Diving seal, Farne Islands, photo by Farne Island Divers

All mammals, including humans, have a diving response. This consists of three physiological changes in the body:

• The heart rate slows down, by about 10% – 30% sometimes more (with training).
• the blood vessels narrow causing reduced blood flow to the limbs
• on deep dives a blood shift takes place allowing plasma and water to pass into the chest cavity to ensure that the pressure stays constant and the organs are not crushed.

These changes allow the body to tolerate a low level of oxygen. It is the diving response that allows otters, seals and other water mammals to stay under water for a long time. The effect is much less pronounced in humans but it exists. The slowing of the heart rate happens very quickly if you put your face (mouth and nose) into water that is colder than about 21 degrees C. The colder the water the quicker the response.

Photo courtesy of Farne Island Divers