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The respiratory system of all animals is that which allows oxygen to pass from the surrounding air or water into the bloodstream of the organism. In addition to this, the system is required to remove carbon dioxide from the blood and release it into the air or water. Amphibians have three types of highly vascularized that are able to be used in respiration. They are :
Buccopharyngeal mucosa (the mouth and throat membrane)
Amphibians have external gills which are capable of performing the functions of respiration while the animal is under water. In frogs, the gills are present only during the tadpole stage, when the individual is confined to the water. As the tadpole transforms into an adult frog, its gills are absorbed into the body and the lungs take over for the process of respiration.
The lungs of amphibians that are required to take over for the gills after metamorphosis are generally already developed before the gills are reabsorbed. They are relatively simple in structure, resembling smooth sacs. The lungs of more terrestrial amphibians, such as the toad, are larger and are equiped with more alveolar respiratory surfaces.
As was mentioned above in the description of amphibian skin, the dermal region is very heavily vascularized to allow for efficient exchange of gases. When the adult amphibian is under water, it will breathe through its skin until it is able to come up to the surface for air.
Many male amphibians have vocal sacs which are basically outpouchings of the mouth cavity that extend ventrally and laterally under the skin and muscles of the throat. When a frog calls, its pouch is filled with air and the function is to assist in the resonation of the sound. Individual frogs also seem to be able to vary the sound of their call by adjusting the action of their vocal sac.
Larval amphibians respire, or exchange carbon dioxide and oxygen, through their gills and skin. Most adult amphibians lose their gills during metamorphosis, but they can respire in two ways: through the lungs and through the skin. Respiration through the lungs is called pulmonary respiration. Amphibians ventilate their lungs with a unique mechanism that pumps air into the lungs; this is called positive-pressure breathing. For example, a frog breathes by changing the volume and pressure of air in its mouth while either opening or closing its nostrils.
Both inhalation and exhalation involve a two-step process during which the floor of the frog's mouth is raised and lowered. The frog controls the direction of air flow by opening or closing its nostrils. Because amphibians have a small surface area in the lungs for gas exchange, respiration is very important to most aquatic and terrestrial amphibians.
Respiration 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 file snakes, family Acrochordidae, and sea snakes, family Hydrophiidae, as well as the soft-shelled turtle, Trionyx, can use their skin for respiration when submerged.
The lungs of reptiles are large, and they are often divided internally into several chambers. The lining of the lungs may be folded into numerous small sacs called alveoli . Alveoli greatly increase the internal surface area of the lungs, thus increasing the amount of oxygen that can be absorbed. In most snakes, only the right lung actively functions. It is elongated and may be half as long as the body. The left lung is either reduced to a small nonfunctional sac or absent entirely.
A reptile fills its lungs be expanding its rib cage. This expansion reduces the pressure within the thorax and draws air into the lungs. When the ribs return to their resting position, pressure within the thorax increases and air is forced out of the LUNGS.