Cornell University Ergonomics Web

DEA3500: Ambient Environment: Thermal Regulation

Heat, Cold, and Performance: we are still remarkably ignorant of the body's thermoregulatory mechanisms!

1. Heat balance - the heat exchange process

The living body constantly produces heat and this must be transferred to the environment. Heat balance (thermal equilibrium) is the balance between the rate of heat production and the rate of heat loss.

Heat balance equation
Heat production = the rate of heat production = M - W where:
M = total rate of energy production which can be found from the rate of oxygen consumption (1 litre O2 = 5 kcal = 20,000 joules)(1cal = 4.184j)(1 kcal = 1000 cal)
W = rate at which external work (force x distance) is being performed.
M - W = total energy - work energy

Heat loss = the rate of heat loss = R + C + E + L + K + S where:
R = radiation (heat loss or gain) between skin or clothing surface and surrounding surfaces e.g. walls, sun etc. At rest, in a thermoneutral environment (21C), 60% heat loss from nude body is by radiation. Mention radiant asymmetry.
C = convection (air close to body absorbs heat ), which is a form of conduction to the surrounding air and is the heat loss (or gain) by the mixing of air close to the body surface.
2 types: natural convection (in still air the body produces an upward flow of warm air) and forced convection (movement of air past the body e.g. wind.
At rest as above, convection accounts for 18% of heat loss.
E = evaporation. The evaporation of water through the outer layers of skin (insensible perspiration) or from the skin surface when this is wetted by sweat (perspiration) or some other external agency.
L = warming and wetting of air which is inhaled and then exhaled (this is sometimes included under E).
K = conduction to the surfaces by direct contact with skin or clothing e.g. sitting on a cold surface etc. This is sometimes included under C. At rest as above, this accounts for 3% of heat loss.
S = rate of storage of heat in the body.

Heat balance exists when
M - W = R + C + E + L + K + S

R + C + K = 72% of heat loss
Eskin = 15% (Excretion of feces and urine = 3%)
Llungs = 7% exhaled, 3% warming inhaled

Ideally S should equal 0 when the body is in heat balance i.e. heat production = heat loss with no storage.

In practice the body rarely attains or maintains heat balance and many factors influence the relative importance of the heat exchange processes.

2. Thermoregulation - the simplest thermoregulatory model divides the body into two components: the core and the shell. Also more complex models e.g. Stolwyck & Hardy's 25 node mode.

Shell - skin temperature varies over a greater range than core temperature. Skin temperature depends primarily on environmental conditions.
Core - core temperature is controlled within a relatively narrow band by thermoregulatory systems. Core temperature depends primarily on work rate.

Heat exchange mechanisms - 3 main physiological mechanisms.

a. Vasomotor - all skin needs some blood supply to keep it alive but skin blood flow can be increased many times this basic level. Increasing skin blood flow raises skin temperature and increases heat transfer to the environment, and cools the core. Decreasing skin blood flow cools the skin and reduces heat transfer to the environment and warms the core. Changes in skin blood flow are most marked at the extremities of limbs (hands and feet) and less marked in the trunk and head. This is why hands and feet frequently feel cold first.

b. Sweating - (max. continued total sweat rate = 1 litre/hour; max. short-term = 10-15 litres per 6 hours; always 650 ml/day.). As skin temperature approaches core temperature, transferring heat from the core to the skin becomes increasingly difficult. In hot environments the evaporation of sweat from the skin surface cools this thereby improving heat transfer from the core.
2 types of sweat:
appocrine - forehead, back, palms of hands, armpits - protein-containing sweat
eccrine - water sweat from all other skin areas (latent heat of water is 600 cal/gm)

c. Shivering - when skin blood flow is minimal there may be excessive heat loss from the core by conduction through the shell tissues. Maintenance of core temperature requires an increase in heat production and shivering is disorganised muscular activity which has this effect, and increases heat production 300-400%. To evaporate max. sweat takes 6000-9000 kcal, which equals heat of a lumberjack in cold weather!.

3. Thermal adjustment systems - when the body changes from one thermal environment to another the following mechanisms are brought into operation:

a. Changing from a warm to a cold environment entails the following:
-skin becomes cool
-blood is routed away from the skin to the core where it is warmed before flowing back to the skin
-core temperature rises slightly then falls with prolonged exposure
-shivering and "gooseflesh" may occur If the body stabilizes then large areas of the skin will receive little blood. If cooling continues then eventually core temperature falls producing hypothermia which may result in death. (Anecdote re: Chris's death from hypothermia following caving incident, not just limited to old people.)

b. Changing from a cold environment to a warm one entails the following:
-more blood is routed from the core to the skin surface thereby raising skin temperature
-core temperature falls but with continued exposure rises again
-sweating begins
If the body stabilizes then large areas of the skin will receive blood and sweating will occur. If warming of the body continues eventually core temperature rises, producing hyperthermia (heatstroke) which may result in death.

4. Acclimatization to heat and cold. Acclimatization consists of a series of physiological adjustments that occur in a person who is habitually exposed to either hot or cold conditions. It has been said that acclimatization consists of two processes: "getting used to it" and "not getting used to it"!

Acclimatization to heat: In hot climates physiological adaptations occur which help to cool the body:
-water intake increases (note it's better to drink warm fluids because these will elevate core temperature and encourage sweating to increase cooling, whereas cold fluids decrease core temperature, which discourages sweating to decrease cooling, which increases discomfort.
-sweating increases. With "training" sweat glands produce more sweat.
-blood volume increases and more blood is diverted to the skin (may even get a "red" face)
-behavioral changes occur: less clothing is worn, or heat is avoided e.g. resorting to an air-conditioned room (an example of getting used to not getting used to it because such behavior does not produce physiological acclimatization).

Heat acclimatization is best achieved by actually doing work in hot climates e.g. exercise. The best results occur when exposure lasts for at least one hour or at least every other or every day for at least 2 to 3 weeks (some acclimatization occurs within 4-7 days, and reasonably in 12-14 days). Discontinuation of heat exposure results in reversion to the unacclimatized state in a few weeks. Booster exposures every week or so can maintain a high level of acclimatization. This process has been confirmed by experimental studies e.g. Lind & Bass, 1963.

Lind & Bass (1963) - Men worked each of 9 days for 100 minutes at a time at an energy expenditure of 300 kcal hr-1 in a hot climate. Core temperature and pulse rate decreased and sweat rate increased with acclimatization.

Acclimatization to cold: The processes of cold acclimatization are less clear. Unlike sweating, the rate of shivering does not increase with prolonged exposure to cold. However, other changes do take place.
-the body core contracts so that much more of the body tissues are in the shell. The contracted core is now better insulated and therefore less heat is needed to maintain core temperature.
-there is a tendency to reduce blood flow to the extremities and so the use of hands is more difficult and is normally reduced. However, prolonged use of the hands in cold climates results in more blood being diverted to these e.g. herring filleters on the North Sea Coast used to work in the open with their hands either immersed in near-freezing water or exposed, wet to the wind. For an unacclimatised person this rapidly produces severe pain, yet the filleters worked all day with little discomfort. Such changes may take a long time (months or years) to occur.

The effects of acute or prolonged exposure to different temperatures on skin and the body core are summarized below:

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