|Respiration - Mechanics
Cetaceans can hold their breath underwater for as long as 15 minutes for a dolphin and up to 75 minutes by a Sperm Whale, if necessary. They begin to exhale just before reaching the surface, inhale above the water (usually taking 3-4 breaths), then close a muscular flap over the blowhole(s) as they submerge again.
The nasal passages are not connected to the throat which prevents water entering the lungs when the mouth is open during swimming or feeding. Instead, the air from the blowhole enters directly into the trachea, then to the lungs.