Part land animals, part water creatures, reptiles have developed unique respiratory systems. All reptiles use lungs to breathe, even when they have gills or permeable skin according to the New World Encyclopedia. Reptile respiration differs among these classes of reptiles: the crocodilians, the squamates (snakes and lizards) and the testudines (turtles).
Squamate Reptile Respiration
Snakes and lizards don't have diaphragm muscles for lung ventilation: muscles used for locomotion are the same used for their respiratory systems. Contracting and flexing body muscles moves their ribs and fill their lungs. Most squamate reptiles must hold their breath when they engage in bursts of intense physical activity. Some lizards complement lung ventilation with buccal pumping. This involves using throat muscles to help fill their lungs.
Crocodilian Reptile Respiration
Crocodilians use a diaphragm muscle for lung ventilation, but their respiratory system differs from mammals. They use a hepatic piston, a process involving muscular movement of the pelvis and liver rather than the rib cage. This organ movement allows the lungs to expand.
Testudine Reptile Respiration
The hard shell that turtles and tortoises have makes lung expansion a bit more difficult. Most must use their limbs for lung ventilation. Some turtle lungs are covered with a sheet of muscle. Pulling their limbs in expels air from the lungs and pushing their limbs out of their shells expands the lungs. The lungs of many turtles attach to both the shell and the internal organs. By contracting and flexing a series of muscles, these turtles can push these organs up and down, contracting and expanding the lungs. Movement of their limbs aids this respiratory process.
Reptile Respiration during Eating and Movement
Most reptiles must hold their breath when they swallow but crocodilians and skinks have a second palate made of bone that allows them to breathe under water. Snakes extend their windpipes so that they don't suffocate during the swallowing process. Turtle respiration while in motion varies among species. Green sea turtles don't breathe when they're moving along nesting beaches, reports the New World Encyclopedia, but the North American box turtle breathes continuously while moving, and this respiration doesn't coincide with their limb movement. Red-eared sliders take smaller breaths while moving than at rest, and box turtles can breathe without extending their limbs.