Dolphins are equipped with blowholes on the crowns of their heads through which they breathe air when they reach the surface of the water. They are capable of holding their breath for up to 10 minutes.
Because they breathe air rather than survive on oxygen from the water, dolphins are considered mammals and not fish. Unlike humans, dolphins are voluntary breathers, which means they must think about breathing rather than breathing subconsciously. Because of this, dolphins are never fully asleep like humans; instead, a dolphin shuts down only half of its brain while the other half remains awake to breathe.