CHAPTER 22 Maintaining body temperature
Introduction
The aim of this chapter is to provide an understanding of the factors and processes involved in thermoregulation, i.e. the maintenance of body temperature at a near-constant level. An understanding of the physical, physiological and behavioural mechanisms will allow for a more rational approach to treatment for patients whose thermoregulatory system is disturbed. There are a number of reasons why body temperature might rise or fall from the ‘normal’ range, but before any decisions are made about treatment, the first step is to ensure that measurement is accurate.
The remarkably narrow range of normal temperature, together with the fact that infections cause body temperature to rise, allows us to use temperature as a measure of disease. However, the practice of temperature measurement and monitoring is often based on ritual rather than on rational decision-making. The responsibility of the nurse is to:
It is for the above reasons that patients should have their temperature measured and assessed by a registered nurse or nursing student under supervision.
Normal body temperature
‘Body temperature’ can be a misleading term because the body is not at a uniform temperature. For example, skin temperature varies considerably. The skin has been described as a mosaic of temperatures and even ‘core’ (deep body) temperature can vary slightly. For this reason it is good practice to use the same body site for each temperature measurement and to report the temperature of the site used, e.g. ‘Axilla temperature of Mr A on admission to hospital was 36.9°C’ or ‘Oral temperature of Mrs B was 37.1°C’.
The temperature of the tissues of the body
Surface temperature
Under most circumstances, the skin surface or ‘body shell’ is the coolest part of the body. The organs, blood and deeper tissues are described as the body ‘core’ (Tortora & Derrickson 2006). Because the temperature of the skin is influenced by the temperature and humidity of the air (Figure 22.1), measurement of skin temperature is not a reliable method to detect temperature changes of the organs and deep body tissue (Box 22.1). This may be most important when caring for patients with critical illness where accurate temperature measurement is likely to be of importance in the diagnosis of infection.

Figure 22.1 Core temperature and temperature of the skin surface at various sites in a hot, a thermoneutral and a cold environment.
(Based on an original figure by Aschoff & Wever, cited in Stainer et al 1984, with additional data from Childs C.)
Differences in temperature between the surface and internal organs
The brain controls the temperature of the body so that it remains close to 37°C. The temperature of the skin, however, can vary.
Activity
Look at the manikins (A, B, C) in Figure 22.1. In each example body temperature (oral) is 36.8°C.
Deep body (core) temperature
The organs of the head and thorax (the main components of core tissues), e.g. heart, liver and brain, are ‘insulated’ from environmental conditions by bone, fat and skin. The liver, kidneys, brain and myocardium have a high metabolic rate and consequently a higher temperature than tissues with a lower rate of metabolic activity such as smooth muscle and skin (Houdas & Ring 1982).
In a recent publication by Moran et al (2007) the authors undertook a prospective observational cohort study to investigate whether tympanic measurements were a reliable measurement technique in the critically ill patient. Different conventional methods for body temperature measurement were compared. Access the paper by Moran and colleagues on the website to find out whether the authors considered tympanic membrane temperature to be a valuable measurement method in critically ill patients.
See website Research abstract 22.1 for information about a study by LeFrant et al (2003) that demonstrated the variation in core temperatures in the different organs, muscles, etc.
Regulation of body temperature
Mammalian thermoregulation is controlled by the brain, specifically the pre-optic region within the anterior portion of the hypothalamus (POAH) (see website Figure 22.1 for anatomy of the hypothalamus). Incoming (afferent) signals to the hypothalamus from thermoreceptors in skin and organs are transmitted to and integrated within the POAH, which then responds by transmitting outgoing (efferent) signals to the body so that heat can be either generated/conserved or lost.
For example, if the temperature of the blood bathing the cells of the hypothalamus starts to rise above ‘normal’ (37°C), as in a fever, the outgoing, or efferent, signals stimulate ‘effector’ mechanisms so that heat is lost from the body. Conversely, if the incoming blood is at a lower temperature than the reference (set-point) temperature (i.e. <37°C), mechanisms to conserve or produce heat will be activated. In this way, deep body temperature is prevented from rising or falling from the biological set-point of 37°C. The pathways that help to maintain body temperature at a constant level are shown in Figure 22.2.

Figure 22.2 Control of body temperature. A. Responses to a cool environment or lowered core temperature. B. Responses to a warm environment or raised core temperature. BAT, brown adipose tissue; C, convection; K, conduction; NST, non-shivering thermogenesis; R, radiation.
Mechanisms of heat conservation and heat production
Heat conservation
If skin or blood temperature falls, signals from peripheral thermoreceptors in skin are interpreted at the POAH and the heat-promoting centre stimulates mechanisms to retain heat as well as to increase the amount of heat produced within the body (endogenous heat production).
Behavioural thermoregulation
The ability of humans to conserve heat by putting on more clothes or seeking shelter from the cold is often overlooked or forgotten, but such behaviours are the most fundamental of the protective thermoregulatory activities and are the ‘first line’ of defence against extreme heat or cold. Very young and very old people, as well as those who are immobile due to illness or who are sedated, are unable to protect themselves from heat or cold and so are more vulnerable to environmental temperature changes. It then becomes the responsibility of the nurse to place the patient in a comfortable environment to ensure that they are not at risk of hyper- or hypothermia.
Peripheral vasoconstriction
As the person begins to feel cold, changes in the flow of blood to the skin also occur. Nerve impulses from the POAH, concerned with heat conservation, cause blood vessels, particularly in the hands, feet, ears and nose, to constrict. Sympathetic stimulation to nerves supplying the blood vessels in these regions results in peripheral vasoconstriction which leads to a reduction in the flow of warm blood from internal organs to the skin. This has the effect of retaining body heat, keeping the core tissues at or close to 37°C.
If body temperature continues to fall despite the above measures, body heat will be produced by a rise in the rate of metabolic heat production (see non-shivering and shivering thermogenesis below).
Heat production
In a healthy young adult, metabolic rate is between 35 and 39 kcal/m2 body surface/h whereas in an infant, metabolic rate is 53 kcal/m2 body surface/h. The fact that an infant has a higher metabolic rate than an adult can be explained by the rapid synthesis of cells in the growing body. However, in both adults and children, metabolic rate may rise well above the normal limit for a given age if the person is sick or injured. The most well recognised explanation for a rise in metabolic heat production is the onset of a fever (Childs & Little 1994, Jenney et al 1995) after an injury such as a burn.
In early studies, patients suffering from serious burns (see Ch. 30) were found to have a very high metabolic rate and also a very high body temperature (Childs 1994). Wilmore (1977) reported that metabolic heat production in severe burns was almost double that of a healthy person of the same age. Later, it was found that improvements in surgical management, wound care and analgesia ameliorated the enormous increases in metabolic rate and demand for energy (Childs 1994) although patients remained febrile. Part of the high metabolic ‘cost’ of burn injury could be reduced by early surgery (to promote healing of the burn wound) and pain management but fever can persist and is a cause of raised metabolic rate, especially if the patient develops a burn wound infection.
Any activity or event that increases the rate of chemical reactions in cells and the rate of oxygen uptake by the cell increases metabolic rate and thus the amount of metabolically produced heat (Frayn 1997). Body heat is produced in two ways: chemically by non-shivering thermogenesis, and physically by the rhythmical contractions of skeletal muscle, i.e. shivering thermogenesis.
Non-shivering thermogenesis
Although peripheral vasoconstriction is very effective in conserving body heat, additional sources of heat may need to be generated to restore body temperature to ‘normal’. This can be achieved by non-shivering thermogenesis (NST). As its name implies, NST does not involve muscular contraction to stimulate endogenous heat production, but relies upon release of the thermogenic hormone, noradrenaline, under the control of the sympathetic nervous system. Although muscle tissue is the most important source of chemically produced heat, other important heat-producing organs are the brain and liver.
Shivering thermogenesis
People who are cold shiver. The heat conservation area of the brain stimulates mechanisms that increase muscle tone, i.e. shivering, such that heat production increases to five times the basal rate. Shivering occurs in most skeletal muscles but the greatest intensity of shivering thermogenesis is in the jaw and neck and is least in the legs. The repetitive contraction of muscle is not, however, a very economical process, as only 40% of the heat generated during shivering is retained by the body and shivering does not continue indefinitely (see Hypothermia, p. 627).
Mechanisms of heat loss
High air temperature and strenuous exercise will raise the temperature of the blood. If the heat gained during the exercise is not matched by an equivalent rate of heat loss, heat-losing mechanisms are initiated and heat-conserving mechanisms are inhibited (Figure 22.2). The series of events that protects the brain and core tissues from reaching dangerously high temperatures is as follows:
Table 22.1 Principles of heat transfer
Route for Heat Loss | Principle | Relevance to Clinical Practice |
---|---|---|
Radiation (R) | Transfer of energy in the form of electromagnetic waves. The human body emits heat as infrared radiation. At the same time all dense objects (furniture, buildings, other people) are also radiating heat. The rate at which heat is emitted from the human body is dependent upon the temperature difference (gradient) between the skin and other objects and surfaces in the room. If the skin is hotter than the average temperature of objects in the room, heat will be lost. If the objects in the room are hotter, the body will gain heat | A person, naked, sitting quietly in a room at 25°C loses between 50 and 70% of heat by R, the major route for heat loss under such conditions. As air temperature increases, the temperature gradient between skin and air falls such that less heat is lost by this route. As air temperature rises towards skin temperature (35°C), the gradient will be so small that very little heat loss can take place by this route. Evaporative heat loss then becomes an important route for heat loss |
Convection (C) | Air (or water) next to the body warms and moves slowly away because warm air is less dense and rises. As it moves away from the body, cooler air replaces it. This process can be speeded up if a strong draught (e.g. electric fan) is used to force the air away and cause a rapid replacement of warmed air with cool air | Nurses frequently increase the rate of heat loss by convection by placing electric fans close to their patients. This can be a very efficient way to lower skin temperature, but frequently the cool stimulus results in an inappropriate response, i.e. peripheral vasoconstriction. Heat retention within core tissues can cause core temperature to rise rather than fall. In febrile patients who have an elevated set-point, the use of electric fans is likely to cool the skin and return heat to the tissue |
Conduction (K) | Heat loss by K involves transfer of thermal energy from atom to atom. The skin must be in contact with cooler or hotter objects for heat exchange to take place by K | Critically ill patients are often nursed on special beds designed to reduce the incidence of pressure ulcers. These beds are often maintained at a constant temperature to help prevent heat loss from the body. Sometimes the thermostat can fail and patients have been known to overheat or to cool because the temperature of the bed is too high or too low. Patients must be protected from body temperature disturbances of such an iatrogenic nature |
Evaporation (E) | Evaporation of water from the skin and respiratory passages is the most important route for heat loss in hot conditions. The evaporation of water occurs when energy transforms water and sweat droplets to a gas. The heat (or thermal energy) needed to drive this process is taken from the body. Thus the more water there is on the skin, the more heat is taken from the body to turn it into a gas (vaporisation). The more heat removed from the body in the process of E, the more the body cools. The function of sweat (produced under the control of the sympathetic nervous system) as an agent for vaporisational heat loss can be enhanced by spraying the body with a fine mist of warm water | Patients with a high core temperature may not always sweat. During a rise in rectal temperature the body responds as though it were too cold and if patients are observed carefully it will be seen that their skin is dry. At this stage the hypothalamic set-point is above normal but the body continues to activate heat-conserving mechanisms to achieve the new set-point temperature. Only when the patient has reached the new central temperature set-point will heat loss by all routes (E included) be activated. Thus when nurses notice that patients with a high core temperature are sweating, it is more likely to indicate that core temperature has reached the new set-point. A fall in body temperature may then follow |
Fluctuations in a healthy person’s temperature
Humans are able to make both physiological and behavioural adjustments to maintain deep body temperature close to 37°C. Like many mammals, humans have the ability to increase the amount of heat in the body as air temperature falls, or to increase heat loss when conditions become uncomfortably hot. It is for this reason that humans are able to live successfully in most climates on Earth.
Normal temperature range
Although central or hypothalamic temperature is ‘set’ at a relatively constant level, fluctuations do occur in healthy people, e.g. following exercise. No harm is done to the cells of the body by a change in temperature, providing core temperature does not rise above or fall below critical limits. Indeed, the ability of the thermoregulatory system to stimulate changes in heat production or heat loss indicates that the system is operating efficiently.
Website Figure 22.2 illustrates the range of ‘normal’ temperature in health.
Circadian rhythm
Body temperature fluctuates in a characteristic pattern over a 24-h period. It is thought that this pattern is a result of regular, but normal, deviations of the body’s own thermostat or hypothalamic set-point temperature. As with many other physiological functions, thermoregulation displays a circadian rhythm (diurnal variation). This rhythm persists during short periods of night work. Eventually, however, regular night work will reverse the normal pattern so that lower temperatures occur during the day and higher temperatures at night.
In general, the lowest temperatures recorded over a 24-h cycle will be approximately 0.5°C lower than the afternoon temperature. By the evening, body temperature may be as much as 1.0°C above the early morning temperature. In the newborn, a circadian rhythm of core temperature is not fully developed (Waterhouse et al 2000).
Other factors affecting a healthy person’s temperature
Babies and young children
As long ago as 1937, Bayley & Stolz showed that rectal temperature begins to rise during the first 7 months of life, remaining fairly constant until the age of 2 years, after which it begins to fall. Average rectal temperature at age 1 month was reported to be 37.1–37.2°C, at 8 months 37.6–37.7°C, and at 18 months 37.7°C. By the time the children in the Bayley & Stolz study approached their third birthday, rectal temperature had settled to 37.1°C.
To counteract increased heat loss, babies have an important source of body heat: brown adipose tissue (BAT), a specialised type of fat cell (Cannon & Nedergaard 2004). BAT is a unique type of adipose tissue, shown by Hull (1976) to be an important source for heat production in small hibernating mammals, and has been described as the ‘hibernating gland’. BAT is also stimulated in the human newborn infant after birth. Under the control of the sympathetic nervous system, the function of BAT is to transfer energy from food into heat (Cannon & Nedergaard 2004) and probably plays a role in the evolutionary success of mammals. BAT can be found around the kidneys, between the shoulder blades, around the great vessels and deep within the axillae (Frayn 1997). BAT is richly supplied with blood and oxygen and this dense blood supply is responsible for its brown colour. It becomes much less important in maintaining the temperature of the body as a baby gets older. In adults, BAT is scarce and probably not functional (Klaus 2004).
Adults and older people
Unlike small infants, who rely principally on ‘switching on’ heat production to maintain a stable deep body temperature, adults are more efficient at conserving body heat. In other words, they rely on preventing body heat from being lost, either by putting on more clothes (behavioural thermoregulation) or by vasoconstriction at the extremities (physiological thermoregulation).
Older people, however, often have a lower body temperature than children or younger adults, for which there are a number of reasons:
Avoiding unnecessary heat loss
After a bath, patients often feel cold and uncomfortable if the nurse is slow in helping them to dry themselves. What is the cause of this feeling of discomfort when the body is wet and the room temperature is low? Why would leaving the bathroom door open make the patient feel cold? How can the nurse improve the patient’s comfort when preparing them for a bath?

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