Human Adaptability
ANT 475/575
Dr. Bindon
[Home | Discussion | Paper]
[ Department of Anthropology | College of Arts and Sciences | University of Alabama ]

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 75 F

Heat Exchange at 86 F

Heat Exchange at 95 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)

Low peripheral heat conductance
Low vasodilation
Increased (5-6X) peripheral heat conductance
High vasodilation and mean blood flow
Increased peripheral heat conductance
High vasodilation and mean blood flow
Low sweat rate
Excessive increase in sweat rate
Excessive sodium loss
Lower sweat output
Enhanced sodium resorption
Lower sweat threshold
10% increase in evaporation
200% increase in non-evaporated sweat
Better distribution of sweat on skin
More efficient evaporation
High skin temperature
High core (rectal) temperature
Lowering of skin temperature
Lowering of rectal temperature
Maintenance of low skin temperature
Maintenance of low rectal temperature
High heart rate
Lowering of heart rate
Maintenance of low heart rate

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

Early vasoconstriction
Late vasodilation
Early onset of cold induced vasodilation
Low peripheral temperature
Low peripheral temperature
Shivering (increase BMR 2-3X)
Nonshivering thermogenesis
Inadequate delivery of heat to periphery
Adequate delivery of heat to the periphery

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
Body size, shape, and composition
Acclimatization--individual responses
Increased physiological efficiency
Circulation, sweating, shivering
Behavioral adjustments

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.

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Date this page last edited: August 20, 2002
e-mail me at: jbindon@tenhoor.as.ua.edu