Vasodilatation (dilation of blood vessels) increases surface blood flow & increases heat loss (when ambient air/water temperature is less than body temperature).
Vasoconstriction (constriction of blood vessels) decreases blood flow to periphery & decreases heat loss.
Sweating - cools the body through evaporative cooling.
Shivering - generates heat through the increase in chemical reactions required for muscle activity. Visible shivering can maximally increase surface heat production by 500%. However, this is limited to a few hours because of depletion of muscle glucose and the onset of fatigue.
Increasing/Decreasing Activity will cause corresponding increases in heat production and decreases in heat production.
Behavioral Responses - putting on or taking off layers of clothing will result in heat regulation
How We Lose Heat to the Environment or Stay Dry = Stay Alive!
Radiation
The loss of heat to the environment due to the temperature gradient (this occurs only as long as the ambient temperature is below 98.6°F). Factors important in radiant heat loss are the surface area and the temperature gradient.
Conduction
Through direct contact between objects, molecular transference of heat energy.
Water conducts heat away from the body 25 times faster than air because it has a greater density (therefore a greater heat capacity).
Steel conducts heat away faster than water. Example: Generally conductive heat loss accounts for only about 2% of overall loss. However, with wet clothes the loss is increased 5 times.
Convection
A process of conduction where one of the objects is in motion. Molecules against the surface are heated, move away, and are replaced by new molecules which are also heated. The rate of convective heat loss depends on the density of the moving substance and the velocity of the moving substance (water convection occurs more quickly than air convection). Wind chill is an example of the effects of air convection. The wind chill table gives a reading of the amount of heat lost to the environment relative to a still air temperature.
|
Wind Speed |
Actual Air Temperature (F) |
||||
|
MPH |
50 |
40 |
30 |
20 |
10 |
|
5 |
48 |
36 |
27 |
17 |
-5 |
|
10 |
40 |
29 |
18 |
5 |
-8 |
|
15 |
35 |
23 |
10 |
-5 |
-18 |
|
20 |
32 |
18 |
4 |
-10 |
-23 |
|
25 |
30 |
15 |
-1 |
-15 |
-28 |
|
30 |
28 |
13 |
-5 |
-18 |
-33 |
|
35 |
27 |
11 |
-6 |
-20 |
-35 |
Evaporation
Heat loss from converting water from a liquid to a gas.
Perspiration - evaporation of water to remove excess heat.
Sweating - body response to remove excess heat.
Insensible Perspiration - body sweats to maintain humidity level of 70% next to skin - particularly in a cold, dry environment you can lose a great deal of moisture this way.
Respiration - air is heated as it enters the lungs and is exhaled with an extremely high moisture content.
It is important to recognize the strong connection between fluid levels, fluid loss, and heat loss. As body moisture is lost through the various evaporative processes the overall circulating volume is reduced which can lead to dehydration. This decrease in fluid level makes the body more susceptible to hypothermia and other cold injuries.
Response to Cold
Heat Retention - (positive factors)
Size/shape (Eskimo vs. Masai)
Insulation (layering/type)
Fat (as insulation)
Shell/core (shunt blood to core) shell acts as a thermal barrier Total = Heat Retention
Heat Production - (positive factors)
Exercise, shivering Limited by:
Fitness
Fuel stores (glycogen)
Fluid status (efficient exercise)
Food intake (kindling, sticks, logs) Total = Heat Production
Cold Challenge - (negative factors)
Temperature
Wet (rain, sweat, water)
Wind (blowing, moving, e.g. biking) Total = Cold Challenge
Heat Retention + Heat Production Less Cold Challenge = Hypothermia