How do cetaceans breathe?

As mammals, cetaceans breathe air with the use of lungs.  Their nostrils, otherwise known as a blowhole, are situated on the top of the head, which allows them to breathe easily even in bad weather and stormy seas.  When the cetacean dives, special muscles close off the blowhole and these same muscles allow the blowhole to open when the animal resurfaces.  At this time all the warm air from the lungs is expelled as nitrogen oxide in a spout of steam and oxygen is taken in.