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Introduction

The body’s ability to regulate temperature is critical to sustaining life. Death is the ultimate result if the body temperature strays to far from the norm. Core body temperature, the temperature of structures below the skin and subcutaneous tissue, should be maintained between 36.4 and 36.7 ° C¹.

The maintenance of body temperature or thermoregulation is a dynamic system: if heat loss is greater than heat production then the core temperature drops. Likewise if heat loss is less than heat production then the core temperature rises. A drop or rise in core temperature is equally dangerous, the situation is ideal when heat loss and heat production occur at the same rate².

This paper looks at the basic mechanisms by which the body generates and loses heat, and then discusses the control mechanisms available to the body when remedial action must be taken to control temperature.

Basic Heat Production

The basal metabolism is the minimal amount of energy the body uses in order to maintain vital processes³. Generally this expenditure of energy is expressed in terms of heat production per unit of body surface per day or the Basal Metabolic Rate (BMR). BMR measures the rate at which a quiet, resting, fasting body breaks down nutrients to liberate energy.

The energy used to produce heat in the body is measured in calories. A calorie is the amount of energy required to raise the temperature of water from 14 to 15° C. The average man has a BMR of 1,700 calories (7.1 Kilojoules)³.

An increase in the metabolic rate increases the production of heat. Factors that affect the metabolic rate include:

Exercise – raises the metabolic rate as much as fifteen times above the resting level. In trained athletes this can be as much as twenty times.

Hormones – the Thyroid hormones (thyroxine and triiodothyromine) are main regulators of BMR. Testosterone and Human Growth Hormone (HGH) also raise BMR.

Nervous System – under stress, the sympathetic nervous system causes the release of norepinephrine. The SNS also stimulates the adrenal medulla to release epinephrine and norepinephrine under stress – both hormones increase the BMR.

Body Temperature – the higher the body temperature the higher the MR. Each 1° C increase, raises the rate of biochemical reactions by 10%. Thus the temperature increases even further.

Ingestion of Food – the MR increases by as much as 10 – 20% when ingesting food. The rate is higher for proteins and less for carbohydrates and fats.

Age – the MR of a child, with respect to size, is two times that of an elderly person. The high rates of reactions are related to growth.

Others – gender (lower MR in females except during pregnancy and lactation), climate (lower MR in the tropics), sleep (lower MR), and in cases of malnutrition (lower MR).

Basic Heat Loss

Having examined some of the basic heat-generating mechanisms of the body we now look at some of the heat-loss mechanisms. These are defined using simple terms borrowed from thermodynamics.

Radiation – heat loss via this mechanism occurs through the emission of InfraRed radiation.

Evaporation – is the conversion of a liquid to a vapour. Every gram of water removed from the surface of the skin removes a great deal of heat from the body (0.58 Kcal) per gram of water. Under normal rest conditions 22% of heat loss occurs through evaporation.

Conduction – is the transfer of heat through physical contact it contributes to about 3% of heat loss.

Convection – is the transfer of heat by movement of liquid or gas between areas of different temperature. Under normal conditions, at rest, approximately 15% of body heat is lost to the air by convection and conduction.

It is seen that about 40% of heat loss is due to Evaporation, Conduction and Convection, whilst under rest conditions at 21° C 60% is due to radiation².

The Metabolic Thermostat

Having defined the basic mechanisms for heat gain and loss we can now look at what happens when there is an imbalance between the two.

The hypothalamus is generally recognised as containing the body’s thermostat. It contains a group of neurons in the anterior portion called the preoptic area. Feedback to this area of the brain is provided by temperature receptors throughout the body. As temperature rises neurons in the preoptic area increase their firing rate, as temperature drops the firing rate slows.

Nerve impulses from the preoptic area interact with the heat-losing and heat-promoting centres in the hypothalamus. The respective centres set in motion the physiological responses to either raise or lower body temperature when stimulated.

Heat Promotion

If the core temperature drops a negative feedback system comes into play promoting temperature increase. The thermoreceptors in the skin and hypothalamus send signals to the preoptic heat-promoting centre. In response impulses from the hypothalamus cause the secretion of thyrotropin-releasing hormone (TRH) which activates several effectors. Most responses are related to sympathetic functions, and these include:

Vasoconstriction – The SNS is stimulated to constrict blood vessels at the periphery, and warm blood is moved deeper within the body preventing heat loss.

Sympathetic Stimulation – The heat-promoting centre stimulates the release of epinephrine and norepinephrine from the adrenal medulla. This increases cellular metabolism thus increasing heat production (chemical thermogenesis).

Skeletal Muscles – Muscle tone is increased inducing shivering (involuntary thermogenesis).

Thyroid Hormones – An increased production of thyroid hormone increases the MR.

Clothing can be added to aid the heating process.

Heat Loss

If the body temperature rises then nerve impulses are sent to the preoptic heat-losing centre, and the following, mostly parasympathetic, control mechanisms are induced in the body:

Vasodilation – blood vessels in the skin vaso-dilate, the skin warms and excess heat goes into the environment.

Metabolic rate – the metabolic rate is lowered thus lowering the production of heat by the body.

Perspiration – the high blood temperature stimulates the hypothalamus to activate the stimulation of sweat glands to produce sweat. The skin is cooled through evaporation.

Clothing can be removed to aid the cooling process.

Conclusion

The body’s ability to adapt to heat and cold are crucial to the maintenance of life. The mechanisms described above are capable of controlling the temperature within defined limits. If external factors become too extreme the body cannot compensate and death may result.

References

Murray, M., Pizzorno, J., The Encyclopaedia of Natural Medicine, Little, Brown and Company, UK, 1995.

Tortora, G.J., Grabowski, S.R., Principles of Anatomy and Physiology - 8^th Edition, Harper Collins, NY, 1996.

Martin, E.A. (Ed.), Oxford Concise Colour Medical Dictionary, Oxford University Press, Oxford, 1994.