A dolphin's respiratory system works like most other mammals, with a few modifications that allow the animal to live in the water. Dolphins breathe using a single blowhole located at the top of their heads. A muscular flap covers the blow hole, allowing a dolphin to hold its breath under water and preventing water from filling its lungs.
A dolphin is not a fish and doesn't have gills that filter oxygen from the water. It must move to the water's surface to breathe. It contracts the muscular flap just prior to surfacing, opening the blowhole.
As the dolphin exhales, a visible spout of water rises from its blowhole. However, this water doesn't come from the lungs. It is simply water on top of the blowhole pushed up with the carbon dioxide and other waste gases the animal expels from its lungs. Like those in a human, the lungs of a dolphin don't tolerate water. Even though dolphins spend their entire lives in the water, if a fishing net traps a dolphin and prevents it from surfacing to breathe, the animal drowns.
The lungs of the dolphin are much more efficient than those of humans. During each respiratory cycle, a dolphin exchanges 80 percent or more of the air in its lungs, while a human only exchanges about 17 percent of the air in his lungs.