MECHANISMS OF HEAT LOSS
When you understand the effects of cold water on the body, and how the body responds, you are far more prepared to make life-saving decisions, either for yourself or in a rescue situation.
It’s actually quite simple: the body attempts to maintain a constant core temperature (Homeostasis) through a balance of heat loss and heat gain. Body heat is normally gained through activities such as exercise and shivering, and also with the application of external heat sources such as heat packs.
Body heat is lost in several ways, and there are four major mechanisms at work:
The heat generated from within the body is given-off to the surrounding atmosphere.
When you sweat or when your skin or clothing gets wet, the evaporation of that liquid (i.e., the change from liquid to vapour form) promotes heat loss, and the natural result is a cooling effect.
Convection is the process of air or water flowing by the skin and carrying away body heat. It’s convective heat loss that you try to prevent by staying as still as possible in the water. Staying still, the boundary layer of water next to the skin is heated by the body and remains undisturbed. If you move around in the water, you disrupt that boundary layer of warmer water, and that increases heat loss.
The body conducts heat to whatever the skin is in direct contact with. Conductive heat loss occurs when the skin is subjected to either cold air or water, but it is especially critical in water, as your body loses heat about 25 times faster in water than in air of the same temperature.
EXAMPLE: Figure (a) is an infrared picture of a person at rest in room air. Notice that the lighter areas (which signify warmer surface temperature and surface blood flow) are well distributed throughout the body.
Once a body has been in cold water for an extended period of time, most of the skin is cool with little blood flow. However, there are critical areas that are lighter (warmer) than the surrounding tissue (Figure b). This is because blood is flowing through major blood vessels, which are near the skin surface. These areas in the neck, armpits and groin are areas of high heat transfer. That means that these areas have high heat loss in the cold but allow heat gain in the heat. This is why, in a rescue scenario, the most effective rewarming often consists of applying external heat directly to the armpits as well as the chest.
As a final note, it’s important to realize that the activity of swimming (which is naturally thought of as producing a heat GAIN), in cold water conditions will result in increasing the blood flow to blood vessels close to the skin, and because of Conduction and Convection, can actually increase the rate of heat LOSS and expedite the onset of hypothermia. Figure (c).