Chapter is written following discussions with my colleague, Leif Vanggaard,
MD, Arctic Institute, Copenhagen.
· To define body core and body shell, heat balance, heat exchange (conduction, conversion,
evaporation and radiation), hyperthermia, hypothermia, mean body temperature,
heat capacity, and thermal steady state.
· To describe fever (pyrogens), benignant and malignant hyperthermia, heat exhaustion, heat
syncope, heat stroke, sun stroke, and hypothermia.
· To describe radiation sickness.
· To calculate one thermal variable, when relevant variables are given.
· To explain the concepts heat exchange, thermogenesis by food and shivering, the human
temperature control system and its function at different environmental
· To use the above concepts in problem solving and case histories.
law of cooling:
The dry heat loss is proportional to the temperature difference between the
human body (shell) and the surroundings.
total energy of a system is conserved in an interaction, not the kinetic energy or the mass (Einstein). If the mass changes during an interaction, there
is a resultant change in kinetic energy, so that the total energy remains
constant. – Heat energy is proportional to molecular movement rates – “heat
energy equals movement.”
The higher the temperature of an object, the more it radiates. The energy
radiated from an object is proportional to the fourth power of its Kelvin
temperature. – The energy radiating from an object and received by the human
body is proportional to the temperature difference between the object and the
skin (see Eq. 21-4). This is because human life
implies relatively small temperature gradients.
core consists of the thermoregulated deeper
parts of the body and the proximal extremity portions of warm-blooded animals
shell refers to those outer parts of the body
(skin and subcutaneous tissue) that change temperature at cold exposure.
· Conductance changes of the shell are used as a measure of
heat loss describes a direct transfer of heat
energy by contact between two bodies of different temperature (eg, skin and
heat loss is defined as the heat loss by contact
between the surface (skin) and a moving medium (air or water).
heat loss is defined as the heat loss by
evaporation from the body surface or lungs.
· Fever occurs when the core temperature of the
body is raised above normal steady state levels. The body reacts as if it
is too cold. Fever implies a disorder resulting in shivering combined with
vasoconstriction, headache, dedolation, and general discomfort (eg, malaria).
· Heat flow is defined as energy exchanged due to a temperature difference.
Heat flow is transmitted along a temperature gradient.
capacity is the amount of heat required to
produce a temperature increase for a given amount of substance.
energy balance in a resting person is a
condition, where the heat production is equal to the heat loss. Thus the body
temperature is constant and the heat storage is zero (thermal steady state). Usually, there is no internal heat energy
flux between body core and shell.
· Hyperthermia is an increase in core temperature above normal.
· Hypothermia refers to a clinical condition with a lowered core temperature (below 35 oC).
body temperature is defined according to Eq.
21-1 (see end of Chapter).
thermogenesis is a rise in metabolism, which is
not related to muscular activity (shivering or exercise).
perspiration (leakage of the skin) is the small
cutaneous evaporation loss, which is unrelated to sweat gland function.
· Insulation refers to resistance to heat transfer.
heat loss is a transfer of heat energy between 2
separate objects at different temperature. Heat energy is transferred via
electromagnetic waves (photons). This heat transfer does not require a medium,
and the temperature of any intervening medium is immaterial.
temperature is the temperature of the outer
parts of the body (measured on the skin surface) and related to cold
· Shivering is a reflex myogenic response to cold with asynchronous or balanced muscle contractions performing no external work.
heat capacity is the relationship between heat
energy exchanged per weight unit of a substance and the corresponding
temperature change. The specific heat
capacity of water is 4.18 and of the human body (blood and tissues) 3.49 kJ kg-1 oC-1,
respectively. The specific heat capacity of atmospheric air is 1.3 kJ (m3)-1 oC-1.
· Temperature is the measurement of heat energy content.
paragraph deals with
1. The temperatures of the body,
2. Body responses to cold, 3. Body
responses to heat, 4. Emotional
sweating, 5. Metabolic
Rate and environmental temperature, 6. Temperature
control, 7. The human
thermo-control system, and 8. Thermoregulatory
1. The temperatures of the
human body consists of a peripheral shell and a central core (Fig.
21-1). The heat content (H or enthalpy) of the human body is reflected by its temperature. By
definition a thermometer only measures the temperature of the thermometer, so
its location is essential. The mean core temperature is 37 oC in
healthy adults at rest, but small children have larger diurnal variations.
The skin is the main heat
exchanger of the body. The skin temperature is determined by the core temperature and by the environment (temperature, humidity, air
velocity). Thus the shell temperature is governed by the needs of the body to
exchange heat energy.
21-1: Heat transfers, body cores and shells temperatures of a naked person
standing in cold and warm air, respectively.
The shell temperature is measured on the skin surface and at the hands
and feet to approach the room temperature of 19oC in a person standing in a cold room for hours (Fig. 21-1, left). The
shell temperature is several degrees lower than the temperature in the central
core. The limbs have both a longitudinal and a radial temperature gradient.
The shell temperature and the size of the shell vary with the environmental
temperature and the termal state of the person. A naked person, standing on a
cold floor in 19oC air has a small core and a thick shell compared
to the same person in a warm environment (Fig. 21-1). The shell temperature of
the skin and distal extremities is difficult to evaluate. The best estimate is
measurement of the infrared heat radiation flux with a radiometer.
The core temperature is the rather
constant temperature in the deeper parts of the body and in the proximal extremity portions (see
the red stippled lines of Fig. 21-1). However, the core temperature may vary
several Centigrades between different regions depending on the cellular
activity. The brain has a radial temperature gradient between its deep and
superficial parts. In a sense, the temperature of the mixed venous blood
represents an essential core temperature.
The rectal temperature
A high core temperature
is found to be constant in the rectum about 10-15 cm from the anus. When
measuring the rectal temperature a
standard depth of 5-10 cm is used clinically. The venous plexus around the
rectum communicate with the cutaneous blood in the anal region. The rectal
temperature falls when the feet are cold, because cold blood passes the rectum
in the veins from the legs for the same reason. The rectal temperature rises
during heavy work involving the legs.
Parents should be
advised to measure the rectal temperature in disease suspect children. The
rectal temperature is a reliable estimate of the core temperature in resting
Sublingual (oral) or
axillary temperatures are unreliable measures of the core temperature - often
more than half a degree lower than the rectal temperature.
cranial temperature (tympanic and nasal)
The main control of
temperature is performed by the anterior hypothalamus, which has a high
bloodflow. Within the cranium the hypothalamus lies over the Circle of Willis,
which supplies it with blood, and close to the cavernous sinus which drains
it. Hypothalamus elicits heat loss responses when stimulated by heat. The
tympanic membrane and areas in the nasal cavity (the anterior ethmoidal
region, part of the sphenoid sinus) are supplied with blood from the internal
carotid artery just like the hypothalamus. These cranial locations then serve as a substitute for the measurement of the
inaccessible hypothalamic temperature.
Intake of 250 g of ice
releases an abrupt fall in the nasal temperature in a warm person, whereas the
change in rectal temperature is smaller and delayed (Fig. 21-2). The cranial
core temperature is more dynamic than the rectal.
21-2: Intake of ice reduces the temperature in a warm person resting at 45oC.
In sports and in
surgical hypothermia dynamic measurements of core temperature are essential.
The cranial temperature is often preferred. During forceful movements the
thermistor may be displaced. In such situations an oesophageal location is applied at heart level. This is an approximative measure of the
temperature of the mixed venous blood of the right heart located close to the
The mean body temperature is defined according to Eq.
21-1. The storage of heat energy in the body can be calculated according
to its heat capacity (3.49 kJ* kg-1*oC-1),
the body weight (kg) and the change in mean body temperature in the period (Eq.
According to the first
law of thermodynamics, the storage of heat energy equals the metabolic energy
change minus the heat loss (Eq. 21-3). Quantification
of thermodynamics in humans is possible using equations 21-1 to 21-7 (later in this chapter).
The body is in heat
energy balance, when the storage is zero. However, the core temperature may
change with internal fluxes of heat energy between core and shell without
storage or loss of heat energy at a constant activity.
Venous blood draining
active muscles and the liver is likely to be warmer than pulmonary venous
blood, since this has undergone evaporative cooling in the alveoli. A patient
with high fever can be in thermal steady
state, with a high constant heat production, if both core- and
shell-temperatures are constant, and no internal energy flux occurs.
Warm-blooded animals, homeotherms such as humans, can change their metabolism in order to keep their heat
production equal to the heat loss. Such animals have a temperature
control system and thereby maintain a rather
constant core temperature. Warm-blooded animals live with the advantage of
an unchanged cell activity and temperature in their core. However, the human core
temperature falls during the oestrogen phase of the menstrual cycle and during
sleep (circadian rhythm). The lowest temperature is between 18 at night and 6
o’clock in the morning (Fig. 21-3). The temperature cycle is part of the
circadian periodicity. Our biological clock seems to be synchronised with the
rotation of the globe. Also meals, light and temperature plays a role.
Ovulation releases a
sharp rise in morning temperature. Progesterone effects seem to explain the
higher temperature in the last phase of the menstrual cycle (Fig. 21-3).
21-3: Variations of the core temperature during 24 hour (above), and
variations related to phases of the menstrual cycle (below).
Cold-blooded animals (poikilotherms)
live with a behavioural temperature rhythm, but have no autonomic temperature
control. The core- and shell-temperatures vary with the environment and the
cellular activity. Reptiles, premature and low weight-premature newborn babies
are cold-blooded. These babies have no thermoregulation (see later). However,
their capacity for heat production is 5-10 times as great per unit weight as
that of adults.
Humans have a warm-blooded (homeothermic) core and a cold-blooded (poikilothermic) shell in a cold environment.
Persons exposed to
general anaesthesia, alcohol, and certain drugs lose the autonomic
thermoregulation. Cold-blooded animals must live with varying core and shell
temperature, whereby the rate of their cellular activities varies with the
surrounding temperature (Fig. 21-4).
21-4: The body core temperature and the environmental body temperature for
a warm-blooded animal (cat) and a cold-blooded animal (lizard).
Convection. The convective heat loss is
calculated by Eq. 21-7. A healthy person in sports
clothes experiences thermal comfort at three times the resting metabolic rate
(3 MET), when the surrounding temperature is 20oC, the humidity is
50% and the wind velocity is 0.5 m*s-1.
(water has a high thermal conductivity) illustrates the importance of
conduction andconvection in heat energy transfer.
The dry diving suit excludes water from contact with the skin and traps
low-conductance air in insulating clothing worn inside the watertight sealing.
The wet suit traps water next to the skin but prevents its circulation.
The water is warmed through contact with the skin, and the high insulation of
the foam rubber wet diving suit, with its many pockets of trapped air,
minimises the rate of heat energy loss to the surrounding water. Air is a poor
heat conductor and thus a good insulator. During deep diving high pressures
compress these air pockets and thus reduce the insulation properties of wet
b) Radiation describes
a transfer of energy between objects in the form of electromagnetic waves
(photons). This includes ultraviolet and visible (sun light) radiation from
the outside and from the body infrared or warm heat radiation.
heat transfer can be calculated for a naked
person according to Eq. 21-4.
When the skin
temperature (Tskin) is less than the temperature of the surrounding
objects, heat is gained by radiation.
At wintertime, heat can
be lost through a window glass by radiation from the body to the cold
environment irrespective of the room temperature. This is because the skin
temperature is higher than the outside temperature.
Conduction. Sitting on a cold stone is a typical
example of conduction loss, just as standing on a cold floor (Fig.
Conduction heat can also be gained, although it is really possible to walk on
glowing coals with speed and a thick epidermal horn layer.
Evaporative heat loss- see sweat secretion
Body-responses to cold
vasoconstriction lowers skin temperature, and
thereby reduces the conductive-convective
heat loss that is determined by the temperature gradient from the skin
surface to the environment. Cutaneous vasoconstriction directs the peripheral
venous blood back to the body core through the deep veins and the commitant
veins. These veins are located around the arteries with warm blood, so that
the venous blood receives part of the heat energy from the arterial blood -
so-called counter current heat exchange (Fig.
21-5). The vasoconstriction is so effective, that the bloodflow through the
arterio-venous anastomoses in the fingers and toes can fall to below one
percent of the flow at normal temperature. The cooling of the shell is
immediate, and the size of the shell increases (Fig.
21-1). Obviously, the
shell is large for a naked person in cold air. The resistance vessels of the
hands may open periodically to nourish the tissues, but the high viscosity of
the cold blood can endanger the tissue nutrition and result in trench foot.
The arterio-venous shunts of the hands and feet are closed, so the
bloodflow to the limbs is a nutritive minimum.
The deep arteries and
veins of the limbs lie in parallel, so the arterial bloodflow loses heat to
the incoming venous blood partially surrounding the arteries (Fig. 21-5). This
is a typical counter-current heat
exchange. In a cold environment, where vasoconstriction and heat exchange
produces cold extremities, the total insulation is increased at the expense of
reduced neuromuscular efficiency.
Fig. 21-5: Counter-current exchange in a human arm conserving heat
energy in a cold climate (left). Superficial venous cooling ribs eliminate
heat energy in a warm climate (right).
a warm climate the high
bloodflow of the extremities ensures an optimal temperature of the deeper
structures (eg, the neuromuscular system). The temperature of the arterial
blood is maintained (Fig. 21-5, right) and the arterio-venous anastomoses are
wide open conveying warm blood to the superficial veins. The superficial veins
also act as cooling ribs and
transfer large amounts of heat to the skin surface, where it is eliminated
from the body by convection, conduction and evaporation (Fig. 21-5, right).
Shivering is a reflex myogenic response to cold with asynchronous or balanced muscle contractions elicited from the
hypothalamus via cutaneous receptors. The activity in agonist and antagonist
muscles balance, so there is no external work. Without outside work, all
energy is liberated as metabolic heat energy. Heat production is also
increased by thyroid gland activity and by release of catecholamines from the
work, such as running, is helpful in maintaining
body temperature when feeling cold. Cold increases the motivation for warm-up
exercises and illustrates the voluntary, cortical (feedforward influence)
on temperature homeostasis. The core temperature increases proportionally to
the work intensity during prolonged steady state work (Fig. 21-6). The mean
skin temperature falls with increasing work intensity at 20oC,
because the sweat evaporation cools the skin.
21-6: Muscular and oesophageal temperature during steady state exercise.
The levels of exercise range from zero to 100% of the maximum oxygen uptake.
The temperature in the
active muscles determines the level of the rectal temperature. Following
marathon rectal temperatures of more than 41oC have been measured
and heat strokes have occurred. A marathon is even more difficult to
accomplish in warm, humid environments and strong sun may cause sunstroke (see
People may adapt to
prolonged exposure to cold by increasing their basal metabolic rate up to 50%
higher than normal. This metabolic
adaptation is found in Inuits (Eskimos) and other people continuously
subject to cold.
temperature, where we maintain our autonomic temperature control, is in the
range of zero to 45oC. Below and above this range we adapt to the
environment by behaviour (adding or removing clothing, warm or cold bath, sun
or shadow). A core temperature above 44oC starts protein
denaturation in all cells and is incompatible with life. Below 32oC
humans lose consciousness and below 28oC the frequency of malignant
cardiac arrhythmia’s is increasing, ending with ventricular fibrillation and
death at a core temperature below 23 oC (Fig. 21-7).
21-7: Environmental temperature variations and temperature control. Lack
of vital signs in the clinic (respiration, heart rate, EEG) must not be taken
as death. Treatment must be instituted until death signs are developed.
Body-responses to heat
million sweat glands produce sweat at a rate of up to 2 litres per hour or more during exercise
in extreme warm conditions. If not compensated by drinking, such high sweat
rates lead to circulatory failure and shock. Sweat resembles a dilute
ultrafiltrate of plasma. Healthy humans cannot maintain their body
temperature, if the environmental air reaches body temperature and the air is
saturated with water vapour. Primary sweat is secreted as an isosmotic fluid
into the sweat duct, and subsequent NaCl reabsorption results in the final
hypo-osmotic sweat. Thermal sweating is abolished by atropine, proving that the postganglionic fibres are
cholinergic. Cholinergic drugs provoke sweating just as adrenergic agonists
do. Evaporation of water on the body surface eliminates 2428-2436 J g-1 at mean shell temperatures of 30-32oC. Evaporation of a large
volume of sweat per time unit (V°sweat)
implies a substantial loss of heat according to Eq. 21-5.
Normally, the skin
temperature falls with increasing work intensity, because the sweat
evaporation cools the skin (Fig. 21-6). Danger occurs when the average skin
temperature and the body core temperature converge towards the same value.
Condensation of water on the skin gains heat energy, which is
stored in the body. This is what happens in a Sauna.
Vasodilatation of skin vessels in warm environments results in increased
cardiac output. The arterio-venous anastomoses in the hands and feet are open, and the
bloodflow can rise up to at least 10 folds. The shell is minimal, when a naked
person is in warm air (Fig. 21-1, right). The skin bloodflow, mainly in the extremities, determines the amount of
heat energy, which is carried from the body core to be lost on the surface.
The heat energy is transported from the large body core to the skin by convection
in the blood. A substantial part of the heat energy is lost through the
superficial veins of the extremities acting as cooling
ribs (Fig. 21-5). The blood of the superficial veins is thus arterialized,
when the person is warm.
piece of steak has the same composition as human skin but of course no blood
flow and no sweat evaporation. Thus the steak will be cooked at an air
temperature that humans can survive. A person can stay in a room with dry air
at 128oC for up to 10 min during which time the steak is partially
is a paradoxical response in
contrast to the thermal sweating of
thermoregulation. Emotional stress elicits vasoconstriction in the hands and feet combined with profuse
sweat secretion on the palmar and plantar skin surfaces.
Metabolic Rate and environmental temperature
total heat loss consists of the evaporative heat loss and the dry heat loss
(ie, the sum of convective, conductive and radiative loss). Newton’s
law of cooling states that the dry heat loss is proportional to the
temperature difference between the human body (shell) and the surroundings.
Let us look at a
healthy, lightly dressed sitting person in thermal balance. His heat loss is
plotted as the ordinate and the environmental temperature as the abscissa (Fig.
21-8). The two types of heat loss are added in order to provide the
total heat energy loss. At a room temperature of 37oC there is a
dry heat loss of zero, and below there is an increasing dry heat loss. Above
37oC the heat loss turns into heat input. Obviously, the dry heat
loss also depends upon conduction and convection of heat inside the body by
contact and by the perfusion. At extremely low environmental temperature the
dry heat loss becomes larger than the metabolic heat liberation and the body
is cooled down.
The person is in thermal
steady state, and the metabolic
rate is almost constant in the thermoneutral
zone between 20 and 30o C (Fig. 21-8). The law of metabolic reduction reflects the tendency for heat
production to match the rate of heat loss. The thermoneutral zone, where
minimal compensatory activity is required, is separated in the lower vasomotor and the upper
sudomotor control zone.
In the lower,
comfortable zone (20-26o C) the total
heat dissipation is maintained equal to the metabolic rate by cutaneous,
vasomotor alterations. The small evaporation loss is termed leakage of the
skin or insensible perspiration,
which is unrelated to sweat gland function and rather constant at the basal
21-8: Metabolic rate and environmental temperature in a fasting dressed
human at rest. The wet and the dry heat loss, as well as the metabolic heat
and the basal metabolic rate (BMR)
is measured in Watts.
In the upper
sudomotor zone above 26o C environmental temperature, the
bloodflow through the skin rises, as does sweat secretion and evaporation. At
37o C the rise in energy loss occurs via evaporation (Fig. 21-8).
When the environmental
temperature falls the metabolic rate increases
- first by increasing muscle tone and then by shivering. The chemical or metabolic
temperature control is in the environmental region from 20o C
and below (Fig. 21-8), where shivering, decreased bloodflow through skin and
non-myogenic heat production take place. Here, metabolism controls the core
temperature by increasing metabolic rate with falling temperature in the environment. Above 20o C, the physical
temperature control takes over, as an autonomic capacity for alterations
in heat loss. In this thermoneutral zone the body temperature is kept constant
almost without either heat-producing mechanisms or sweat secretion. - The thermal
comfort for light clothed,
seated persons is about an air temperature of 26oC when the
humidity is 50%, 30oC for nude persons, and about 36oC
sitting in water to the neck.
In the metabolic zone, the total heat loss rises with falling environmental
temperature, but below 5 oC in the environment, the dry heat loss
exceeds the metabolic rate, and the body is cooled down (Fig.
21-8). This is
the zone of hypothermia, where cold
death is inevitable without treatment.
The zone of hyperthermia begins at an environmental temperature of 37 oC, where humans soon reach the maximal capacity for evaporation and
there is an unbalanced heat influx to the body ending in heat death.
The metabolism varies
with the shell and the core temperature. These relations were elucidated by
series of similar experiments. A person was placed in stirred water to the
neck, where the water temperature thus defined the mean shell temperature. By
pre-treatment in other baths an array of core-and shell- temperature
combinations were obtained and measured simultaneously with the metabolic rate
(metabolic heat liberation). The
core temperature was reduced by intake of up to 2 l of crushed ice in water.
At a shell temperature
of 20 oC, intake of ice water reduces the core temperature below
37.1 oC (here called
the set point) and the metabolic rate increases by shivering (Fig. 21-9) with
falling core temperature.
21-9: The metabolic rate as a function of the core and the shell
Warmer skin makes the
set point fall and the rise in metabolic rate per oC (of core
temperature fall) is less steep. Core
temperatures above 37.1 oC all have a low metabolic rate,
regardless of the shell values. At a shell temperature of 30 oC the
set point decreases from 37.1 to 36.7 oC. The hypothalamus
functions as a thermostat using the rest of the body to stabilise its own temperature. With
rising core temperature the metabolic rate is maintained low and the body
tries to cool down. When the body core temperature falls below set point, the
metabolic rate increases by shivering and the heat energy storage as well. The cutaneous cold receptors are maximally active at 20 oC
and they are silent above 33 oC - they can only trigger shivering,
when the core temperature is below set point. Shivering ceases immediately,
when we take a warm shower and the skin is warmed. The hypothalamic, preoptic
heat receptors inhibit shivering, and shivering is totally blocked above the
Another series of
experiments were directed towards heat loss. The heat loss from evaporated
sweat was recorded during exercise at different shell temperatures.
21-10: Evaporative heat loss as a function of the core and the mean skin
temperature during different intensities of exercise.
cutaneous cold receptors with connections to the hypothalamus (Fig.
At mean skin
temperatures of 33-39 oC, where the cutaneous cold receptors are
silent, the person is unable to sweat before the core temperature is above
36.9 oC (the set point
of Fig. 21-10). Below the set point, the evaporation is low (perspiratio
insensibilis). With increasing muscle activity and core temperature the
evaporative heat loss may rise towards 20 kJ each min.
Mean skin temperatures
below 33 oC reduce the evaporative heat loss. The rising
hypothalamic temperature releases the sweat secretion, but the local secretion
is inhibited by the cutaneous cold receptors.
Heat has a direct effect
on preoptic heat receptors in the hypothalamus. At increased core temperature,
the preoptic heat receptors totally block shivering, although the hypothalamic
centre also receives shivering signals from cutaneous cold receptors in cold
surroundings with mean skin temperatures below 33 oC. The falling
mean skin temperature can also act directly to lower cutaneous bloodflow and
have a rather constant core temperature although
the metabolism and environmental temperature may vary considerably. This
implies that control is exerted.
21-11: Thermoregulation by dynamic gain and set point systems.
Information about the
environmental temperature is provided by peripheral thermo-sensors, which are located in the skin, abdominal
organs, and muscles. Internal or blood temperature is monitored by central thermo-sensors in the preoptic hypothalamus and the medulla.
A rise in hypothalamic
temperature causes vasodilatation in the skin and reduces muscular tone.
The person looses motivation for physical activity and reduces clothing. Then
thermal sweat is observed, and after some time, reduced activity of the
adrenal cortex and of the thyroid gland is also observed.
A fall in hypothalamic temperature by cooling of the shell and core
releases cutaneous vasoconstriction together with increased muscular tone and
shivering. There is a sympathetic activation with secretion of catecholamines,
oxidation of fatty acids and glucose, and increased secretion of the thyroid
and adrenal gland. The muscle tone is increased, and shivering is triggered
reflexly as asynchronous muscle contractions without external work
(movements), so all the metabolic energy is released as heat. The capacity for
muscular thermogenesis by shivering is high. Up to five folds basal metabolic
rate is observed, which corresponds to heavy industrial work.
Shivering may be
suppressed voluntarily at the beginning. Transmission signals for shivering
passes the rubrospinal pathways to a-
neurons of antagonist muscles.
gain and set point control
A: A dynamic gain system responds continuously
to feedback signals -
regardless of the core temperature. With rising tissue temperature, the neural
activity of heat sensors increases linearly, whereas the activity of cold
sensors decreases (Fig. 21-11A).
This combined sensory
input to the hypothalamus increases the core temperature and thus increases
the activity of heat loss effectors, while inhibiting heat production
effectors (Fig. 21-11 A). This determines the reference signal.
The dynamic gain
system has a floating reference signal moving with the continuous heat loss and heat production (Fig. 21-11A).
set-point system does not respond to a rise in temperature before a
certain set point is reached. The set point is the core temperature at which
neither heat loss mechanisms or heat production mechanisms are active.
When a thermal
disorder reaches a certain set-point in
the hypothalamus, signals passes to the effectors. The desired core
temperature (tset = set point
temperature) is compared to the actual value (tcore in Fig.
21-11B). The caudal hypothalamus works as a thermostat. Error signals - a
deviation from tset -
evoke responses that tend to restore core and hypothalamic temperature toward
the set point. When the actual core and hypothalamic temperature rises above
the desired set point such as 37 oC, effectors are turned on, and
the compensatory heat energy loss is almost linear (Fig. 21-11B). These
compensatory mechanisms (vasodilatation, sweat, reduced muscle tone) do not
turn off until the temperature drops to the set point (ie, an all-or-non
When the actual core
and hypothalamic temperature is just below the set point, the compensatory
mechanisms (vasoconstriction and shivering) are relatively inactive.
The hypothalamic set
point change with the physiological conditions and is elevated in fever by pyrogens from microorganisms. The rise in metabolism is mainly accomplished by
The human thermo-control system
temperature control exhibits both dynamic
gain and set point characteristics.
The control system implies widespread cutaneous and deep sensors. Their
afferents converge towards the hypothalamic integrator, which acts as a thermostat. The hypothalamus also contains thermosensors in the preoptic region, and
inhibitory neurons perform crossing
inhibition (Fig. 21-12). The central
heat drive from the preoptic
hypothalamus is maintained (Fig. 21-12). The stability of the core
temperature is maintained by the large
heat capacity of the body mass, and by the deep thermosensors, which are
Shivering is released from cutaneus cold sensors firing maximally at 20 oC.
These cold sensors are silent above 33 oC. The cold shell (Fig.
21-12) activates deep cold sensors in the preoptic hypothalamus. This
increases heat production by shivering. The preoptic thermostat simultaneously
reduces heat loss by crossing inhibition.
21-12: The hypothalamic thermostat and its connections.
secretion is released by preoptic warm sensors
as soon as their temperature is 37 oC or above (tset).
Cutaneous cold sensors inhibit sweat secretion at shell temperatures below 33 oC,
since they are silent above this temperature.
In conclusion, preoptic
warm sensors show set point characteristics below the set point, and preoptic
cold sensors show set point characteristics above the set point.
Apart from that, preoptic sensors show dynamic gain: With rising tissue temperature, the neural activity of
heat sensors increases linearly, whereas the signal frequency of cold sensors
increases with falling temperature (Fig. 21-11A).
sympathetic vasodilatation is probably also
released by preoptic warm sensors above set point (Fig. 21-12). A fall in skin
temperature below 33 oC will reduce skin bloodflow by crossing
inhibition (Fig. 21-12).
Alcohol seems to off set
the thermocontrol mainly by inhibition of the hypothalamic thermostat (Fig.
Newborns, down to
premature babies above 1000 g, possess certain thermoregulatory functions. The
newborn can increase thermogenesis by a factor of three without shivering
shows vasomotor reactions, sweat secretion, and reduces the surface area in
cold air. However the baby has special problems: The surface-volume ratio is
3-fold higher than that of an adult. The baby has a thin shell due to the thin
subcutaneous fat layer, so even a maximal vasoconstriction cannot limit heat
loss with a capacity comparable to that of an adult. Newborns are specially
equipped to perform non-shivering thermogenesis (chemical thermogenesis).
Non-shivering thermogenesis is any rise in metabolism, which is not related to
shivering. In babies this form of thermogenesis is particularly large in their
brown adipose tissue. This tissue
is abundant around vital organs, in neck and mediastinum, between scapulae and
in the armpits. Brown adipose tissue cells contains multilocular droplets in
the fat phase and many mitochondria. The tissue receives sympathetic
innervation and is stimulated by catecholamines and thyroid hormones. Cooling
increases the bloodflow and temperature of brown adipose tissue. Noradrenaline
injections cause vasodilatation via b-receptors,
and increase the metabolism to the same extent. The cause of the increased
metabolism is an increased cell membrane permeability for Na+- and
K+-ions, whereby the Na+- K+-pump (high ATP
demand) is activated.
21-13: Thermocontrol in the newborn.
Newborns are in thermal
balance at a minimal metabolism only when the surrounding temperature is high
(32-34 oC). With other words, newborns have an extremely high lower
threshold for the thermoneutral zone, namely 32 oC. The threshold for maximal use of shivering is approximately 23 oC
compared to a naked adult about 5 oC. In general, these threshold
values for newborn increase with falling body weight in premature. Premature
below 1000 g has hardly any thermoneutral zone (Fig. 21-13). They are actually
cold-blooded and their temperature control is maintained with a couveuse.
The sympathetic and the somatomotor
nervous system participate in thermoregulation (Fig.
sympathetic neurons control the bloodflow
through fingers, hands, ears, lips and nose. Arterioles contract and arteriovenous
anastomoses close (thermal insulation) following an increase in
sympathetic tone, and dilatate following a decrease in tone. When arterioles
and arteriovenous anastomoses open, the bloodflow is markedly increased and
thus the convective heat loss from
the skin is increased.
21-14: The thermoregulatory feedback system.
fibres control sweat secretion. The vasodilatator bradykinin is liberated in the skin. Thus, profuse sweat secretion
is always accompanied by vasodilatation.
releases thyroid hormones from the
thyroid gland and catecholamines from the adrenal medulla. These hormones liberate fatty acids and glucose for
combustion. A reduced sympathetic tone also reduces the activity of the
adrenal and the thyroid gland.
The thermogenic response
to cold also involves a non-myogenic or non-shivering component probably in adipocytes. Non-shivering heat
production is controlled by the sympathetic nervous system via adrenergic b-receptors.
The noradrenaline (NA in Fig. 21-14) released at the nerve terminals close to
the adipocytes, stimulates the liberation of free fatty acids and their
subsequent oxidation. Non-myogenic heat
production includes a contribution from the brown fat of babies, but is
insignificant in adults.
Shivering is induced by way of the motor system. The central
shivering pathway passes from the hypothalamus to the motor neurons in the
spinal cord. Shivering is abolished by blockade of the neuromuscular end plate
behaviour such as fanning and adding or removing
clothing is effective in changing the thermal insulation. Several layers of
clothing with trapped air act as a good insulator.
In healthy persons,
heat energy is liberated from cellular metabolism and transferred to the
environment through the skin by sweating and vasodilatation. Sweating occurs
during exercise, and its evaporation is the most important mechanism in
maintaining the core temperature as close to 37o C as possible. The
thermoregulatory centre in the hypothalamus controls all processes.
21-15: Heat and cold adaptation
adaptation is found among Australian aborigines and Inuits
in Greenland. Inuits have relatively more sweat glands in the face and less on
sleep naked on the ground even at low temperature. Inuits have a basal
metabolic rate 50% higher than persons living in a temperate climate do. The
threshold for shivering is shifted towards the left in cold-adapted persons,
but they maintain normal function at the new set point. Very old people may
show the same phenomenon, and live with a core temperature of 35 oC
without shivering. Obviously, cold adaptation implies non-shivering
thermogenesis, which is oeconomic metabolic heat liberation (Fig.
acclimatization is actually a sweat
gland adaptation, and a 2-week process following arrival to a hot climate
such as the tropics or a desert. Gradually sweat-evaporation is increased and
the NaCl loss is reduced. The sweat secretion capacity may reach 4 l each hour
with a thin sweat. The adaptation is caused by increased aldosterone secretion
from the adrenal cortex. Aldosterone increases the reabsorption of NaCl and
the secretion of K+ from the sweat, during its passage of the sweat
gland tubules. The larger sweat loss the thirstier one feels. This is because
the large sweat secretion reduces the time period for NaCl- reabsorption in
the sweat gland tubules. The resulting high NaCl concentration in the plasma
implies thirst, so heat adapted persons have to drink a lot. Thirst is an
extremely late indicator of dehydration during work in a warm climate.
inhabitants are of course heat-adapted. They have an increased core
temperature (set point) and their threshold for sweat and vasodilatation is
typically 0.5oC higher than that of a person living in the temperate
zone (Fig. 21-15).
paragraph deals with 1. Heat
cramps, 2. Heat exhaustion and heat stroke, 3. Malignant
hyperthermia, 4. Hypothermia, 5. Frostbite, and 6. Fever and
paragraph concerning nuclear energy radiation is given at the end.
cramps in the leg muscles occur following exercise, when athletes run too fast
in a hot climate. Heat cramps respond to salt and water replenishment in a
normal diet, and the cramps are probably caused by hyponatraemia. During
prolonged sweating, the runner is losing salt and water. If only the water
loss is replenished, the result is water-induced hyponatraemia - a parallel to
the classical miner’s cramps.
Heat Exhaustion and Heat Stroke
the water-salt balance is at risk in a hot climate, it is always a threat to
the circulation. Profuse sweat secretion, in a subject who is not
acclimatized, results in salt- and water depletion, with a daily loss of more
than one mol of NaCl and more than 6 l of water. Within a period of one-hour
strenuous working endurance athletes have lost up to 8 l of water.
extracellular fluid volume and increasing body and brain temperature to above
40oC elicit severe symptoms and signs. As the volume and salt
depletion develops, the sweat production goes down in spite of extreme
vasodilatation. The falling blood pressure stimulates the high-pressure
baroreceptors resulting in a rising heart rate. The dehydration with imminent
shock frequently results in cerebral, renal and hepatic failure. The low brain
bloodflow through an overheated brain leads to fatigue (heat
exhaustion), confusion and unconsciousness or syncope (heat
syncope). The confusion may develop into a veritable delirium (an acute impairment of consciousness) with brain oedema.
stroke - in the sun it is called sunstroke - is heat collapse that occurs suddenly, hereby creating a
life-threatening condition. This heat collapse often occurs in warm, humid
environments, when an unacclimatized subject exercises. The subject - without
sweating (hypothalamic failure) -suddenly falls into coma, if not preceded by
a short period of confusion and delirium. The condition is fatal, if not
relieved by rapid cooling.
hyperpyrexia is often caused by a genetic
defect (autosomal dominant) in the sarcoplasmic reticulum of skeletal
muscles. General anaesthesia (often
halogen-substituted ethane) triggers an allergic reaction with sudden opening
of Ca2+ - channels in the muscle cells. The following influx of Ca2+ elicits generalized and maintained muscle contraction (rigidity), which
liberates enormous quantities of heat energy. This condition is life
threatening and often results in sudden death during or just after
is a fall in core temperature to values below 35 oC. Hypothermic
subjects lose consciousness, when the core temperature falls below 32 oC
- a potentially lethal condition called severe
During anaesthesia and
surgery the core temperature of the patient falls and stabilise around 34-35 oC
after 4-5 hours. Such a surgery
hypothermia increases the risk of cardiac complications, bleeding tendency
and prolonged wound healing.
The condition is a
prominent cause of death in climbers and skiers, as well as in persons being
immersed in cold water or living in the Antarctis. The climbers and skiers are
often exposed to a cold, wet and windy environment carrying insufficient
clothing. As the neuromuscular function suffers, they can no longer move.
General hypothermia develops and they die.
In mild hypothermia the
subject can still take action to rewarm by exercise and clothing, but as
consciousness is lost the core temperature falls further, because shivering is
abolished. The patient feels cold to touch, is in a developing coma, the
circulation and respiration fall, as does the metabolic rate. The dissociation
curve of oxyhaemoglobin is moved to the left, and the solubility of gasses
increases as the blood temperature falls.
The heart is the target
organ in hypothermia. Below 30 oC, spontaneous remission is
practically impossible, and death ensues from ventricular fibrillation
occurring around 28 oC.
Careful monitoring of
all vital functions is required during rewarming, which is performed either
passively (rewarming by the patients own metabolism) with insulation and space
blankets or actively by a warm water bath, while monitoring the patient. A
good strategy in treating hypothermic victims is the slogan: “They are not dead until they are warm and dead.”
An arterial blood sample
from a hypothermic patient is routinely analysed at 37 oC. The PaO2 and pHa is falsely
higher, the PaCO2 is
falsely lower, than in the circulating blood of the hypothermic patient. This
is because more CO2 is bound also as carbamino-haemoglobin, and
less O2 is bound in the cold blood. Base Excess is defined at 37 oC and thus a true metabolic
variable (see Chapter 17).
induced hypothermia is used in brain- and
heart-surgery, where the usual thermocontrol is inactivated by general
anaesthesia. The procedure becomes dangerous, when it elicits ventricular
Frostbite is a local cold injury due to the formation of extracellular ice crystals in
the skin and other tissues. This leads to extracellular dehydration and hyperosmosis,
whereby the cells lose water until they die. As the skin temperature falls below
+ 7 oC the subjects have lost their sensory functions, and thus
do not recognise the developing frostbite.
The most effective
treatment is warming of the still
frozen area by immersion in 40-42 oC hot water following
hospitalisation. The patient experiences severe pain (above + 7 oC) and morphine must be administered. Tissue which is no
longer frozen must be treated sterile.
Fever and hyperthermia
Fever occurs when the core temperature of the
body is raised above normal steady state levels. The body reacts as if it
is too cold, while the temperature rises up to the new higher set point. Fever
attacks imply shivering combined with vasoconstriction, headache, dedolation
pains, and general discomfort. Fever is the result of one of two phenomena:
the set point may be set to a higher level, or the efficacy of the temperature
control system may be impaired. Fever implies hyperthermia; however many cases of hyperthermia do not constitute fever. Fever results
from the action of endogenous pyrogens on
the hypothalamic heat control centre (they
increase the set point for the core temperature via prostaglandins). Exogenous
pyrogens from microbes cause these endogenous
polypeptides to be released from the defence cells of the body (ie, the
reticuloendothelial system, RES). Antipyretic drugs inhibit cyclo-oxygenase
activity, hereby interfering with the synthesis of prostaglandins and
Following an attack of fever, vasodilatation and sweat evaporation
reduces the core temperature.
Physiologic hyperthermia is an increase in core temperature caused by extreme heat stressor exercise.
During work the body temperature rises up to 39oC without clinical
consequences.Hereby, the heat loss capacity is exceeded. During hyperthermia
the heat loss effectors are strained
to the utmost. The high body temperatures of exercise activate cooling
mechanisms and elicit sweat loss, which strive to return the core temperature
to its normal level.In extreme hyperthermia the core temperature may rise to
more than 41o C (heat stroke). Irreversible protein denaturation
occurs above 44oC with brain oedema and destroyed thermoneurons in
the hypothalamus. Clinically, the brain damage is shown with disorientation,
lack of sweat secretion, delirium and universal
cramps before death.
radiation implies destruction of tissue molecules.
Dosages of absorbed
radiation is measured in Joules per kg (1 J kg -1 is known as a gray,
Gy). One Gy is equivalent to 100 rads. Radioactivity is measured as the number of degradations
per second in bequerels or Bq.
One degradation per s from radioactive material equals one Bq.
The absorbed dose of
radiation is balanced for damaging ionizing tissue effect, since different
types of radiation have different density of ionization. A dose equivalent
termed sievert (Sv) causes a rather large damage. The annual background
radiation is 2.5 milli-Sv (2.5 mSv).
Non-penetrating radiation occurs from alpha- and beta particles. These
particles are stopped by paper, but when they enter a tissue - such as the
bone marrow – they stay there and spoil everything.
consists of either gamma rays (neutron) or X-rays. Survivors from nuclear
power plant accidents, with whole body absorption greater than 100 rads, are
threatened by acute and chronic radiation sickness.
radiation sickness appears as vomiting and
malaise following exposure to 1 Gy (100 rad) or more. Lymphocyte production is
reduced immediately, soon followed by leucopenia and thrombocytopenia with
bleeding. The villi of the gastrointestinal tract are destroyed, absorption of
nutrients is impaired, and new villous cells are not produced. Diarrhoea,
often with blood loss, results in dehydration and anaemia. The skin is red and
blistering, and the hair is loosed. The immunodeficiency system is destroyed,
and secondary infections have a high mortality - especially pulmonary
infections. Cerebral oedema may kill
the victim within hours when exposed to 35 Gy or more.
radioactive iodine is treated by immediate intake of potassium iodide, which
block the major part of the thyroid absorption. The treatment of seriously
exposed radiation victims is supportive and frustrane.
radiation sickness or late
radiation damage implies an increased rate of mutagenesis, which includes
a high frequency of leucaemia, cancer of the brain, the thyroid and the
salivary glands, infertility and cataract (ie, an eye disease where the vision is blurred by an opaque lens). The
radiation liberates large amounts of highly reactive ions in the cells. The
reactive ions rupture DNA strands and cause mutations with production of
cancer cells. Cancer cells multiply exponentially and their energy demand
approach the total nutritive energy available, causing malnutrition and death.
The nuclear power plant
accident in Tjernobyl, Ukraine, had
many victims from radiation exposure. The nearby town, Pripatja, previously
with 40,000 residents is now abandoned.
The frequency of thyroid
cancer among children in Ukraine is more than 100 times the expected. Most of
the children and young persons exposed have developed leucaemia or cancer.
Their airway epithelium and respiration is seriously affected, and they have
an unusual high frequency of cerebral haemorrhage.
· The mean body temperature (Tbody)
is calculated with the following equation:
21-1: Tbody = (0.7 × Tcore + 0.3 × Tshell),
mean shell temperature (Tshell ) is estimated from a series of
representative skin temperatures. The obvious assumption in this equation is
that 70% of the body weight is core (Tcore), and the balance is
shell, but the size of the shell varies with the environmental temperature.
· The storage of heat energy in the body is
calculated as follows:
21-2: DH STORE = 3.49*BODY WEIGHT* (T2 - T1)
the unit kJ per hour.
· According to the first law of thermodynamics,
the following relation is valid:
21-3: DH STORE = DH METABOLIC - (DH RADIATION +DH CON + DH EVAPORATION )
where DH CON is the change in heat loss by
convection and conduction.
· Radiative heat loss from a warm object (Tobj ) can be calculated for a naked person with a known skin temperature (Tskin ):
21-4: DH RADIATION = 0.5 × A × (Tskin - Tobj)
0.5 is kJ min-1 m-2 K-1, A is the area of the
human body (radiating or receiving radiation), and Tobj is the
temperature of the object exchanging energy. This equation is an approximation
of Stefan-Boltzmanns rule.
· Evaporation of water on the body surface
eliminates 2430 J g-1. Evaporation of large volume rates of sweat (V°sweat)
implies a substantial loss of evaporative energy (J/min) according to the
Eq. 21-5: DH EVAPORATION (J min-1) = 2430 (J g-1) × V°sweat (g min-1).
· Changes in heat conduction through the shell
(conductanceshell) reflect changes in skin bloodflow. The
conductance can be calculated by the formula:
21-6: Conductanceshell = Metabolic Rate/(Tcore - Tshell).
A large temperature difference
implies an effective isolation, whereas a small difference implies a low
· The heat loss of a naked person resting in
quiet air by convection (including a minor part by conduction without
movement) is given by the following equation:
21-7: HCON = 0.5 *(Tshell - Tair) in kJ per min.
equation is an approximation of Newton’s law of cooling.
of the following five statements have True/False options:
can survive at a temperature that would cook a piece of steak, because the
steak cannot dissipate core heat energy.
conductance of air at high pressure exceeds that at low pressure, whereby more
heat energy is lost from the divers body by conduction through air inside a
diving bell deep under water than at one atmosphere of pressure.
homeostasis is present when heat energy production equals heat loss.
metabolic rate (BMR) is lower before than after a meal.
E. The neutral environmental temperature defines the level, where the
resting metabolic rate is minimal.
Each of the following five statements have True/False options.
A. Water-induced hyponatraemia is called miners cramps.
B. Substantial influx of Ca2+ to the cells is probably involved
in some cases of malignant hyperthermia.
C. Hypothermia is a fall in core temperature below 32oC.
D. One degradation per second from radioactive material equals one
bequerels (1 Bq), which is also equal to one curie.
E. Acute radiation reduces leucocyte and
A male with a body weight of 70 kg stops his malaria
prophylaxis with primaquine when leaving the endemic area. Three weeks later,
a sudden attack of fever increases his core temperature from 37 to 40oC
within 30 min. The heat capacity of the human body is 3.47 kJ/kg and per oC.
The metabolic rate of the subject increased substantially during the rise in
temperature. Following the cold stage with uncontrollable shivering, the
patient develops a delirious condition with severe headache. Two hours later
the patient develops a profuse sweating. Partially evaporation of the water in
32 ml sweat/min (25% evaporates) occurs from the body surface (eliminating
2,436 J/g), during which body temperature drops.
the extra heat energy stored in his body after 30 min.
the smallest possible metabolic rate in the 30-min period.
causes the extra heat energy stored in the body?
the reduction in body temperature by ingestion of one L of ice water, when the
fever is at its highest level.
the time it takes to lose the accumulated heat energy by evaporation.
A male sedentary person, weight 70 kg, has a daily
food intake of 400 g carbohydrate (17.5 kJ/g), 100 g fat (39 kJ/g), and 100 g
protein (17 kJ/g). There is a metabolic water formation of 32 mg/kJ. The man
excretes 2100 ml of water (i.e., 1200 ml in the urine, 100 ml in faeces, and
800 ml through lungs and skin).
1. Calculate the metabolic
rate (in kJ/day or MJ/day).
2. The man is in water
balance by a water intake of 1,700 ml as a total. Explain this apparent
A 79-year old male is found apparently dead in the
snow following a winter storm, where all traffic was arrested by snow. His
muscles are stiff, and the heart rate is not palpable. The tendon reflexes are
depressed, and the pupillary and other brainstem reflexes are lost.
The body is placed in a chapel at the hospital until
the funeral. The next day the personnel are disturbed by noises from the
chapel. Obviously, the man is alive.
1. What has awakened the man?
2. Suggest a likely core temperature, at the time where the man was
admitted to the hospital.
A 20-year old person, with a body surface area of 1.8
m2, is at rest with a metabolic rate (MR, or heat energy production
and transfer) of 80 Watts. His rectal temperature is 37 oC and his
mean skin temperature is 33 oC. - Suddenly, a malaria attack
develops with a 7-fold rise in MR, and the patient reach a temperature plateau
of 40 oC, with a mean skin temperature of 34 oC.
The conductance of the shell (Cshell) is a
measure of skin bloodflow. This can be calculated by the formula: Cshell = MR/(Tcore - Tshell ).
1 Calculate the
conductance of the shell at rest.
2. Calculate the
conductance of the shell at the fever plateau.
3. How does the body
accomplish this increase?
to solve the problems before looking up the answers..
core temperature is 37 oC in healthy adults at rest, but small children have larger
· Parents should be advised to measure the rectal temperature in disease suspect
children. The rectal temperature is a reliable estimate of the core
temperature in resting persons.
constant core temperature does not guarantee heat energy balance, because the energy content of the
shell may change with the surroundings.
human core temperature falls during the oestrogen phase of the menstrual cycle and during sleep. The
lowest temperature is between 18 at night and 6 o’clock in the morning.
Progesterone effects seem to explain the higher temperature in the last phase
of the menstrual cycle
temperature cycle is part of the circadian periodicity. Our biological clock seems to be
synchronised with the rotation of the globe. Also meals, light and temperature
plays a role.
thermal comfort point for light-clothed, seated persons is about 26oC when the humidity
is 50%, 30oC for nude persons, and about 36oC sitting in
water to the neck.
in warm environments permits increased skin bloodflow as the cardiac output
also begins to rise. The arterio-venous anastomoses are dilatated, and the
bloodflow can rise to at least 10 folds in hands and feet.
the metabolic zone, the total heat
loss rises with falling temperature, but below 5 oC the dry heat
loss exceeds the metabolic rate, and the body is cooled down. This is the zone
of hypothermia, where cold death is inevitable without therapy.
Three million sweat glands produce sweat at a rate of up to 2 litres per hour
or more during extreme conditions. Such high sweat rates lead to circulatory
failure and shock. Sweat resembles a dilute ultrafiltrate of plasma.
zone of hyperthermia begins at an environmental temperature of 37 oC in humid air, where
we soon reach the maximal capacity for sweat secretion, and there is a heat
influx to the body ending in heat death.
hypothalamus functions as a thermostat using the rest of the body to stabilise its own
dynamic gain system responds continuously to feedback signals, regardless of the size of the core
temperature. With rising tissue temperature, the neural activity of heat
sensors increases linearly, whereas the activity of cold sensors decreases
set-point system does not respond to a rise in temperature before a certain set point is
reached. The set point is the core temperature at which neither heat loss
mechanisms or heat production mechanisms are active.
temperature control exhibits both dynamic gain and set point characteristics. The control system
implies widespread cutaneous and deep sensors. Their afferents converge
towards the hypothalamic integrator and thermostat.
thermogenic response to cold also involves a non-myogenic or non-shivering component probably in
adipocytes. Non-shivering heat production is controlled by the sympathetic
nervous system via adrenergic b-receptors.
thermogenesis is any rise in metabolism, which is not related to muscular activity. In
babies this form of thermogenesis is particularly large in their brown adipose
tissue. The brown adipose tissue of babies is abundant around vital
organs, in neck and mediastinum, between scapulae and in the armpits.
· Newborns are in thermal balance at a minimal metabolism only when the surrounding
temperature is high (32-34 oC).
behaviour such as fanning and adding or removing clothing is effective in changing the
thermal insulation. Several layers of clothing with trapped air are good
acclimatization is actually a sweat gland adaptation, and a 2-week process following arrival
to a hot climate. Gradually sweat-evaporation is increased and the NaCl loss
is reduced. The adaptation is caused by increased aldosterone secretion from
the adrenal cortex.
· Hypovolaemia with low brain bloodflow through an overheated brain leads to fatigue (heat
exhaustion), confusion and unconsciousness or syncope (heat syncope). The
confusion may develop into a veritable delirium with brain oedema.
stroke - in the sun called sunstroke - is heat collapse with high brain temperature
that occurs suddenly, hereby creating a life-threatening condition.
· Hypothermia is a fall in core temperature to values below 35 oC. Hypothermic
subjects lose consciousness, when the core temperature falls below 32 oC
- a potentially lethal condition called severe hypothermia.
induced hypothermia is used in brain- and heart-surgery, where the usual thermocontrol is
inactivated by general anaesthesia.
radiation consists of either gamma rays (neutrons) or X-rays. Survivors from nuclear
power plant accidents with whole body absorption greater than 100 rads are
threatened by acute and chronic radiation sickness.
radiation sickness appears as vomiting and malaise following exposure to 1 Gy (100 rad) or more.
Lymphocyte production is reduced immediately, soon followed by leucopenia and
thrombocytopenia with bleeding. The villi of the gastrointestinal tract are
destroyed, absorption of nutrients is impaired, and new villous cells are not
produced. Diarrhoea, often with blood loss, results in dehydration. The skin
is red and blistering, and the hair is loosed.
radiation sickness or late radiation damage implies an increased rate of mutagenesis, which
includes a high frequency of leucaemia, cancer of the brain, the thyroid and
the salivary glands, infertility and cataract.
American. Monthly journal published by
Scientific American Inc., 415 Madison Avenue, N.Y., USA.
Benzinger, T. H. "The human thermostat." Sci.
Am. 204: 134-147, 1961.
Hong, S. K. News
in Physiol. Sci. 2: 79, 1987.
Wasserman, D.H. and A.D. Cherrington.
"Hepatic fuel metabolism during muscular work: Role and regulation." Am. J. Physiol. 260: E811, 1991.
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