Physiology
Body temperature and its regulation

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Abstract

Humans are homeotherms, i.e. they fix their temperature regardless of their environment. This is vital for normal cellular function and for metabolism to be independent of external temperature. The body has a warm ‘core’ and a cooler peripheral ‘shell’ whose role is to regulate heat transfer in and out of the core. Body temperature is controlled by a feedback system with both peripheral and central sensors, and an integrator located in the hypothalamus. Anaesthesia exposes patients to thermoregulatory challenges due to enhanced heat loss from the core to the shell to the environment, and interference with the hypothalamic temperature ‘set-point’. In extreme circumstances, deliberate hypothermia may provide benefits that outweigh the risks.

Section snippets

Why is body temperature important to maintain?

Man is a homeotherm: an organism that maintains a fairly constant core body temperature, largely independent of its environment. In our case, this core temperature is usually between 36.1 °C and 37.8 °C, and it varies with a well-defined circadian rhythm, being 0.5 °C cooler in the early hours of the morning than it is in the late afternoon. In women there is also an element of monthly rhythm to body temperature, with core temperature rising around 1 °C following ovulation.

The reason for

The shell and the core

To understand the regulation of body temperature, one must think of the body as two separate compartments:

  • a core compartment containing the main heat producing organs at rest; the brain, thoracic and abdominal compartments

  • a peripheral shell comprising the skin and subcutaneous fat, most importantly that of the limbs, that is cooler and acts as insulation for the core compartment.

The core compartment contains those organs most susceptible to temperature damage, and the relative importance of

How is a change in temperature sensed?

Body temperature regulation functions as a feedback loop system. The set point is defined by the hypothalamus, and it is here where integration of error signals from the central and peripheral temperature receptors is thought to occur, and most effector responses originate (see Figure 3).

Situated in the walls of the third ventricle, superior to the pituitary gland, the hypothalamus contains the most important central temperature receptors, but there are also contributions from temperature

How is body temperature altered?

To explain how these mechanisms of heat loss or conservation occur, it is necessary to understand the various modes by which heat transfer can occur. These phenomena are listed below, with the percentage heat loss each contributes to a naked human in an ambient temperature of 21–25 °C given in parentheses.

  • Radiation (60%) – this involves the transfer of energy by infrared rays from a hotter body to a cooler one. Although a major component of heat loss for naked man at 21–25 °C, man can also

Heat conservation for naked man

A cold stimulus causes a peripheral vasoconstriction to reduce the temperature of the skin and therefore the conductive, convective and radiant heat loss from it. This is triggered by the hypothalamic stimulation of the sympathetic nervous system.

As well as decreasing the total amount of warm blood flowing through the skin vessels, the remaining blood is redirected. Arteriovenous (AV) anastomoses that exist to allow blood to flow from arteries into more superficial skin veins are constricted,

Heat generation

  • Basal heat production: the energy cost at rest of breathing, heartbeat etc. amounts to between 1000 and 2000 kcal/24 hours dependent on sex (less in women, as they have proportionally more fat than men), declines with age (decrease in lean mass), and is proportional to body size.

  • Muscle activity: is the main mechanism for extra heat production in the cold. This can be involuntary in the form of shivering, which can increase heat production to four times the normal level, or voluntary where

Heat dissipation in a naked human

When a warm stimulus is encountered, the reverse of some of the above processes occurs; vasodilation of vessels in the skin warms the periphery and accommodates convective, conductive and radiant heat loss as long as the ambient temperature is lower than that of the body. The AV anastomoses and superficial veins also open to allow the blood to be as close to the interface between skin and environment as possible, to increase heat loss.

When the magnitude of the warm stimulus has reached a

The thermoneutral zone (TZ) and behaviour

The ambient temperature boundaries, within which we need only employ vasomotor control to maintain a constant core body temperature, is called the thermoneutral zone of ambient temperatures (see Figure 2). Outside of this zone, as explained above, we employ other mechanisms of heat dissipation (sweating) and heat generation. However, we can easily shift our thermoneutral zone by altering our clothing, or our degree of exposure.

Hyperthermia and hypothermia

Now the mechanisms of body temperature regulation have been explored in resting physiology, it is important to explain some common circumstances in which these mechanisms might be overwhelmed or modulated.

Body temperature regulation in anaesthesia and surgery

Deliberate hypothermia in surgery has some benefits for the same reasons as explained above.

  • Cooling using extracorporeal circulation in neurosurgery is useful for operations on usually inaccessible aneurysms, as the circulation can be stopped for around 15 minutes while protecting against ischaemic injury to tissue.

  • ‘Cold cardioplegia’ (i.e. cooling the heart rapidly by flushing with a solution high in potassium) reduces the metabolism of the cardiomyocytes, while also depolarizing the membrane

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