Reptiles' lungs are the main component of their respiratory system, just as they are in humans, birds and land-dwelling amphibians. However, among the main types of reptiles, there are some distinct differences in the details of their respiratory system functioning.
Reptile Respiration Basics
Regardless of the differences, most reptile lungs work in essentially the same way. Like mammalian lungs, reptile lungs work like suction pumps. Muscles controlling the lungs cause them to expand. The expansion causes the pressure in the lungs to be lower than the pressure outside the lungs; therefore, air fills the lungs (link = ref 5). Although all reptiles use their lungs as the primary source of air, some can use their skin to bring oxygen into their bodies. Sea snakes, for example, can take in nearly half of the air they need through their skin (5).
Scaled Reptile Respiration
The most diverse group of reptiles are those with scales – a group called Squamata. Squamates include snakes and lizards. In these reptiles, the muscle that controls their lungs also controls their movement. Consequently, many reptiles must hold their breath during rapid periods of movement, such as running after prey or away from a predator. A few reptiles in these groups have developed ways around this problem. For example, the Tegu lizard's proto-diaphragm allows him to inflate his lungs more fully, while monitor lizards can use their throat muscles to handle respiration during movement.
The Crocodilian group includes crocodiles, alligators, caimans and gharials. These reptiles have developed a more efficient respiration method than that seen in lizards and snakes. Members of this reptile group have a diaphragm muscle, sometimes referred to as the heptatic piston, attached to their liver; the opposite side of the liver is attached to the lungs. When these reptiles contract that muscle, the liver moves downward in their bodies to give the lungs more room to expand so they can hold more air.
The Testudine group of reptiles includes turtles, tortoises, and terrapins. Of all the reptiles, this group has shown the most diversity in methods of respiration. Unlike other reptiles, most turtles and their cousins have rigid shells that are not capable of expansion. But some turtles can expand or contract the space within their body cavities to allow for breathing. Indian flapshell turtles have a muscular sheet around their lungs that can expand and contract to allow for respiration. Like snakes, some turtles, including the green sea turtle, must hold their breath while moving. American box turtles, on the other hand, can breathe and walk at the same time, while red-eared slider turtle take smaller breaths while moving. Some aquatic turtles even have gills on their bodies to assist them in respiration.