Because dolphins are mammals, they need to breathe air, just like humans. 

 

Dolphins breathe through their blowhole on the top of their head.

 

Each time humans breathe in, we  exchange about 10% of the air in our lungs. Each time a dolphin breathes in, the dolphin exchanges 80% of 90% of the air in their lungs.

 

Dolphins get more oxygen out of the air they breathe and can store more oxygen in their muscles than we can. Dolphins can breathe in and out very quickly.

 

Bottlenose dolphins have been known to remain under water for eight to ten minutes.

 

It is probably easy for a dolphin to dive to some 500 feet and a trained dolphin has been known to dive to 1,795 feet.

 

Whales and dolphins are conscious breathers, meaning that they think before they breathe, while humans breathe automatically.

 

If a dolphin loses consciousness, he will not be able to keep his blow-hole clear of the water and will suffocate and die. Other dolphins will jostle a dolphin at risk in this way to keep him awake, and will hold the dolphin to the surface so that he can breathe. 

 

Helping other dolphins seems to be so strongly ingrained that that may be why dolphins sometimes come to the assistance of humans in trouble in the water.