More on the mammalian dive reflex 

The mammalian dive relflex — or response — consists of a set of reflexes that are activated by holding one’s breath or being exposed to cold water on the face. It is a physiological mechanism that enables the body to handle and control a lower level of oxygen within the body. 

The natural response has the following effect on the body:  

1. Bradycardia – the slowing of the heart rate. The heart rate can slow from 10-30 percent, or even more for experienced individuals, in order to conserve energy and oxygen and enter a calmer state of being beneath the surface.
2. Peripheral Vasoconstriction – the narrowing of the blood vessels. This reduces blood flow to the limbs and ensures oxygen-sensitive areas — such as the heart and brain — receive the oxygen they need.
3. Blood Shift – the change in blood plasma and water passing through organs and the circulatory walls. This protects organs from the increase in pressure at extreme depths. 

Other reflexes within the body include the spleen, which contains an excess of red blood cells. During extended periods of diving or following long dives, the spleen contracts, releasing more red blood cells into the blood stream, allowing more oxygen to be stored and replenished throughout the body. This action helps the body to regain normal balance of oxygen-rich blood cells following dives quickly.