Mammilian Dive Reflex: The Human Body's Amazing Adaptation in Cold Water

For decades it was thought that drowning was drowning and that was all there was to it. That the human body, once underwater for a prolonged period of time was dead and that no resuscitation could bring them back. However, for some years now scientists and health officials have pointed out that some people have survived up to an hour underwater and without any physical damage resulting while in cold water. (WebMD, 2006). So what is it that cold water triggers in the human body causing it to preserve itself longer than in warm water?

The adaptation is called Mammilian Dive Relex and occurs only when the body is suddenly submerged in cold water of about 32 degrees Fahrenheit and above. (WebMD, 2006). However, if the body goes into hypothermia slowly, the Mammilian Dive Relex won't kick in, and normal hypothermia and drowning will occur. It seems a fragile balance between survival by adaptation and death by adverse environment.

When the human body is submerged head and chest in cold water a great deal of heat is lost. About 50% of body heat is lost through the head and neck (Sutherland, 2006), so being fully submerged in water creates an enormous amount of body heat loss. Furthermore, swimming or struggling expends more energy and creates a higher amount of body heat loss severely cutting survival time. Initially after entering cold water the heart rate quickens as does breathing. Both of these symptoms can cause cardiac arrest and hyperventilation respectively. (Sutherland, 2006). Additionally, the arms and leg become stiff and numb making swimming an extremely difficult task. Not to mention that as water enters the respiratory tract, the airway goes into spasm often making it difficult to speak and call for help. (WebMD, 2006).