The blowhole of a dolphin is actually an adapted nose which has moved to the top of its head. Dolphins must tell themselves when to breathe; it's not an automatic response like it is for humans.

Dolphins are mammals and breathe air into their lungs, just like we do. Dolphins cannot breathe under water like fish can as they do not have gills. Dolphins breathe through a nostril, called a blowhole, located right on top of their heads. This allows them to take breaths by exposing just the top of their heads to the air while they are swimming or resting under the water. After each breath, the blowhole is sealed tightly by strong muscles that surround it, so that water cannot get into the dolphin’s lungs.