through the mouth or nose, or into or out of the alveoli are tabulated below, together with how they are calculated.
The number of breath cycles per minute is known as the respiratory rate. In mammals , inhalation at rest is primarily due to the contraction of the diaphragm. This is an upwardly domed sheet of muscle that separates the thoracic cavity from the abdominal cavity. When it contracts the sheet flattens, i. The contracting diaphragm pushes the abdominal organs downwards. But because the pelvic floor prevents the lowermost abdominal organs moving in that direction, the pliable abdominal contents cause the belly to bulge outwards to the front and sides, because the relaxed abdominal muscles do not resist this movement Fig.
This entirely passive bulging and shrinking during exhalation of the abdomen during normal breathing is sometimes referred as "abdominal breathing", although it is, in fact, "diaphragmatic breathing", which is not visible on the outside of the body. Mammals only use their abdominal muscles only during forceful exhalation see Fig. Never during any form of inhalation.
As the diaphragm contracts, the rib cage is simultaneously enlarged by the ribs being pulled upwards by the intercostal muscles as shown in Fig. All the ribs slant downwards from the rear to the front as shown in Fig.
Thus the rib cage's transverse diameter can be increased in the same way as the antero-posterior diameter is increase by the so-called pump handle movement shown in Fig. The enlargement of the thoracic cavity's vertical dimension by the contraction of the diaphragm, and its two horizontal dimensions by the lifting of the front and sides of the ribs, causes the intrathoracic pressure to fall.
The lungs' interiors are open to the outside air, and being elastic, therefore expand to fill the increased space. The inflow of air into the lungs occurs via the respiratory airways Fig. In health these airways starting at the nose or mouth, and ending in the microscopic dead-end sacs called alveoli are always open, though the diameters of the various sections can be changed by the sympathetic and parasympathetic nervous systems.
During exhalation the diaphragm and intercostal muscles relax. This returns the chest and abdomen to a position determined by their anatomical elasticity. This is the "resting mid-position" of the thorax and abdomen Fig. The volume of air that moves in or out at the nose or mouth during a single breathing cycle is called the tidal volume. During heavy breathing hyperpnea , as, for instance, during exercise, inhalation is brought about by a more powerful and greater excursion of the contracting diaphragm than at rest Fig.
In addition the " accessory muscles of inhalation " exaggerate the actions of the intercostal muscles Fig. These accessory muscles of inhalation are muscles that extend from the cervical vertebrae and base of the skull to the upper ribs and sternum , sometimes through an intermediary attachment to the clavicles. Seen from outside the body the lifting of the clavicles during strenuous or labored inhalation is sometimes called clavicular breathing , seen especially during asthma attacks and in people with chronic obstructive pulmonary disease.
During heavy breathing, exhalation is caused by relaxation of all the muscles of inhalation. But now, the abdominal muscles, instead of remaining relaxed as they do at rest , contract forcibly pulling the lower edges of the rib cage downwards front and sides Fig. This not only drastically decreases the size of the rib cage, but also pushes the abdominal organs upwards against the diaphragm which consequently bulges deeply into the thorax Fig. The end-exhalatory lung volume is now well below the resting mid-position and contains far less air than the resting "functional residual capacity".
However, in a normal mammal, the lungs cannot be emptied completely. In an adult human there is always still at least 1 liter of residual air left in the lungs after maximum exhalation. The automatic rhythmical breathing in and out, can be interrupted by coughing, sneezing forms of very forceful exhalation , by the expression of a wide range of emotions laughing, sighing, crying out in pain, exasperated intakes of breath and by such voluntary acts as speech, singing, whistling and the playing of wind instruments.
All of these actions rely on the muscles described above, and their effects on the movement of air in and out of the lungs. Although not a form of breathing, the Valsalva maneuver involves the respiratory muscles. It is, in fact, a very forceful exhalatory effort against a tightly closed glottis , so that no air can escape from the lungs. The abdominal muscles contract very powerfully, causing the pressure inside the abdomen and thorax to rise to extremely high levels.
The Valsalva maneuver can be carried out voluntarily, but is more generally a reflex elicited when attempting to empty the abdomen during, for instance, difficult defecation, or during childbirth. Breathing ceases during this maneuver. The primary purpose of the respiratory system is the equilibration of the partial pressures of the respiratory gases in the alveolar air with those in the pulmonary capillary blood Fig.
This process occurs by simple diffusion ,  across a very thin membrane known as the blood—air barrier , which forms the walls of the pulmonary alveoli Fig. It consisting of the alveolar epithelial cells , their basement membranes and the endothelial cells of the alveolar capillaries Fig. The air contained within the alveoli has a semi-permanent volume of about 2. This ensures that equilibration of the partial pressures of the gases in the two compartments is very efficient and occurs very quickly.
This marked difference between the composition of the alveolar air and that of the ambient air can be maintained because the functional residual capacity is contained in dead-end sacs connected to the outside air by fairly narrow and relatively long tubes the airways: This typical mammalian anatomy combined with the fact that the lungs are not emptied and re-inflated with each breath leaving a substantial volume of air, of about 2.
Thus the animal is provided with a very special "portable atmosphere", whose composition differs significantly from the present-day ambient air. The resulting arterial partial pressures of oxygen and carbon dioxide are homeostatically controlled. A rise in the arterial partial pressure of CO 2 and, to a lesser extent, a fall in the arterial partial pressure of O 2 , will reflexly cause deeper and faster breathing till the blood gas tensions in the lungs, and therefore the arterial blood, return to normal.
The converse happens when the carbon dioxide tension falls, or, again to a lesser extent, the oxygen tension rises: This is very tightly controlled by the monitoring of the arterial blood gases which accurately reflect composition of the alveolar air by the aortic and carotid bodies , as well as by the blood gas and pH sensor on the anterior surface of the medulla oblongata in the brain.
There are also oxygen and carbon dioxide sensors in the lungs, but they primarily determine the diameters of the bronchioles and pulmonary capillaries , and are therefore responsible for directing the flow of air and blood to different parts of the lungs.
If more carbon dioxide than usual has been lost by a short period of hyperventilation , respiration will be slowed down or halted until the alveolar partial pressure of carbon dioxide has returned to 5. If these homeostats are compromised, then a respiratory acidosis , or a respiratory alkalosis will occur. Oxygen has a very low solubility in water, and is therefore carried in the blood loosely combined with hemoglobin. The oxygen is held on the hemoglobin by four ferrous iron -containing heme groups per hemoglobin molecule.
The reaction is therefore catalyzed by carbonic anhydrase , an enzyme inside the red blood cells. The total concentration of carbon dioxide in the form of bicarbonate ions, dissolved CO 2 , and carbamino groups in arterial blood i.
Ventilation of the lungs in mammals occurs via the respiratory centers in the medulla oblongata and the pons of the brainstem. This information determines the average rate of ventilation of the alveoli of the lungs , to keep these pressures constant. The respiratory center does so via motor nerves which activate the diaphragm and other muscles of respiration.
The breathing rate increases when the partial pressure of carbon dioxide in the blood increases. This is detected by central blood gas chemoreceptors on the anterior surface of the medulla oblongata. Exercise increases the breathing rate due to the extra carbon dioxide produced by the enhanced metabolism of the exercising muscles. Information received from stretch receptors in the lungs limits tidal volume the depth of inhalation and exhalation. The alveoli are open via the airways to the atmosphere, with the result that alveolar air pressure is exactly the same as the ambient air pressure at sea level, at altitude, or in any artificial atmosphere e.
With expansion of the lungs through lowering of the diaphragm and expansion of the thoracic cage the alveolar air now occupies a larger volume, and its pressure falls proportionally , causing air to flow in from the surroundings, through the airways, till the pressure in the alveoli is once again at the ambient air pressure.
The reverse obviously happens during exhalation. This process of inhalation and exhalation is exactly the same at sea level, as on top of Mt. Everest , or in a diving chamber or decompression chamber.
However, as one rises above sea level the density of the air decreases exponentially see Fig. This is achieved by breathing deeper and faster i.
There is, however, a complication that increases the volume of air that needs to be inhaled per minute respiratory minute volume to provide the same amount of oxygen to the lungs at altitude as at sea level. During inhalation the air is warmed and saturated with water vapor during its passage through the nose passages and pharynx.
Saturated water vapor pressure is dependent only on temperature. In dry air the partial pressure of O 2 at sea level is At the summit of Mt. This reduces the partial pressure of oxygen entering the alveoli to 5. The reduction in the partial pressure of oxygen in the inhaled air is therefore substantially greater than the reduction of the total atmospheric pressure at altitude would suggest on Mt Everest: A further minor complication exists at altitude.
If the volume of the lungs were to be instantaneously doubled at the beginning of inhalation, the air pressure inside the lungs would be halved. This happens regardless of altitude. The driving pressure forcing air into the lungs during inhalation is therefore halved at this altitude. However, in reality, inhalation and exhalation occur far more gently and less abruptly than in the example given.
All of the above influences of low atmospheric pressures on breathing are accommodated primarily by breathing deeper and faster hyperpnea. The exact degree of hyperpnea is determined by the blood gas homeostat , which regulates the partial pressures of oxygen and carbon dioxide in the arterial blood. This homeostat prioritizes the regulation of the arterial partial pressure of carbon dioxide over that of oxygen at sea level.
If this switch occurs relatively abruptly, the hyperpnea at high altitude will cause a severe fall in the arterial partial pressure of carbon dioxide, with a consequent rise in the pH of the arterial plasma.
This is one contributor to high altitude sickness. On the other hand, if the switch to oxygen homeostasis is incomplete, then hypoxia may complicate the clinical picture with potentially fatal results.
There are oxygen sensors in the smaller bronchi and bronchioles. In response to low partial pressures of oxygen in the inhaled air these sensors reflexly cause the pulmonary arterioles to constrict. At altitude this causes the pulmonary arterial pressure to rise resulting in a much more even distribution of blood flow to the lungs than occurs at sea level.
At sea level the pulmonary arterial pressure is very low, with the result that the tops of the lungs receive far less blood than the bases , which are relatively over-perfused with blood. It is only in middle of the lungs that the blood and air flow to the alveoli are ideally matched.
This is a further important contributor to the acclimatatization to high altitudes and low oxygen pressures. When the oxygen content of the blood is chronically low, as at high altitude, the oxygen-sensitive kidney cells secrete erythropoietin often known only by its abbreviated form as EPO  into the blood.
In other words, at the same arterial partial pressure of O 2 , a person with a high hematocrit carries more oxygen per liter of blood than a person with a lower hematocrit does. High altitude dwellers therefore have higher hematocrits than sea-level residents. Irritation of nerve endings within the nasal passages or airways , can induce a cough reflex and sneezing.
These responses cause air to be expelled forcefully from the trachea or nose , respectively. In this manner, irritants caught in the mucus which lines the respiratory tract are expelled or moved to the mouth where they can be swallowed. This increases the expired airflow rate to dislodge and remove any irritant particle or mucus.
Respiratory epithelium can secrete a variety of molecules that aid in the defense of the lungs. These include secretory immunoglobulins IgA , collectins , defensins and other peptides and proteases , reactive oxygen species , and reactive nitrogen species. These secretions can act directly as antimicrobials to help keep the airway free of infection.
A variety of chemokines and cytokines are also secreted that recruit the traditional immune cells and others to site of infections. Surfactant immune function is primarily attributed to two proteins: These proteins can bind to sugars on the surface of pathogens and thereby opsonize them for uptake by phagocytes. It also regulates inflammatory responses and interacts with the adaptive immune response.
Surfactant degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection. Most of the respiratory system is lined with mucous membranes that contain mucosa-associated lymphoid tissue , which produces white blood cells such as lymphocytes.
The lungs make a surfactant , a surface-active lipoprotein complex phospholipoprotein formed by type II alveolar cells. It floats on the surface of the thin watery layer which lines the insides of the alveoli, reducing the water's surface tension.