Thermoregulation
Homeothermy
Humans, like all mammals, are homeotherms
We have a homeostatic or balance seeking temperature regulation system
Poikilotherms, like fish and frogs, have their body temperatures dictated by the environment
When it is cold, they are sluggish, when it is warm they become more active
Human Thermoregulation
Heat Production
Basal Metabolic Rate (BMR) is the minimum amount of energy required to sustain the body’s vital functions
This is the basic level of heat production by the body based solely on the chemical reactions of metabolism
Muscular activity produces heat as a side effect of the work that is done
Strenuous physical activity can increase the BMR up to 5 times the normal rate
Several hormones can increase BMR
The stress hormones of the sympathetic nervous system, epinephrine and norepinephrine secreted by the adrenals, and thyroxine secreted by the thyroid gland all produce an increase in BMR
Dietary Intake causes an increase in metabolic rate as the body works to digest foods
Heat Loss
Radiation consists of infra-red emanations
Direction and rate is a function of the temperature differential between the body and surrounding objects
Greater surface area yields higher radiation
Conduction refers to the transfer of heat by direct contact, molecule to molecule
Proportional to surface area, temperature differential, and density of surrounding medium (air vs. water)
Convection involves the movement of molecules from warm to cool areas
This is the "wind chill" effect
Proportional to temperature gradient and air flow
Evaporation of sweat from the skin reduces heat by 0.58 kcal/ml
Evaporation rate is proportional to surface area, temperature gradient
Inversely proportional to humidity
Water vapor in expired air also cools the body
Heat Balance
Heat Exchange at 75º F
Heat Exchange at 86º F
Heat Exchange at 95º F
Sweat Glands
Humans are the most prolific sweaters in the entire animal kingdom
Sweating is accomplished through specialized eccrine sweat glands
These glands are found in the dermis and epidermis, distributed all over the body, except for the margins of the limbs, sex organs, and ear drums
They average between 150 and 340 glands per square centimeter of skin for a total of between 2,000,000 and 5,000,000
The sweat glands are innervated by the sympathetic nervous system
When a rise in core temperature is detected by the hypothalamus, impulses to the sympathetic system cause an increase in sweat output
The sweat gland consists of a deep coiled portion and a duct that opens on the skin
The duct aids in the resorption of electrolytes, mainly sodium and chloride, in the sweat so that the fluid discharged onto the skin has had the electrolyte concentration reduced by a factor of about 20
Heat Transfer by Circulation
The peripheral circulation relies on beds of capillaries that transfer blood between the arterial and venous systems
These arteriovenous anastomoses can change the rate of blood flow from internal organs to the periphery by as much as 30%
Vasoconstriction (reducing the diameter of the capillaries) reduces the blood flow from the core to the periphery
Vasodilation (increasing the diameter) increases the flow
Core-Shell Model
Countercurrent Exchange System
Heat Stress
The sympathetic nervous system directs the body’s attempts to regulate temperature by rerouting circulation and sweating
Red skin indicates vasodilation and the pooling of blood near the surface for release of heat
Sweating causes evaporative cooling
The individual reduces heat stress by lowering physical activity levels, scheduling work during cool times of the day, wearing less clothes, using fans and air conditioning
Acclimation
Nonacclimated |
Acclimation (1 - 4 days) |
Full Acclimation (> 10 days) |
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Ethnicity and Acclimation
The basic physiological adaptation to heat appears very ancient in the hominid line
There is no discernable difference in heat tolerance between different ethnic groups that is not attributable to differences in acclimation
Body size, shape, and composition variability of different groups may also contribute to measured differences in heat tolerance
Bergmann's Rule (1847)
Within a polytypic warm-blooded species, the body size of the sub-species usually increases with decreasing mean temperature of its habitat
An increasing body size means several things
There is an increase in the amount of metabolizing tissue, therefore internal heat gain is greater
There is an overall decrease in the surface area to mass ratio, meaning that heat loss is reduced
Bergmann's Rule
Bergmann’s Rule in Humans
Allen's Rule (1877)
In warm-blooded species, the relative size of exposed portions of the body decreases with decrease of mean temperature
An increase in exposed portions of the body (increasing surface area) also has several natural concommitants
Keeping mass constant, surface area is increased by assuming a more linear form--taller, with long, slender arms and legs
This is especially important for humans since heat loss from evaporation of sweat is greater than in any other animal, and evaporative loss is directly proportional to the amount of exposed surface area
Allen's Rule
Allen’s Rule in Humans
Ruff’s Cylindrical Model of Body Form
Female Bi-iliac Breadth and Latitude
Male Bi-iliac Breadth and Latitude
Frisancho’s Explanation of Population Differences
Cold Stress: Peripheral Rewarming
Acclimation to Cold
Nonacclimated |
Acclimation |
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Factors Affecting Thermal Acclimation
Age
Both infants and elderly have lessened ability to acclimatize to heat or cold
Body size and shape
The surface area to weight ratio will affect the level of acclimatization attainable
Body composition
Subcutaneous adipose deposits insulate the core and make it more difficult to dissipate heat in hot or easier to retain heat in the cold
Cold Stress
The body attempts to increase and conserve body heat by rerouting circulation and shivering
Vasoconstriction causes the blood to pool internally to conserve organ heat
Shivering causes the temperature to increase due to muscular activity
Individuals respond to cold stress by increasing muscular activity, wearing more clothes, or heating their living space
Responses to Cold Stress
Summary
 | Aptitudes--adaptation at the population level
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| Acclimatization--individual responses
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Selected References
Frisancho AR (1993) Human adaptation and accomodation. Ann Arbor, University of Michigan Press.
Roberts DF (1978) Climate and human variability, 2nd ed. Menlo Park, California, Cummings Publishing Co.
Ruff CB (1994) Morphological adaptation to climate in modern and fossil hominids. Yearbook of Physical Anthropology 37:65-107.