A. Radiation: is direct heat loss from the body to its surroundings. If the surroundings are colder than the body, the net result is heat loss. A nude man loses about 60% of his total body heat by radiation. Specifically, heat is lost in the form of infrared radiation. Infrared targeting devices work by detecting radiant heat loss.
B. Conduction: is the direct transfer of heat from one object in contact with a colder object.
(1) Most commonly conduction occurs when an individual sits or rests directly upon a cold object, such as snow, the ground, or a rock. Without an insulating layer between the Marine and the object (such as an isopor mat), one quickly begins to lose heat. This is why it's important to not sit or sleep directly on cold ground or snow without a mat or a pack acting as insulation.
C. Convection: is heat loss to the atmosphere or a liquid.
Air and water can both be thought of as "liquids" running over the surface of the body. Water or air, which is in contact with the body, attempts to absorb heat from the body until the body and air or water is both the same temperatures. However, if the air or water is continuously moving over the body, the temperatures can never equalize and the body keeps losing heat.
D. Evaporation . Heat loss from evaporation occurs when water (sweat) on the surface of the skin is turned into water vapor. This process requires energy in the form of heat and this heat comes from the body.
(1) This is the major method the body uses to cool itself down. This is why you sweat when you work hard or PT. One quart of sweat, which you can easily produce in an hour of hard PT, will take about 600 calories of heat away from the body when it evaporates.
E. Respiration. When you inhale, the air you breathe in is warmed by the body and saturated with water vapor. Then when you exhale, that heat is lost. That is why breath can be seen in cold air. Respiration is really a combination of convection (heat being transferred to moving air by the lungs) and evaporation, with both processes occurring inside the body.
3. PHYSICAL RESPONSES TO HEAT. When the body begins to create excess heat, it responds in several ways to rid itself of that heat.
A. Initially, the blood vessels in the skin expand, or dilate. This dilation allows more blood to the surface where the heat can more easily be transferred to the surroundings.
B. Soon afterwards, sweating begins. This contributes to heat loss through convection and evaporation.
4. PHYSICAL RESPONSES TO COLD. Almost the opposite occurs as with heat.
A. First, blood vessels at the skin surface close down, or constrict. This does two things:
(1) Less blood goes near the surface of the body so that less heat is lost to the outside.
(2) More blood goes to the "core" or the center of the body, to keep the brain, heart,lungs, liver, and kidneys warm. This means fingers and toes tend to get cold.
B. If that is not enough to keep the body warm, the next step is shivering. Shivering is reflexive regular muscular contractions, this muscular activity causes heat production. As mentioned before, shivering can only last for a short time before exhaustion occurs. With shivering you will either warm up, as usually occurs, or continue to get colder and start to become hypothermic. Hypothermia will be discussed later.
5. ENVIRONMENTAL INJURIES (WSVX.02.14b) are cold weather injuries, dehydration, and altitude related illnesses.
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