| What is freediving?

A little bit about the physiology of freediving.
Mammals and birds that live and feed underwater have developed a reflex to conserve oxygen while underwater. This so called "diving reflex" consists of the blood vessels constricting in certain parts of the body (vascular constriction) and a decrease in heartbeat (bradycardi). Vascular constriction acts in parts of the body that tolerate a decrease in oxygen and can function with anaerobic metabolism during a dive. The organs that demand a constant supply of oxygen are the brain and heart and the "diving reflex" does all it can so that these organs receive a constant supply of oxygen rich blood.

Because of this decrease in circulation the work that the heart must do also decreases thus decreasing the amount of oxygen used.
The "diving reflex" begins when the animal holds its breath (apn?) and is reinforced when its head goes underwater. The reflex stops when the animal starts to breath again. After a dive the animal remains above the surface until the lactic acid that has built up during the dive breaks down.

This reflex functions with people just like any other marine mammal.
It starts automatically when you dive or hold your breath. Bradycardi varies greatly from diver to diver.

The decrease of pulse rate of an untrained diver is normally about 10 to 30% while diving. The decrease in pulse rate in a professional diver is 50% or more which is about the same as many semi-aquatic animals.