A-level Applied Science/Energy Transfer Systems/Body Temperature

The body generates heat as a result of chemical reactions taking place in it. Energy is constantly lost from the surface of the body through the skin as heat. A small amount of heat is also lost with expired air, urine and faeces.

The rate at which heat (energy) is lost from the body depends on conditions such as the temperature of the surroundings, how much skin is exposed and the level of activity of the body.

Normally, body temperature is regulated by homeostatic mechanisms and maintained at a more or less constant value. Variation from this value may indicate a failure of these mechanisms and a probable health problem. However, the body can produce a physiological response to conditions such as fever and will raise the body temperature to aid recovery.

The range of body temperatures measured in the mouth: normal 36.8 °C ; range 36.5.37.2 °C death below 25 °C hypothermia 32 °C fever above 37.2 °C heat exhaustion or heat stroke likely if above 38 °C in absence of infection high temperatures that would lead to death above 43 °C

19• how temperature is controlled and regulated by the nervous system, the circulatory system and the skin. You should be able to describe the temperature control mechanisms of sweating, vasodilation, vasoconstriction and shivering; 20• the circumstances in which particular individuals may be at risk of hypothermia, heat exhaustion or heat stroke.

Thermoregulation

Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when temperature surrounding is very different. This process is one aspect of homeostasis: a dynamic state of stability between an animal's internal environment and its external environment.

If the body is unable to maintain a normal temperature and it increases significantly above normal, a condition known as hyperthermia occurs. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia.

Heat production and heat loss vary considerably in different parts of the body, although the circulation of the blood tends to bring about a mean temperature of the internal parts. Hence it is important to determine the temperature of those parts which most nearly approaches to that of the internal organs. Also for such results to be comparable they must be made in the same situation. The rectum gives most accurately the temperature of internal parts. Occasionally that of the urine as it leaves the urethra may be of use. More usually the temperature is taken in the mouth, axilla or groin.

Temperature regulation

conduction - heat escapes from or enters into your body e.g.when lying on a cold or hot surface.
convection - cooler air currents remove heat from the surface of your skin, warmer air currents make the skin hotter.
evaporation - evaporative cooling occurs when water (from perspiration or swimming) leaves the skin surface as a vapour, lowering the body temperature by taking the heat from the body.
radiation - e.g. acquisition of heat from solar radiation or losing heat from the skin.

Types of thermoregulation

There are two types of thermoregulation that are used by animals:

physiological regulation: Endothermy - This is when an organism changes its physiology to regulate body temperature. For example, many mammals tend to sweat in order to lower temperature. Another example is when humans get cold, muscles may shiver in order to produce heat.
behavioral regulation: Ectothermy - This is when an organism changes its behaviour to change its body temperature. For example, when animals warm up in direct sunlight, they may wish to find shade to cool down.

Thermoregulation in vertebrates

Human thermoregulation (simplified)

Almost all birds and mammals have a high temperature almost constant and independent of that of the surrounding air. This is called homeothermy.

To regulate body temperature, an organism may need to prevent heat gains in arid environments. Evaporation of water, either across respiratory surfaces or across the skin in those animals possessing sweat glands (mammals) helps in cooling body temperature to within the organism's tolerance range.

Skin often has a continuous layer of insulating fat beneath the dermis — in marine mammals like whales this is referred to as blubber. Organisms when presented with the problem of regulating body temperature not only have behavioural, physiological and structural adaptations, but also a feedback system to trigger these adaptations to regulate temperature accordingly. The main features of this system are; Stimulus, Receptor, Modulator, Effector and then the feedback of the now adjusted temperature to the Stimulus. This cyclical process aids in homeostasis.

In cold environments, mammals employ the following adaptations and strategies to minimize heat loss:

using small smooth muscles (arrector pili) which are attached to hair shafts; this shivering thermogenesis distorts the surface of the skin as the hair shaft is made more erect (called goose bumps or pimples)

In warm environments, birds and mammals employ the following adaptations and strategies to maximize heat loss:

behavioral adaptations
evaporative cooling by perspiration

Variations in the temperature of human beings and some animals

Chart showing diurnal variation in body temperature, ranging from about 37.5 °C from 10 a.m. to 6 p.m., and falling to about 36.3 °C from 2 a.m. to 6 a.m.

Variations from thermometer placement

As stated above, the temperature of warm-blooded animals is maintained with but slight variation. In health under normal conditions the temperature of humans varies between 36.5 °C and 37.5 °C, or if the thermometer be placed in the axilla, between 36.25 °C and 37.5 °C. In the mouth the reading would be from 0.25 °C to 1.5 °C higher than this; and in the rectum some 0.9 °C higher still. The temperature of infants and young children has a much greater range than this, and is susceptible of wide divergencies from comparatively slight causes.

Variations associated with development

The average temperature falls slightly from infancy to puberty and again from puberty to middle age, but after that stage is passed the temperature begins to rise again, and by about the eightieth year is as high as in infancy.

Variations due to circadian rhythms

In humans, a diurnal variation has been observed dependent on the periods of rest and activity, the maximum ranging from 10 a.m. to 6 p.m., the minimum from 11 p.m. to 3 a.m. These observations indicate that body temperature is partially regulated by circadian rhythms.

Variations due to women's menstrual cycles

During the follicular phase (which lasts from the first day of menstruation until the day of ovulation), the average basal body temperature in women ranges from 36.45 - 36.7 °C. Within 24 hours of ovulation, women experience an elevation of 0.15 - 0.45 °C due to the increased metabolic rate caused by sharply elevated levels of progesterone. The basal body temperature ranges between 36.7 - 37.3 °C throughout the luteal phase, and drops down to pre-ovulatory levels within a few days of menstruation. Women can chart this phenomenon to determine whether and when they are ovulating, or to aid conception or contraception.

Variations due to fever

Fever is a regulated elevation of the set point of core temperature in the hypothalamus, caused by circulating pyrogens produced by the immune system. To the subject, a rise in core temperature due to fever may result in feeling cold in an environment that people without fever do not.

Limits compatible with life

There are limits both of heat and cold that a warm-blooded animal can bear.

The effect of too extreme a cold is to lessen metabolism, and hence to lessen the production of heat. Both catabolic and anabolic changes share in the depression, and though less energy is used up, still less energy is generated. This diminished metabolism tells first on the central nervous system, especially the brain and those parts concerned in consciousness. Both heart rate and respiration rate become diminished, drowsiness supervenes, becoming steadily deeper until it passes into the sleep of death. Occasionally, however, convulsions may set in towards the end, and a death somewhat similar to that of asphyxia takes place.

On the other hand, too high a temperature hurries on the metabolism of the various tissues at such a rate that their capital is soon exhausted. Blood that is too warm produces dyspnea and soon exhausts the metabolic capital of the respiratory centre. Heart rate is increased, the beats then become arrhythmic and finally cease. The central nervous system is also profoundly affected, consciousness may be lost, and the patient falls into a comatose condition, or delirium and convulsions may set in. All these changes can be watched in any patient suffering from an acute fever. The lower limit of temperature that man can endure depends on many things, but no one can survive a temperature of 45 °C or above for very long. Mammalian muscle becomes rigid with heat rigor at about 50 °C, and obviously should this temperature be reached the sudden rigidity of the whole body would render life impossible.

Human temperature variation effects

Hot

Fevers are not to be confused with heat stroke. In fever the person can feel cold at high body temperatures since the body is fooled into thinking it is cold by the infectant microbe affecting the point that the body thermostat is set at. It is literally set higher than usual.

37 °C (98.6 °F) - Normal body temperature (which varies between about 36.12-37.5 °C (96.8-99.5 °F)
38 °C (100.4 °F) - Sweating, feeling very uncomfortable, slightly hungry.
39 °C (102.2 °F) (Pyrexia) - Severe sweating, flushed and very red. Fast heart rate and breathlessness. There may be exhaustion accompanying this. Children and epileptics may be very likely to get convulsions at this point.
40 °C (104 °F) - Fainting, dehydration, weakness, vomiting, headache and dizziness may occur as well as profuse sweating.
41 °C (105.8 °F) - (Medical emergency) - Fainting, vomiting, severe headache, dizziness, confusion, hallucinations, delirium and drowsiness can occur. There may also be palpitations and breathlessness.
42 °C (107.6 °F) - Subject may turn pale or remain flushed and red. They may become comatose, be in severe delirium, vomiting, and convulsions can occur. Blood pressure may be high or low and heart rate will be very fast.
43 °C (109.4 °F) - Normally death, or there may be serious brain damage, continuous convulsions and shock. Cardio-respiratory collapse will occur.
44 °C (111.2 °F) or more - Almost certainly death will occur; however, patients have been known to survive up to 46.5 °C (115.7 °F).^[1]

Cold

36 °C (97.0 °F) - Normal body temperature (which varies between about 36-37.5 °C (96.8-99.5 °F)
36 °C (96.8 °F) - Mild to moderate shivering (it drops this low during sleep). May be a normal body temperature.
35 °C (95.0 °F) - (Hypothermia) is less than 35 °C (95.0 °F) - Intense shivering, numbness and bluish/greyness of the skin. There is the possibility of heart irritability.
34 °C (93.2 °F) - Severe shivering, loss of movement of fingers, blueness and confusion. Some behavioural changes may take place.
33 °C (91.4 °F) - Moderate to severe confusion, sleepiness, depressed reflexes, progressive loss of shivering, slow heart beat, shallow breathing. Shivering may stop. Subject may be unresponsive to certain stimuli.
32 °C (89.6 °F) - (Medical emergency) Hallucinations, delirium, complete confusion, extreme sleepiness that is progressively becoming comatose. Shivering is absent (subject may even think they are hot). Reflex may be absent or very slight.
31 °C (87.8 °F) - Comatose, very rarely conscious. No or slight reflexes. Very shallow breathing and slow heart rate. Possibility of serious heart rhythm problems.
28 °C (82.4 °F) - Severe heart rhythm disturbances are likely and breathing may stop at any time. Patient may appear to be dead.
24-26 °C (75.2-78.8 °F) or less - Death usually occurs due to irregular heart beat or respiratory arrest; however, some patients have been known to survive with body temperatures as low as 14.2 °C (57.5 °F).^[1]

Hypothermia

Hypothermia refers to any condition in which the temperature of a body drops below the level required for normal metabolism and/or bodily function to take place. In warm-blooded animals, core body temperature is maintained at or near a constant level through biologic homeostasis. When the body is exposed to colder temperatures, however, its internal mechanisms may be unable to replenish the heat that is being lost to the body's surroundings.

Hypothermia is the opposite of hyperthermia. Because the words sound alike, they are easily confused.

Stages of Hypothermia in Humans

Stage 1: Body temperature drops by 1 °C - 2 °C below normal temperature. Mild to strong shivering occurs. Unable to perform complex tasks with the hands; the hands become numb. Blood vessels in the outer extremities contract, lessening heat loss to the outside air. Breathing becomes quick and shallow. Goose bumps form, raising body hair on end in an attempt to create an insulating layer of air around the body (a vestigial response, but useful in other species).
Stage 2: Body temperature drops by 2 °C - 4 °C. Shivering becomes more violent. Muscle miscoordination becomes apparent. Movements are slow and laboured, accompanied by a stumbling pace and mild confusion, although the victim may appear alert. Surface blood vessels contract further as the body focuses its remaining resources on keeping the vital organs warm. Victim becomes pale. Lips, ears, fingers and toes may become blue.
Stage 3: Body temperature drops below approximately 32 °C (normal is 37 °C). Shivering usually stops below 32 °C; difficulty speaking, sluggish thinking, and amnesia start to appear; inability to use hands and stumbling are also usually present. Cellular metabolic processes shut down. Below 30 °C the exposed skin becomes blue and puffy, muscle coordination very poor, walking nearly impossible, and the victim exhibits incoherent/irrational behavior including terminal burrowing behaviour or even a stupor. Pulse and respiration rates decrease significantly but fast heart rates (ventricular tachycardia, atrial fibrillation) can occur. Major organs fail. Clinical death occurs. Because of decreased cellular activity in stage 3 hypothermia, the body will actually take longer to undergo brain death.

Hyperthermia

Hyperthermia (hyperpyrexia), in its advanced state referred to as heat stroke or sunstroke, is an acute condition which occurs when the body produces or absorbs more heat than it can dissipate. It is usually due to excessive exposure to heat. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to effectively deal with the heat, and body temperature climbs uncontrollably. This is a serious medical emergency that requires immediate attention.

Hyperthermia is the opposite of hypothermia. Because the words sound alike, they are easily confused.

Progression

Body temperatures above 40 °C are life-threatening. At 41 °C, brain death begins, and at 45 °C death is nearly certain. Internal temperatures above 50 °C will cause rigidity in the muscles and certain, immediate death.

Heat stroke may come on suddenly, but usually follows a less-threatening condition commonly referred to as heat exhaustion or heat prostration.

Signs and symptoms

Heat prostration, or heat exhaustion, is characterized by mental confusion, muscle cramps, and often nausea or vomiting. At this stage the victim will likely NOT be sweating. With continued exposure to ambient heat, which sometimes is facilitated by the mental confusion, temperature may rise into the 39 to 40 °C range, and lead to full-blown heat stroke.

One of the body's most important methods of temperature regulation is sweating. This process draws heat from inside, allowing it to be carried off by radiation and/or convection. Evaporation of the sweat furthers the cooling process, since this is an endothermic process that draws yet more heat from the body. When the body becomes dehydrated enough to prevent the production of sweat, this method of heat reduction is stopped. When the body is no longer capable of sweating, core temperature begins to rise quickly.

Victims may become confused, hostile, often experience headaches, and may seem drunk. Due to dehydration, blood pressure may drop significantly, leading to possible fainting or dizziness], especially if the victim stands suddenly. As blood pressure drops, heart rate and respiration rate will increase (tachycardia and tachypnea) as the heart attempts to supply enough oxygen to the body. The skin will become red as blood vessels dilate in an attempt to increase heat dissipation. As heat stroke progresses, the decrease in blood pressure will cause blood vessels to contract, resulting in a pale or bluish skin colour. Complaints of feeling hot may be followed by chills and trembling, as is the case in fever. Some victims, especially young children, may suffer convulsions. Acute dehydration such as that accompanying heat stroke can produce nausea and vomiting; temporary blindness may also be observed. Eventually, as body organs begin to fail, unconsciousness and coma will result.

References

↑ ^a ^b Excerpt: Humans, Body Extremes. Guinness World Records, 2004. Last accessed November 27, 2006.

Handbook of Physiology, Kirkes, (Philadelphia, 1907)
Simpson, S. & Galbraith, J.J. (1905) Observations on the normal temperatures of the monkey and its diurnal variation, and on the effects of changes in the daily routine on this variation. Transactions of the Royal Society of Edinburgh 45: 65-104.

External links

From w:Thermoregulation, w:Hypothermia, w:Hyperthermia

[Body_Temp_Records-1] Excerpt: Humans, Body Extremes. Guinness World Records, 2004. Last accessed November 27, 2006.

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