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Temperature regulation (also called thermoregulation) is an important feature of human homeostasis. This is because the body has to balance two needs. It needs enough heat for the chemical reactions in cells to work properly. It also needs to avoid too much heat, because enzymes (proteins that speed up reactions) then lose their shape and stop working (denature). This is done through a negative feedback loop, as shown in the diagram below. To learn more about feedback loops, see our article Introduction to Endocrinology in our Endocrinology series.
Diagram - Negative feedback shown diagrammatically, with regulation of body temperature used as an example
Creative commons source by OpenStax [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The ideal body temperature is 37°C ± 0.5°C. This is the range at which the body can work most efficiently, and the temperature will vary slightly within this range as the negative feedback loop keeps correcting it.
The sensors of temperature (thermoreceptors, nerve cells that detect temperature) exist in the skin (they sense the temperature at the body surface). They also exist in the hypothalamic preoptic area (a small region of the hypothalamus that acts as the body's thermostat and detects core body temperature). The hypothalamic preoptic area measures the temperature of the blood directly. It also receives impulses from the thermoreceptors in skin and mucous membranes (the moist linings of areas such as the mouth and nose). There are also core temperature sensors in the spinal cord and the internal organs (viscera).
If the body is too hot, the following things occur to attempt to cool it:
- Reduced sympathetic vasoconstrictor tone and activation of cholinergic sympathetic fibres, leading to increased cutaneous vasodilation (widening of the blood vessels in the skin). Here 'tone' means the usual background level of nerve signals, and cholinergic fibres release acetylcholine. The wider vessels bring more warm blood to the skin, so more heat is lost.
- Sweating - sweat glands activated all over the body. As the sweat evaporates, it takes heat away from the skin.
If the body is too cold, the following things occur to attempt to warm it:
- Increased sympathetic noradrenergic activity leading to increased vasoconstriction (narrowing of the blood vessels in the skin, so less warm blood reaches the surface and less heat is lost). Noradrenergic means releasing noradrenaline.
- Shivering - Repeated muscle contraction requires more ATP, so more respiration, which releases heat as a by-product.
Humans can also employ voluntary mechanisms, like putting on a jumper to warm up or seeking shade to cool down.
Classifications of Body Temperature
- <28°C - Severe hypothermia (an abnormally low body temperature that is life-threatening)
- <35°C - Hypothermia overall: mild is 32 to below 35°C and moderate is 28 to below 32°C
- 36.5-37.5°C - Typical normal adult core body temperature range
- 38°C or above - Fever (pyrexia): the hypothalamus raises its set point (the temperature it aims for), usually because of infection
- >40°C - Hyperthermia: the temperature rises beyond what the hypothalamus can control, as in heat stroke (a life-threatening illness in which the body overheats in hot weather or during hard exercise, with confusion or seizures)
- >41°C - Hyperpyrexia (an extremely high temperature)
These temperatures are for adults, and they are only a guide because the bands overlap. A fever and hyperthermia are told apart by the cause, not by the number alone: in a fever the hypothalamus has raised its set point, but in hyperthermia the temperature rises beyond what the hypothalamus can control. It is important to know these definitions (hypo- means too low and hyper- means too high), because they point to different problems that need different treatment. Hypothermia is treated by rewarming, a fever by looking for its cause (often infection), and heat stroke by rapid cooling.
Difficulty Managing Body Temperature
There are many different groups of people that can struggle to regulate body temperature.
Changes occur in the body with age that make it more difficult to return to a normal temperature after exposure to very hot or very cold conditions.
- Sweat glands become less active - meaning that the ability to perspire is reduced, making it harder for the elderly to cool themselves.
- Decreased subcutaneous fat deposition - decreased ability to retain heat.
- Weaker nerve control of the blood vessels in the skin - the vessels narrow (vasoconstrict) less well in the cold, so heat is retained less effectively. They also widen (vasodilate) less well in the heat, so the body cools less effectively.
- Loss of muscle mass - shivering is less effective.
The very young struggle to control their temperature for several reasons, including body systems that have not yet matured:
- Babies have limited regulatory mechanisms and rely heavily on their brown fat (a special fat that burns energy to make heat) to keep warm. However, making heat from brown fat uses a lot of energy, so it is not a long term option for heat production.
- Babies have immature neurological pathways, meaning they cannot yet shiver effectively. This means they struggle to warm themselves up.
- Babies have a very large surface area to volume ratio, meaning that they have a large area of skin for their size available for heat loss. They also have fewer cells able to produce heat.
- Babies also cannot carry out voluntary actions such as putting on a jumper, and cannot tell anyone that they are cold.
Disease processes can also contribute to difficulties with thermoregulation.
- Those with poorly controlled and long-standing diabetes can have damage to their autonomic nerves (the nerves that control things like sweating and the width of blood vessels without conscious effort). They can also have damage to their smallest blood vessels (microvascular complications). This means that their vessels may not vasoconstrict or vasodilate well in response to temperature, and they may sweat less, so temperature regulation can be difficult.
- Very hot or very cold temperatures can cause flare ups of Multiple Sclerosis (MS). This can increase rigidity and worsen symptoms of MS. Patients with MS can also suffer from an impaired ability to thermoregulate due to demyelination (loss of myelin, which slows or blocks nerve signals) affecting central nervous system pathways involved in temperature regulation, such as those that control sweating.
Edited by: Dr. Ben Appleby
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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