Lungs beneath the carapace
The paired lungs lie dorsally, attached beneath the carapace, and are multicameral: divided into chambers whose spongy walls are folded into small pockets called faveoli where gas exchange occurs. Because they sit at the top of the body cavity, the lungs also serve as a buoyancy organ — by adjusting the volume of air held, a turtle can float at the surface, hang in mid-water, or rest on the bottom.
Breathing inside a rigid shell
The ribs are fused into the shell, so the chest wall cannot move, and there is no muscular diaphragm. Air is moved instead by antagonistic sheets of abdominal muscle at the limb openings, which enlarge and compress the body cavity against the fixed lungs; limb strokes and the surrounding water pressure assist. A turtle surfaces with the nostrils leading, exchanges air in a few quick breaths, and submerges again — breathing is intermittent by design.
Diving and accessory gas exchange
Between breaths a turtle tolerates long submergence, during which heart rate falls and metabolism is sustained at a low ectothermic level. Some freshwater turtles supplement the lungs while submerged with gas exchange across vascularized linings of the throat and of paired cloacal bursae, thin-walled sacs opening off the cloaca. These accessory surfaces extend time underwater, particularly in cool water, but the lungs remain the primary respiratory organ.