Dolphins are mammals and they breathe air, just like humans do. However, since they live in water, they have adapted to their environment by developing a specialized respiratory system that allows them to stay underwater for extended periods of time.
Dolphins breathe through a blowhole, which is a single nostril located on the top of their head. The blowhole is covered by a muscular flap of skin that opens and closes to allow the dolphin to inhale and exhale. When the dolphin surfaces to breathe, it quickly opens its blowhole and exhales a burst of air, followed by a quick inhalation of fresh air. Dolphins can empty and refill their lungs in just a fraction of a second, which allows them to take in large amounts of air in a short period of time.
Unlike humans, dolphins do not have conscious control over their breathing. Instead, they have a reflex that automatically triggers the inhalation and exhalation of air when they surface. This allows them to breathe even while sleeping, as their blowhole is located on top of their head and remains above the water's surface.
Dolphins are also able to store oxygen in their muscles and blood, which allows them to dive to great depths and stay underwater for several minutes at a time. They can slow their heart rate and reduce blood flow to non-essential organs, which helps to conserve oxygen and allows them to stay submerged for longer periods of time.