One major factor is their ability to conserve oxygen. First of all, the lungs of dolphins are relatively small, but they are highly adapted with thousands of very fine aveoli and capillaries where oxygen is exchanged into the blood. They also are very good at making sure each breath is very efficient. Another adaptation in dolphins is a high amount of a substance called myoglobin in their muscles. This substance works as an oxygen store in their muscles, so that even when the animal is not breathing during a dive, the muscles are still provided with oxygen and the animal can still swim.
Dolphins are also be capable of slowing down the systems of their body, such as their heart rate.
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