Physiology of Thermoregulation

AUTHOR: BETH NORMAN AND DR LUCY LONGBOTTOM

Homeostasis of thermoregulation

Broadly speaking, as with all homeostatic mechanisms, the process of thermoregulation relies upon the following components, with neural pathways connecting each of the separate elements:

• Controlled variable (temperature in this case)
• Receptors (peripheral and central)
• Processor and pre-determined set point (hypothalamus)
• Effector mechanisms (behavioural and physiological responses)

Applying the above to thermoregulation, a change in external environmental temperature is detected by peripheral and central thermoreceptors. Peripheral thermoreceptors are free nerve endings of both A and C type fibres which are found in the dermis and detect a change in external body temperature, whereas central thermoreceptors are located in the preoptic nucleus of the anterior hypothalamus and detect change in internal body temperature. From here, both peripheral and central thermoreceptors transmit action potentials directly to the posterior hypothalamus. The posterior hypothalamus could be considered the processor or integrating centre for all temperature related stimuli; it also functions as the body’s ‘thermostat’, determining the set point for temperature (2). Deviations from this set point result in the initiation of effector mechanisms to either promote heat loss or heat gain, depending on the initial stimulus. Effector mechanisms comprise both behavioural changes and physiological responses, depending on the external environment. If thermoregulation is successful these behavioural and physiological responses result in effective heat gain or heat loss, which, via a negative feedback loop, either inhibits or stimulates thermoreceptors and therefore input to the hypothalamus.

Before exploring specific mechanisms of heat loss or gain, it is important to appreciate the four main ways in which heat may be exchanged from the surface of the skin (3)

• Radiation
  • Transfer of heat via infrared waves without direct contact, occurring where a temperature difference exists. For example, this is how the sun heats the earth.

• Evaporation
  • Transfer of heat by the evaporation of water. For example, perspiration.

• Conduction
  • Transfer of heat by two objects which are in direct contact with each other. For example, sitting on a cold rock on a warm day.

• Convection
  • Transfer of heat to the air surrounding the skin. For example, movement of air due to wind prevents the build up of heat near the skin surface by displacing warm air with cooler air. Wind chill increases the speed of heat loss through convection and can thus lead to greatly reduced relative temperatures.

Mechanisms for heat loss

Although the body can tolerate larger deviations in temperature decrease, relatively mild increases in internal body temperature result in massive change in enzymatic function, multiorgan failure and ultimately death (4). It is therefore of critical importance that thermoregulation maintains the delicate balance between environmental temperature and core body temperature through heat loss mechanisms. 

Mechanisms of heat loss can be divided into physiological and behavioural adaptations. Depending on the external temperature, some or all of these heat loss mechanisms may be employed.

Physiological changes

• Vasodilation of arterioles
  • Inhibition of sympathetic centres in the posterior hypothalamus lead to vasodilation of blood vessels (2)
  • This results in an increase in blood flow to capillaries, bringing warmer blood closer to the skin
  • Heat may then be lost via radiation to the environment, ensuring a cooler temperature of venous return
• Sweating
  • When the external environment becomes warmer than the skin, the body gains heat through both radiation and conduction. This means that evaporation is the main way in which heat loss can occur
  • Stimulation of the anterior hypothalamus by heat results in the activation of sweat glands. These sweat glands can also be stimulated by adrenaline and noradrenaline during exercise
  • There are two types of sweat glands; eccrine and apocrine. Eccrine glands are located throughout most of the body, with the majority on the soles of the feet and function by releasing water to the skin surface with sweat ducts (5). Apocrine sweat glands produce malodorous sweat and are located predominantly in the axillary and perineal regions. For further information on this process, please see this excellent article by Yousef, Ahanger and Varacallo (2)
  • Evaporative heat loss results in cooling of the skin and underlying blood (5)
  • The energy required for evaporation is called ‘the latent heat of vapourisation’; this describes the process of turning water into gas and requires a significant amount of heat energy. In this way, sweating is one of the most efficient forms of heat dissipation in humans, in environments with low humidity (6)
  • In humid environments, sweating becomes much less effective due to the relativity of water content in the air
  • Although evaporation heat loss can be an incredibly useful mechanism for heat loss, it is important to always be aware of the accompanying risk of dehydration

Behavioural changes

• Moving into the shade
• Removing layers of clothing
• Stopping exertion and resting
• Ensuring adequate hydration

Mechanisms for heat gain

After the hypothalamus has received impulses from cold peripheral thermoreceptors and central thermoreceptors, it sets into motion a number of heat gain mechanisms. Similarly to heat loss, these can be divided into physiological and behavioural responses.

Physiological responses

• Vasoconstriction
  • Stimulation of the posterior hypothalamus leads to vasoconstriction of blood vessels
  • This minimises radiation heat loss by reducing blood flow to peripheral tissues including the skin
• Skeletal muscle contraction (also known as shivering)
• • Increases the metabolic rate, leading to heat production by the combustion of metabolic substrates (7)
• Piloerection (also known as goosebumps)
• • Arrector pili muscles at the base of hairs in the skin contract causing hairs to straighten and trap a layer of air around the body, reducing heat loss by convection
• Non-shivering thermogenesis (3)
• • An increase in metabolic heat production which is not associated with muscle activity
  • This is predominantly associated with an increase of metabolism of brown fat which is found in neonates. For further information on brown fat metabolism, please see Luginbuehl & Bissonnette (2009) (8) here

Behavioural changes

• Moving around more 
• Adding layers of clothing
• Curling up into a small shape to minimise exposed surface area
• Standing near to a heat source

Application to expedition medicine and global health

A clear understanding of the process of thermoregulation and conditions that may arise when this homeostatic mechanism fails are crucial in the arsenal of an expedition medic. Heat illness occurs throughout all environments and may occur when least expected, for example hypothermia in the desert at night. Consideration must also be given to other causes of impaired thermoregulation which may not be due to environmental factors, for example medications (eg. neuroleptic malignant syndrome), traumatic brain injuries, spinal cord injuries, and alcohol to name a few. It is also important for the medic to remember to also look after themself in extreme temperatures – please see our article on hypothermia which discusses this concept further here.

Are you interested in learning more about thermoregulation?

If so, why not check out our Altitude Medicine in Practice: Alpine Ski Touring Expedition course? Whilst you’re there, why don’t you take a look at our other courses too?

Useful resources

• Wilderness Medical Society Clinical Practice Guidelines for Prevention and Treatment of Heat Illness (2019). Access here: https://www.wemjournal.org/article/S1080-6032(18)30199-6/fulltext

• Resus Room podcast on ‘Heat Illness’. Listen here: https://www.theresusroom.co.uk/heat-illness/