Dolphins breathe in air through their blowhole. The blowhole is covered by a muscular flap which provides a watertight seal. The flap opens during contraction and closes during muscle relaxation. For them to be able to breathe, they have to rise to the surface and breathe in the oxygen. When a dolphin breaths in air it goes directly into their lungs, exhaling and inhaling rarely takes more than 0.3 seconds and they take about 2-3 breathes per minute on average. During each respiration a dolphin exchanges 80% or more of its lung air.
When diving, the dolphin reduces its request for oxygen. Their heart beats slower and the amount of blood pumped around the body decreases. During a deep dive only important organs such as the heart and brain get oxygen rich blood flowing through them.