Human Ecology

Humans are the most geographically distributed species on the planet.

Therefore humans are subjected to an extremely wide range of environmental conditions.

However the internal environment of the bodies of all humans are very consistent. The maintenance of the internal environment is known as homeostasis.

Included in the factors homeostatically controlled are: temperature, water content, ion composition, blood glucose levels and oxygen levels.

You need to know the effect on humans of extreme environmental temperature and of life at high altitudes.

Temperature

Humans are endotherms; we can regulate our body's core temperature i.e. the temperature deep within the body where the vital organs are located.

The core temperature has a very narrow range of fluctuation; between 36 and 37.5⁰C.

This diurnal rhythm is for the typical individual. Humans whose lifestyle is the reverse of the normal: active during the night and asleep during the day show the opposite temperatures

Temperature is generated within the body by metabolic processes at a fairly steady rate. Additional heat is generated during exercise and by shivering (which is an involuntary form of muscular activity). Heat can also be gained from he environment and from hot food.

Heat is lost by the body by conduction (in contact with a colder object) and convection (loss of energy into the air), as radiation and by evaporation of sweat.

All of these processes involve temperature loss across the skin and the skin has many features for regulating temperature loss.

Heat loss through the skin by conduction, convection or radiation requires blood to near the surface of the skin. The volume of blood near the surface can be controlled.

Vasoconstriction and Vasodilation

The arterioles and venules leading to and from the surface are linked by shunt vessels (aka shunt veins).

Vasoconstriction

To decrease flow of blood to the surface the arteries immediately after the opening to the shunts are constricted (by contraction of the muscles in the tunica media) and blood is forced along the shunt, keeping it away from the surface.

Vasodilation

To increase the flow of blood to the surface the arterioles are dilated (by relaxation of the muscles in the tunica media) and so blood which would otherwise pass along the shunts is directed to the surface.

Sweating

Heat loss through the skin by evaporation requires the production of sweat by the sweat glands. This fluid is then vaporized by heat energy from the skin and carries that energy away into the environment.

Sweat also contains dissolved salt (sodium chloride).

Reducing conduction

When the body becomes too cold the hair follicles become erect as a result of contraction of the erector muscles (aka arrector pili muscles). The hairs trap a layer of air next to the skin and, because air is a poor conductor of heat, this reduces heat loss from the skin. This mechanism is much less important in humans because we have considerably less fur than other mammals.

The adipose layer (fat layer under the skin) acts as an insulator because fat is a poor conductor.

Temperature receptors and the hypothalamus

Skin temperature is detected by thermoreceptors in the dermis. A signal is passed to the hypothalamus in the hindbrain. The hypothalamus also monitors the temperature of blood passing through it (and so the temperature of the sore organs). The hypothalamus coordinates the responses of effector organs which act to correct temperature change. The hypothalamus can be divided into two: the anterior hypothalamus is the heat loss centre; the posterior hypothalamus is the heat gain centre

When the heat loss centre detects an increase in blood temperature or the heat receptors in the skin are stimulated it:

• increases vasodilation
• increases heat loss by radiation, convection and conduction
• increases sweating
• decreases metabolic activity
• decreases thickness of air layer by flattening hair

When the heat gain centre detects a decrease in blood temperature or the cold receptors in the skin are stimulated it:

• increases vasoconstriction
• decreases heat loss by radiation, convection or conduction
• inhibits sweating
• increases metabolic activity through shivering and release of thyroxine and adrenaline (both hormones increase heart rate)
• increases thickness of air layer by action of hair muscles

The process of temperature regulation is under negative feedback control.

Responses to extremes of temperature

An individual's response to extreme temperatures (both very hot and very cold) vary according to whether the individual is a native of the region (and is therefore adapted to the conditions) or is a visitor.

The response to high temperature

Natives of hot areas show the following adaptations:

structural

• natives of hot areas are typically taller and slimmer with longer limbs as shown in the picture of Masai tribesmen from East Africa (doing some really cool jumping). This maximizes surface area to volume ratio allowing for good heat loss

picture taken from http://anthro.palomar.edu/adapt/adapt_2.htm

• darker skin (because of a higher level of melanin in the skin which absorbs UV-B radiation from the sun and thus protects against melanoma) is also associated with hotter environments but the adaptive value of this is unclear 

physiological

• vasodilation so blood is nearer the surface - note that because the external temperature is higher than the internal most temperature loss is by sweating

behavioural

• lower (relative to natives of temperate areas) physical activity to minimize metabolic heat generated
• lower (relative to natives of temperate areas) calorific intake to minimize metabolic heat generated
• clothing is loose (allows evaporation of sweat but traps layer of air as insulation against external heat)
• covers all of body (protects from radiation)
• light coloured (reflects heat)
• housing designed to keep interior cool

They may adopt the behavioural characteristics of natives given above. In addition:

• foods with higher water contents may be selected to overcome loss of fluid through sweating

The physiological effects of acclimatization include:

• increase in circulating blood volume
• increase in sweat rate
• decrease in time to onset of sweating
• reduction in sodium content of sweat

For a further discussion of the adaptation of natives to hot temperatures see the piece taken from Steve Jones' book In the Blood: God, Genes and Destiny

The response to cold temperature

Natives of cold areas show the following adaptations:

structural

• natives of cold areas are typically short and stocky. This minimizes the surface area to volume ratio and so minimizes heat loss
• typically natives of cold areas have lighter skin - this can be explained by the negative effect of melanin in areas of low sunlight. Vitamin D is synthesized by the skin in response to sunlight since melanin absorbs sunlight it reduces the synthesis of Vitamin D. 

physiological

• food is typically high calorie resulting in a high metabolic rate and so increased heat generation
• vital organs are surrounded by a layer of fat - in natives of cold areas this is typically relatively thick offering greater insulation
• in direct contrast to natives of hot areas vasoconstriction is typical so less blood passes near the surface reducing heat loss
• Shivering can be an effective short-term warming technique. The increased muscle activity in shivering results in some heat production

behavioural

• the use of insulating clothing, houses, and fires.  
• limiting outdoor activities to warmer times of the day.  
• sleeping in groups in order to minimize heat loss.

They may adopt the behavioural characteristics of natives given above. 

The physiological effects of acclimatization include:

• vasoconstriction
• shivering

Heat Stress and Cold Stress

Acute exposure to heat or cold have a number of effects of varying seriousness.

The symptoms of Heat Stress include:

• salt loss, heat cramp and dehydration

The symptoms of Cold Stress include:

• trenchfoot, frostbite and hypothermia

Heat Stress

Heat stress is the result of an increase in sweating. This results in a reduction in body fluids (dehydration) and hence a loss in salts (electrolytes).

Dehydration can result in loss of blood volume. This makes the blood more viscous and harder to pump so heat loss is impaired. In addition less blood reaches the brain which can result in mental impairment and, in extremes, confusion, unconsciousness or death.

The derangement of electrolyte levels results in abnormal contraction of muscles (cramps). Note that cramps can be caused by high and low electrolyte levels.

Cold Stress

Cold stress is caused by low temperatures but can be worsened by:

• wind (the wind chill effect)
• water

Wind chill: 

The wind chill temperature is how cold people feel when outside. Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it FEEL much colder.

Trenchfoot

is a very serious non-freezing cold injury, which develops when skin of the feet is exposed to moisture and cold for prolonged periods (twelve hours or longer, usually many days or weeks). 

The combination of cold and moisture softens skin, causing tissue loss and, often, infection.

Trenchfoot is so-called because of its association with WWI soldiers wearing rubberized or tight-fitting boots in wet trenches. However it is not necessary for the weather to be damp for trenchfoot to occur because sweat accumulates inside boots and keeps the feet wet

Frostbite

is the result of freezing of (usually) the extremities. Tissue fluid freezes forming ice crystals and causing the dehydration of cells. This dehydration results in enzyme denaturation and the inhibition of DNA synthesis. Ultimately cells and tissues become damaged and necrotic.

Hypothermia

Hypothermia is the clinical syndrome that results from reduced core temperature. Hypothermia is always the product of loss of heat to the environment in excess of the rate of heat production by the body.

The result of hypothermia is a reduction in blood pressure and heart rate so pathology is the result of insufficient blood (and oxygen) to vital organs. This can be fatal.

 

Question

A subject in a warm (40^oC) chamber swallows ice and the following physical effects are observed:

• skin temperature - remains constant for 10 minutes then increases over 10 minutes before falling back to normal
• hypothalamus temperature - remains constant for 5 minutes then decreases over 5 minutes before rising back to normal
• rate of sweating - remains constant for 5 minutes then decreases over 10 minutes before rising back to normal

a. Sketch graphs of these findings

b. Explain how the fall in hypothalamus temperature is brought about (2)

c. Describe how the hypothalamus brings about the change in sweating rate (2)

d. Suggest a reason for the rise in skin temperature (2)

Revision Activity

This article includes many responses to abnormal internal temperature levels. Summarise these in a table under the headings (where applicable):

responses to an increase in temperature

responses to a decrease in temperature

desired result of response

undesirable result of response

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