Turtles breathe air; they need to come to the surface to breathe oxygen. But they have adaptations that allow them to stay underwater for long periods of time. They even have an adaptation to absorb small amounts of oxygen without breathing.
Turtles breathe air through external nares located above their mouths. Air moves through the glottis and into the trachea, made of a series of rings of cartilage. A turtle's trachea is elongated and flexible, allowing the turtle to move his head in and out of his shell. The trachea splits near the heart into two bronchi that deliver air to the lungs, where it is absorbed into the body.
Since a turtle's shell cannot expand and contract the way a person's ribs do, turtles have muscles inside their shells that expand and contract to move air in and out of the lungs. Moving their limbs also helps with breathing by altering the pressure in the lungs.
Since turtles are cold-blooded, their activity level depends on the external temperature. They are more active in warm temperatures and less active when it is cold. As temperatures drop, activity levels decrease, as do turtles' metabolism. When temperatures drop, turtles go into hibernation. Some turtles hibernate, without breathing, in the mud at the bottom of the pond.
While in hibernation, they don't move and their heart rates slow. They "breathe" anaerobically, using fats stored during the summer months. This process maintains the turtles' low metabolism and cell function but results in buildup of lactic acid. The turtles' shells release carbonates into their systems, neutralizing the acid and preventing it from becoming deadly.
Oxygen From Water
Some species of turtles can absorb oxygen from the water, allowing them to stay underwater for long periods of time without coming up for air. The length of time they can stay underwater depends on species and temperature. Sea turtles, for example, can remain underwater for four to seven hours at rest. Hibernating turtles can stay underwater for several months.
The cloaca is an opening in a turtle's rear end where the rectum and urinary systems empty. Expanding and contracting muscles forces water in and out of the cloaca. In some turtle species, such as the eastern painted turtle, the cloaca has a high density of blood vessels, allowing the turtle to absorb oxygen from the water through the skin. Some species, such as the musk turtle, can absorb oxygen into the blood vessels in the throat cavity.
Despite their adaptations, turtles can drown if they become stuck underwater. Unless the temperature is cold enough to allow a turtle to enter hibernation, the turtle will need to breathe to get oxygen, and may breathe water into his lungs.
Turtles may also asphyxiate underwater if they use up all of the oxygen available in their systems. Under stressful situations, turtles may struggle, using up available oxygen quickly. Lactic acid builds up as anaerobic respiration switches on, and it can quickly become toxic. In such cases, turtles who cannot reach the surface to breathe may die in minutes.