Answer:
Dolphins breathe through a hole in the top of their head which is called blowhole.
The blowhole can empty and refill the lung in one-fifth of a second through an explosive blow which can reach velocities up to one hundred miles per hour.
The blowhole is closed tightly by a group of strong muscles surrounding it, which avoid water from getting in.
The air introduced through the blowhole, is conducted to the lungs for getting oxygen, which is a huge difference with fishes who use gills to breathe.