Body Temperature and Temperature pdf - Metabolism and Temperature Regulation

the cells of the body; (2) extra rate of metabolism caused by muscle activity,

tant of these factors are listed again here: (1) basal rate of metabolism of all

The most impor-

metabolic rate of the body.

rate of heat production, called the

summarizes body energetics, we discuss the different factors that determine the

Heat production is a principal by-product of metabolism. In Chapter 72, which

controls each of these.

decrease. Most of the remainder of this chapter is concerned with this balance

Conversely, when heat loss is greater, both body heat and body temperature

heat is being lost, heat builds up in the body and the body temperature rises.

When the rate of heat production in the body is greater than the rate at which

Heat Production Against Heat Loss

Body Temperature Is Controlled by Balancing

fall to values below 96°F.

versely, when the body is exposed to extreme cold, the temperature can often

exercise, the temperature can rise temporarily to as high as 101° to 104°F. Con-

are not perfect. When excessive heat is produced in the body by strenuous

extremes of the surroundings, because the temperature regulatory mechanisms

The body temperature increases during exercise and varies with temperature

1°F higher when measured rectally.

ally considered to be between 98.0° and 98.6°F when measured orally and about

(36°C) to over 99.5°F (37.5°C). The average normal core temperature is gener-

temperatures measured orally, as shown in Figure 73–1, from less than 97°F

Normal Core Temperature.

surroundings.

portant temperature when we refer to the skin’s ability to lose heat to the

with the temperature of the surroundings. The skin temperature is the im-

core temperature,

skin temperature,

The

it operates in health and in disease.

designed control system. The purpose of this chapter is to discuss this system as

The mechanisms for regulating body temperature represent a beautifully

air and still maintain an almost constant core temperature.

develops a febrile illness. Indeed, a nude person can

0.6°C), day in and day out, except when a person

the body—remains very constant, within

ture of the deep tissues of the body—the “core” of

The tempera-

Core Temperature and Skin Temperature.

Normal Body Temperatures

Regulation, and Fever

Body Temperature, Temperature

889

±1°F

(

be exposed to temperatures as low as 55°F or as

high as 130°F in dry

in contrast to the

rises and falls

No single core temperature can be considered normal,

because measurements in many healthy people have shown a range of normal

between heat production and heat loss and the mechanisms by which the body

Heat Production

Figure 73–3 shows quantitatively the effect of envi-

the heat conduction from the core to very little.

core of the body to the skin with great efficiency,

great as 30 per cent of the total cardiac output. A high

The rate of blood flow into the skin venous plexus

anastomoses.

arteriovenous

areas of the body—the hands, feet, and ears—blood is

capillaries, shown in Figure 73–2. In the most exposed

skin. Especially important is a continuous venous

Provides Heat Transfer

Blood Flow to the Skin from the Body Core

to approach the temperature of the surroundings.

ture, even though it allows the temperature of the skin

The insulation beneath the skin is an effective

women, this insulation is even better.

the insulating properties of a usual suit of clothes. In

organs to the skin, the insulating properties of the

When no blood is flowing from the heated internal

as readily as other tissues.

one third

insulator for the body. The fat is important because it

The skin, the subcutaneous tissues, and especially the

insulates the core from the skin surface.

roundings. Let us begin by discussing the system that

is determined almost entirely by two factors: (1) how

surroundings. Therefore, the rate at which heat is lost

tissues to the skin, where it is lost to the air and other

heart, and in the skeletal muscles during exercise. Then

the deep organs, especially in the liver, brain, and

needed for digestion, absorption, and storage of food

temperature increases; and (6) extra metabolism

activity in the cells themselves, especially when the cell

epinephrine, and sympathetic stimulation on the cells;

metabolism caused by the effect of epinephrine, nor-

hormone and testosterone) on the cells; (4) extra

(and, to a less extent, other hormones, such as growth

including muscle contractions caused by shivering; (3)

Metabolism and Temperature Regulation

890

Unit XIII

extra metabolism caused by the effect of thyroxine

(5) extra metabolism caused by increased chemical

(thermogenic effect of food).

Heat Loss

Most of the heat produced in the body is generated in

this heat is transferred from the deeper organs and

rapidly heat can be conducted from where it is pro-
duced in the body core to the skin and (2) how rapidly
heat can then be transferred from the skin to the sur-

Insulator System of the Body

fat of the subcutaneous tissues act together as a heat

conducts heat only

normal male body are about equal to three quarters

means of maintaining normal internal core tempera-

Blood vessels are distributed profusely beneath the

plexus that is supplied by inflow of blood from the skin

also supplied to the plexus directly from the small
arteries through highly muscular

can vary tremendously—from barely above zero to as

rate of skin flow causes heat to be conducted from the

whereas reduction in the rate of skin flow can decrease

ronmental air temperature on conductance of heat

104

102

100

A few normal adults

Cold weather, etc.

A few normal adults

Oral

Rectal

Hard exercise

Hard work, emotion

Many active children

Usual range

of normal

Early morning

Usual range
of normal

Early morning

Emotion or
moderate exercise

Many active children

(Redrawn from DuBois EF: Fever. Springfield, IL: Charles C

Estimated range of body “core” temperature in normal people.

Figure 73–1

Thomas, 1948.)

Epidermis

Capillaries

Arteries

Veins

Venous plexus

Arteriovenous
anastomosis

Artery

Dermis

Subcutaneous tissue

Skin circulation.

Figure 73–2

and then by air convection away from the body.

without gross air movement, about 15 per cent of his

fore, in a nude person seated in a comfortable room

cent to the skin to rise as it becomes heated. There-

around the body because of the tendency for air adja-

currents.

Actually, the heat must first be

heat loss by

The removal of heat from the body by con-

with the skin, a phenomenon called

new, unheated air is continually brought in contact

unless the heated air moves away from the skin,

conducted from the air to the body. Therefore, con-

in this way, because now an equal amount of heat is

temperature of the skin, no further loss of heat occurs

ing the velocity of the air molecules’ motion. Once the

to the air if the air is colder than the skin, thus increas-

energy of molecular motion, and the molecules of

per cent) even under normal conditions.

a sizable proportion of the body’s heat loss (about 15

however, represents

conduction to air,

solid objects,

quantities of heat, about 3 per cent, are normally lost

As shown in Figure 73–4, only minute

the body than is radiated to the body.

roundings, a greater quantity of heat is radiated from

and other objects toward the body. If the temperature

human body radiates heat rays in all directions. Heat

not at absolute zero temperature radiate such rays.The

times the wavelengths of light rays. All objects that are

have wavelengths of 5 to 20 micrometers, 10 to 30

infrared heat rays, a type of electromagnetic wave.

sitting inside at normal room temperature, about 60

As shown in Figure 73–4, in a nude person

radiation, conduction,

They include

skin to the surroundings are shown in Figure 73–4.

The various methods by which heat is lost from the

Skin Surface

Basic Physics of How Heat Is Lost from the

hypothalamus.

temperature. This is discussed later in the chapter in

that supply blood to the venous plexus of the skin. This

Nervous System.

radiator” system,

Therefore, the skin is an effective

constricted state and the fully vasodilated state.

into the air, demonstrating an approximate eightfold

Body Temperature, Temperature Regulation, and Fever

Chapter 73

891

from the core to the skin surface and then conductance

increase in heat conductance between the fully vaso-

controlled “heat

and the flow of blood to the skin is a

most effective mechanism for heat transfer from the
body core to the skin.

Control of Heat Conduction to the Skin by the Sympathetic

Heat conduction to the skin by the

blood is controlled by the degree of vasoconstriction
of the arterioles and the arteriovenous anastomoses

vasoconstriction is controlled almost entirely by the
sympathetic nervous system in response to changes in
body core temperature and changes in environmental

connection with control of body temperature by the

and evaporation,

which are explained next.

Radiation.

per cent of total heat loss is by radiation.

Loss of heat by radiation means loss in the form of

Most infrared heat rays that radiate from the body

rays are also being radiated from the walls of rooms

of the body is greater than the temperature of the sur-

Conduction.

from the body by direct conduction from the surface
of the body to

such as a chair or a bed.

Loss of heat by

It will be recalled that heat is actually the kinetic

the skin are continually undergoing vibratory motion.
Much of the energy of this motion can be transferred

temperature of the air adjacent to the skin equals the

duction of heat from the body to the air is self-limited

so that

air convection.

Convection.

vection air currents is commonly called
convection.

conducted

to the air and then carried away by the convection air

A small amount of convection almost always occurs

or her total heat loss occurs by conduction to the air

100 110

Vasodilated

Vasoconstricted

120

Heat conductance through skin

(times the vasoconstricted rate)

Environmental temperature (

Physiology. New York: Dowden, Hutchinson & Ross, 1980.)

Benzinger TH: Heat and Temperature Fundamentals of Medical

ductance from the body core to the skin surface. (Modified from

Effect of changes in the environmental temperature on heat con-

Figure 73–3

Walls

Conduction to
objects (3%)

Evaporation (22%)

Radiation (60%)

heat waves

Conduction to air (15%)

Air currents

(convection)

Mechanisms of heat loss from the body.

Figure 73–4

of the sweat gland. Cholinergic sympathetic nerve

The precursor secretion is an active secretory

As is true of so many other glands, the secretory

secretes the sweat, and (2) a

two parts: (1) a deep subdermal

In Figure 73–5, the sweat

muscles.

during exercise, when these hormones are secreted by

do not have adrenergic innervation. This is important

lating in the blood, even though the glands themselves

fibers). These glands can also be stimulated to

cholinergic

thetic outflow to the skin everywhere in the body.

causes sweating. The nerve impulses from this area

Nervous System

less effective thereafter as an insulator.

because sweating in one’s clothes makes them much

careful not to become overheated even temporarily,

the clothing to become wet. Indeed, one must be

cloth 20-fold or more. Therefore, one of the most

ing becomes wet, because the high conductivity of

The effectiveness of clothing in maintaining body

about half.

effective than otherwise. Using this technique, clothing

gold, which reflects radiant heat back to the body,

fore, coating the inside of clothing with a thin layer of

conducted across the small intervening space. There-

half that from the nude body, but arctic-type clothing

duction and convection is greatly depressed. A usual

quently, the rate of heat loss from the body by con-

decreasing the flow of convection air currents. Conse-

private zone

above that of the body.

orative refrigeration system, they cannot prevent a rise

well as normal people can, but they are likely to die of

glands. These people can stand cold temperatures as

temperature to rise. This occurs occasionally in human

Therefore, anything that prevents adequate evapo-

heat is by evaporation.

the only means by which the body can rid itself of

tions,

by both radiation and conduction. Under these condi-

of the skin, instead of losing heat, the body gains heat

lost by radiation and conduction. But when the tem-

than the temperature of the surroundings, heat can be

Air Temperatures.

Evaporation Is a Necessary Cooling Mechanism at Very High

chapter.

ing the rate of sweating, which is discussed later in the

evaporation of sweat

the skin and respiratory surfaces. However, loss of heat

16 to 19 Calories per hour. This insensible evaporation

700 ml/day. This causes continual heat loss at a rate of

person is not sweating, water still evaporates

each gram of water that evaporates. Even when a

surface, 0.58 Calorie (kilocalorie) of heat is lost for

When water evaporates from the body

of heat loss to air.

lator zone” as occurs in air. Therefore, the rate of heat

a thin layer of water next to the body to form an “insu-

in air. Consequently, it is impossible for the body to heat

greater quantities of heat than air can. Also, heat con-

sand times as great as that of air, so that each unit

Water has a specific heat several thou-

pended in Water.

effective for cooling as a wind of 1 mile per hour.

instance, a wind of 4 miles per hour is about twice as

For

square root of the wind velocity.

and heat loss by convection increases accordingly. The

is replaced by new air much more rapidly than normally,

wind, the layer of air immediately adjacent to the skin

When the body is exposed to

Cooling Effect of Wind.

Metabolism and Temperature Regulation

892

Unit XIII

cooling effect of wind at low velocities is about propor-
tional to the

Conduction and Convection of Heat from a Person Sus-

portion of water adjacent to the skin can absorb far

ductivity in water is very great in comparison with that

loss to water is usually many times greater than the rate

Evaporation.

insensi-

bly from the skin and lungs at a rate of about 600 to

through the skin and lungs cannot be controlled for
purposes of temperature regulation because it results
from continual diffusion of water molecules through

can be controlled by regulat-

As long as skin temperature is greater

perature of the surroundings becomes greater than that

ration when the surrounding temperature is higher
than the skin temperature will cause the internal body

beings who are born with congenital absence of sweat

heatstroke in tropical zones because without the evap-

in body temperature when the air temperature is

Effect of Clothing on Conductive Heat Loss.

Clothing

entraps air next to the skin in the weave of the cloth,
thereby increasing the thickness of the so-called

of air adjacent to the skin and also

suit of clothes decreases the rate of heat loss to about

can decrease this heat loss to as little as one sixth.

About half the heat transmitted from the skin to the

clothing is radiated to the clothing instead of being

makes the insulating properties of clothing far more

for use in the arctic can be decreased in weight by

temperature is almost completely lost when the cloth-

water increases the rate of heat transmission through

important factors for protecting the body against cold
in arctic regions is extreme caution against allowing

Sweating and Its Regulation by the Autonomic

Stimulation of the anterior hypothalamus-preoptic
area in the brain either electrically or by excess heat

that cause sweating are transmitted in the autonomic
pathways to the spinal cord and then through sympa-

It should be recalled from the discussion of the

autonomic nervous system in Chapter 60 that the
sweat glands are innervated by

nerve

fibers (fibers that secrete acetylcholine but that run
in the sympathetic nerves along with the adrenergic

some extent by epinephrine or norepinephrine circu-

the adrenal medullae and the body needs to lose
excessive amounts of heat produced by the active

Mechanism of Sweat Secretion.

gland is shown to be a tubular structure consisting of

coiled portion that

duct portion that passes

outward through the dermis and epidermis of the skin.

portion of the sweat gland secretes a fluid called the
primary secretion or precursor secretion; the concen-
trations of constituents in the fluid are then modified
as the fluid flows through the duct.

product of the epithelial cells lining the coiled portion

fibers ending on or near the glandular cells elicit the
secretion.

tory passages; this cools the blood in the respiratory

When an animal pants, it breathes in and out rapidly,

the pons.

One of these signals initiates panting. The actual panting

neurogenic signals to decrease the body temperature.

blood becomes overheated, the hypothalamus initiates

thermoregulator centers of the brain. That is, when the

The phenomenon of panting is “turned on” by the

mechanism, is used by many lower animals as a means

from the skin. A substitute mechanism, the

most lower animals is not supplied with sweat glands,

faces are usually covered with fur, and (2) the skin of

the surfaces of their bodies, for two reasons: (1) the sur-

tization, the loss is usually 3 to 5 g/day.

day for the first few days. After 4 to 6 weeks of acclima-

unacclimatized

fluid and plasma. An

tical glands, which results from a slight decrease in

tion of body salt. Most of this effect is caused by

the sweat, which allows progressively better conserva-

capability.

production. This increased effectiveness of the sweat-

duction to as much as 2 to 3 L/hour. Evaporation of

more profusely, often increasing maximum sweat pro-

weather for 1 to 6 weeks, he or she begins to sweat

sweat per hour, when this person is exposed to hot

Although a normal, unacclimatized

as follows.

ing capacity, once a person has become acclimatized,

is much less electrolyte loss, despite increased sweat-

sweat when a person is unacclimatized to heat. There

There is a significant loss of sodium chloride in the

sium about 1.2 times.

that in the plasma, lactic acid about 4 times, and potas-

that little of the water is reabsorbed. Therefore, the

half the concentrations in plasma. Furthermore, the

maximum of about 50 to 60 mEq/L, slightly less than

unacclimatized

sodium chloride; the concentrations of sodium and

amounts of precursor secretion are formed, and the

stimulated by the sympathetic nervous system, large

Conversely, when the sweat glands are strongly

of sweating, such constituents as urea, lactic acid, and

most of the other constituents. Therefore, at low rates

of the water is also reabsorbed, which concentrates

falls to as low as 5 mEq/L. This reduces the osmotic

ions are reabsorbed, and the concentration of each

this instance, essentially all the sodium and chloride

the precursor fluid passes through the duct slowly. In

When the sweat glands are stimulated only slightly,

the rate of sweating, as follows.

ride ions. The degree of this reabsorption depends on

flows through the duct portion of the gland, it is mod-

other solutes of plasma. As this precursor solution

104 mEq/L, with much smaller concentrations of the

is about 142 mEq/L and that of chloride is about

contain plasma proteins. The concentration of sodium

similar to that of plasma, except that it does not

The composition of the precursor secretion is

Body Temperature, Temperature Regulation, and Fever

Chapter 73

893

ified by reabsorption of most of the sodium and chlo-

pressure of the sweat fluid to such a low level that most

potassium ions are usually very concentrated.

duct may reabsorb only slightly more than half the

chloride ions are then (in an

person) a

sweat flows through the glandular tubules so rapidly

other dissolved constituents of sweat are only moder-
ately increased in concentration—urea is about twice

Acclimatization of the Sweating Mechanism to Heat—Role
of Aldosterone.

person seldom produces more than about 1 liter of

this much sweat can remove heat from the body at a
rate more than 10 times the normal basal rate of heat

ing mechanism is caused by a change in the internal
sweat gland cells themselves to increase their sweating

Also associated with acclimatization is a further

decrease in the concentration of sodium chloride in

increased secretion of aldosterone by the adrenocor-

sodium chloride concentration in the extracellular

person who

sweats profusely often loses 15 to 30 grams of salt each

Loss of Heat by Panting

Many lower animals have little ability to lose heat from

which prevents most of the evaporative loss of heat

panting

of dissipating heat.

process is controlled by a panting center that is associ-
ated with the pneumotaxic respiratory center located in

so that large quantities of new air from the exterior
come in contact with the upper portions of the respira-

Gland

Duct

Pore

Epidermis

Dermis

Sympathetic

nerve

Primary

secretion,

mainly

protein-

free

filtrate

Absorption,

mainly

sodium and chloride ions

the duct, leaving a dilute, watery secretion.

glandular portion, but most of the electrolytes are reabsorbed in

is formed by the

primary protein-free secretion

Sweat gland innervated by an acetylcholine-secreting sympa-

Figure 73–5

thetic nerve. A

thorax. These deep receptors function differently from

spinal cord,

already occurring; and (3) by promoting skin vaso-

(2) by inhibiting the process of sweating, if this is

providing a strong stimulus to cause shivering, with a

the temperature of the body in several ways: (1) by

When the skin is chilled over the entire body, imme-

detecting cool and cold instead of warm temperatures.

10 times as many in many parts of the skin. Therefore,

receptors. There are far

of the body.

regulation. This is especially true of temperature

ful in controlling body temperature, receptors in other

Detection of Temperature by Receptors in the

mostatic body temperature control center.

ited. Therefore, it is clear that the hypothalamic-

In addition, any excess body heat production is inhib-

to cause the body to lose heat, thereby helping to

become greatly dilated. This is an immediate reaction

When the preoptic area is heated, the skin all over

body temperature falls.

neurons, by contrast, increase their firing rate when the

10°C increase in body temperature. The cold-sensitive

trolling body temperature. The heat-sensitive neurons

as many cold-sensitive neurons. These neurons are

Using the thermode, the anterior hypothalamic-

hypothalamus.

water through it, or it is cooled by cold water. The

This small, needle-like

thermode.

of Temperature

Area in Thermostatic Detection

Role of the Anterior Hypothalamic-Preoptic

temperature becomes either too high or too low.

back mechanisms to operate, there must also be

For these feed-

hypothalamus.

entirely by nervous feedback mechanisms, and almost

The temperature of the body is regulated almost

where between 97° and 100°F.

person in dry air between 55° and 130°F is capable of

even the nature of the surroundings. In general, a nude

ment of the air, the amount of moisture in the air, and

air ranging from 30° to 160°F. The precise

Figure 73–6 shows what happens to the body “core”

Temperature—Role of the

the atmosphere.

shallow; therefore, most of the air that enters the alveoli

of the blood gases, because each breath is extremely

from the tongue. Yet panting does not increase the alve-

the mucosal surfaces, especially evaporation of saliva

Metabolism and Temperature Regulation

894

Unit XIII

passage mucosa as a result of water evaporation from

olar ventilation more than is required for proper control

is dead-space air mainly from the trachea and not from

Regulation of Body

Hypothalamus

temperature of a nude person after a few hours’ expo-
sure to dry
dimensions of this curve depend on the wind move-

maintaining a normal body core temperature some-

all these operate through temperature-regulating
centers located in the

temperature detectors to determine when the body

Experiments have been performed in which minute
areas in the brain of an animal have been either heated
or cooled by use of a
device is heated by electrical means or by passing hot

principal areas in the brain where heat or cold from
a thermode affects body temperature control are the
preoptic and anterior hypothalamic nuclei of the

preoptic area has been found to contain large numbers
of heat-sensitive neurons as well as about one third

believed to function as temperature sensors for con-

increase their firing rate 2- to 10-fold in response to a

the body immediately breaks out in a profuse sweat,
while the skin blood vessels over the entire body

return the body temperature toward the normal level.

preoptic area has the capability to serve as a ther-

Skin and Deep Body Tissues

Although the signals generated by the temperature
receptors of the hypothalamus are extremely power-

parts of the body play additional roles in temperature

receptors in the skin and in a few specific deep tissues

It will be recalled from the discussion of sensory

receptors in Chapter 48 that the skin is endowed
with both cold and warmth
more cold receptors than warmth receptors—in fact,

peripheral detection of temperature mainly concerns

diate reflex effects are invoked and begin to increase

resultant increase in the rate of body heat production;

constriction to diminish loss of body heat from the
skin.

Deep body temperature receptors are found mainly

in the

in the abdominal viscera, and in or

around the great veins in the upper abdomen and

110

130

150

170

Body temperature (

Atmospheric temperature (

120

100

in atmospheric temperature.

internal body temperature remains stable despite wide changes

duration on the internal body “core” temperature. Note that the

Effect of high and low atmospheric temperatures of several hours’

Figure 73–6

This area is

primary motor center for shivering.

explanation, which follows.

heat production, and thyroxine secretion. These

by promoting shivering, sympathetic excitation of

air” next to the skin, so that transfer of heat to

allows them to entrap a thick layer of “insulator

in lower animals, upright projection of the hairs

stance. This is not important in human beings, but

to contract, which brings the hairs to an upright

on end.” Sympathetic stimulation causes the

Piloerection means hairs “standing

hypothalamic sympathetic centers.

This is caused by stimulation of the posterior

Skin vasoconstriction throughout the body.

system institutes exactly opposite procedures. They

When the body is too cold, the temperature control

Body Is Too Cold

Temperature-Increasing Mechanisms When the

and chemical thermogenesis, are strongly

cause excess heat production, such as shivering

The mechanisms that

(98.6°F). An additional 1°C increase in body

the blue curve in Figure 73–7, which shows a

The effect of increased body

Sweating.

hypothalamus that cause vasoconstriction. Full

intensely dilated. This is caused by inhibition

areas of the body, the skin blood vessels become

Vasodilation of skin blood vessels.

The temperature control system uses three important

When the Body Is Too Hot

Temperature-Decreasing Mechanisms

experience, but special features are the following.

or temperature-increasing procedures. The reader is

low, they institute appropriate temperature-decreasing

When the hypothalamic temperature centers detect

Temperature

tions of the body.

mitted into this posterior hypothalamic area. Here the

lary bodies. The temperature sensory signals from the

thalamus. The area of the hypothalamus that they

in peripheral receptors, these signals contribute to

and Peripheral Temperature Sensory Signals

preventing low body temperature.

—that is,

they detect mainly cold rather than warmth. It is prob-

temperature. Yet, like the skin temperature receptors,

Body Temperature, Temperature Regulation, and Fever

Chapter 73

895

the skin receptors because they are exposed to the
body core temperature rather than the body surface

able that both the skin and the deep body receptors
are concerned with preventing hypothermia

Posterior Hypothalamus Integrates the Central

Even though many temperature sensory signals arise

body temperature control mainly through the hypo-

stimulate is located bilaterally in the posterior hypo-
thalamus approximately at the level of the mammil-

anterior hypothalamic-preoptic area are also trans-

signals from the preoptic area and the signals from
elsewhere in the body are combined and integrated to
control the heat-producing and heat-conserving reac-

Neuronal Effector Mechanisms
That Decrease or Increase Body

that the body temperature is either too high or too

probably familiar with most of these from personal

mechanisms to reduce body heat when the body tem-
perature becomes too great:
1.

In almost all

of the sympathetic centers in the posterior

vasodilation can increase the rate of heat transfer
to the skin as much as eightfold.

temperature to cause sweating is demonstrated by

sharp increase in the rate of evaporative heat
loss resulting from sweating when the body core
temperature rises above the critical level of 37°C

temperature causes enough sweating to remove 10
times the basal rate of body heat production.

3. Decrease in heat production.

inhibited.

are:
1.

2. Piloerection.

arrector pili muscles attached to the hair follicles

the surroundings is greatly depressed.

3. Increase in thermogenesis (heat production). Heat

production by the metabolic systems is increased

methods of increasing heat require additional

Hypothalamic Stimulation of Shivering.

Located in the

dorsomedial portion of the posterior hypothalamus
near the wall of the third ventricle is an area called
the

36.4

36.6

36.8

37.0

37.2

37.4

37.6

Calories per second

Head temperature (

Heat production
Evaporative heat loss

heat production reaches a minimum stable level.

ical temperature level at which increased heat loss begins and

activity and shivering. This figure demonstrates the extremely crit-

the body and on heat production caused primarily by muscle

Effect of hypothalamic temperature on evaporative heat loss from

Figure 73–7

changes about 1°C for each 25° to 30°C change in

1.0 (see Chapter 1 for this formula). Experiments

back gain

greatly from day to day or even hour to hour. The

core temperature to change as little as possible, even

temperature control, it is important for the internal

effectiveness of a control system. In the case of body

sented in Chapter 1. Feedback gain is a measure of the

Feedback Gain for Body Temperature Control.

That is, all the temperature control mechanisms con-

the “set-point” of the temperature control mechanism.

37.1°C level. This crucial temperature level is called

of heat production is greater than that of heat loss, so

37.1°C level. At temperatures below this level, the rate

tion, so the body temperature falls and approaches the

heat production. At temperatures above this level, the

ical body core temperature of about 37.1°C (98.8

In the example of Figure 73–7, it is clear that at a crit-

for Temperature Control

Concept of a “Set-Point”

cold climates than in those who live in warm climates.

rates. Further, the continuous stimulatory effect of

develop increased metabolic rates; some Inuit

cold is in the human being.

Therefore, we still do not know, quantitatively, how

20 to 40 per cent. However, human beings seldom

This increase in metabolism does not

thermogenesis.

chemical

the body, which is yet another mechanism of

explained in Chapter 76. The increased thyroxine

by the thyroid gland, as

thyroxine

Thyroid-stimulating hormone in turn stimulates

stimulating hormone.

itary gland, where it stimulates secretion of

by the hypothalamus. This hormone is carried by way

Increased Thyroxine Output as a Long-Term Cause of Increased

maintaining normal body temperature in neonates.

per cent, which is probably an important factor in

of brown fat in the interscapular space, chemical ther-

cent. However, in infants, who

fat, it is rare for chemical thermogenesis to increase

In adult human beings, who have almost no brown

increase in food intake.

with an increase of perhaps one third as much. This

trast to the unacclimatized animal, which responds

heat production when acutely exposed to cold, in con-

chemical thermogenesis; some animals, such as rats,

described in Chapter 72; these cells are supplied by

mitochondria where uncoupled oxidation occurs, as

in the animal’s tissues. This is a

brown fat

The degree of chemical thermogenesis that occurs

oxidative phosphorylation, which

chemical thermogenesis.

increase in the rate of cellular metabolism. This effect

pointed out in Chapter 72, an increase in either sym-

During maximum shivering, body heat

stretch reflex mechanism, which is discussed in

certain critical level, shivering begins. This probably

anterior motor neurons. When the tone rises above a

Instead, they increase the tone of the skeletal muscles

rhythmical and do not cause the actual muscle shaking.

the anterior motor neurons. These signals are non-

the lateral columns of the spinal cord, and finally to

ing through bilateral tracts down the brain stem, into

ture level. It then transmits signals that cause shiver-

duction” (see the red curve in Figure 73–7), this center

fore, as shown by the sudden increase in “heat pro-

by cold signals from the skin and spinal cord. There-

Metabolism and Temperature Regulation

896

Unit XIII

normally inhibited by signals from the heat center in
the anterior hypothalamic-preoptic area but is excited

becomes activated when the body temperature falls
even a fraction of a degree below a critical tempera-

throughout the body by facilitating the activity of the

results from feedback oscillation of the muscle spindle

Chapter 54.
production can rise to four to five times normal.

Sympathetic “Chemical” Excitation of Heat Production.

pathetic stimulation or circulating norepinephrine and
epinephrine in the blood can cause an immediate

is called

It results at least par-

tially from the ability of norepinephrine and epineph-
rine to uncouple
means that excess foodstuffs are oxidized and thereby
release energy in the form of heat but do not cause
adenosine triphosphate to be formed.

in an animal is almost directly proportional to the
amount of
type of fat that contains large numbers of special

strong sympathetic innervation.

Acclimatization greatly affects the intensity of

that have been exposed to a cold environment for
several weeks exhibit a 100 to 500 per cent increase in

increased thermogenesis also leads to a corresponding

the rate of heat production more than 10 to 15 per

do have a small amount

mogenesis can increase the rate of heat production 100

Heat Production.

Cooling the anterior hypothalamic-

preoptic area also increases production of the neu-
rosecretory hormone thyrotropin-releasing hormone

of the hypothalamic portal veins to the anterior pitu-

thyroid-

increased output of

increases the rate of cellular metabolism throughout

occur immediately but requires several weeks’ expo-
sure to cold to make the thyroid gland hypertrophy
and reach its new level of thyroxine secretion.

Exposure of animals to extreme cold for several

weeks can cause their thyroid glands to increase in size

allow themselves to be exposed to the same degree of
cold as that to which animals are often subjected.

important the thyroid mechanism of adaptation to

Isolated measurements have shown that military

personnel residing for several months in the arctic

(Eskimos) also have abnormally high basal metabolic

cold on the thyroid gland may explain the much higher
incidence of toxic thyroid goiters in people who live in

∞F),

drastic changes occur in the rates of both heat loss and

rate of heat loss is greater than that of heat produc-

the body temperature rises and again approaches the

tinually attempt to bring the body temperature back
to this set-point level.

Let us recall

the discussion of feedback gain of control systems pre-

though the environmental temperature might change

feed-

of the temperature control system is equal

to the ratio of the change in environmental tempera-
ture to the change in body core temperature minus

have shown that the body temperature of humans

behavioral control of temperature,

potent. This is

body temperature control, the body has another

Temperature

Behavioral Control of Body

ture and prevents this.

actually “anticipates” a fall in internal body tempera-

duction were increased. Thus, a cold skin temperature

itself is still on the hot side of normal. Here again, one

Figure 73–9. That is, when the skin becomes cold,

A similar effect occurs in shivering, as shown in

low; otherwise, the combined effect of low skin

value of such a system, because it is important that

perature was low. One can readily understand the

temperature was high, sweating began at a lower

ture had fallen to 29°C. Therefore, when the skin

figure, the hypothalamic set-point increased from

decreases. Thus, for the person represented in this

temperatures on the set-point for sweating, showing

Figure 73–8 demonstrates the effect of different skin

This effect is shown in Figures 73–8 and 73–9.

point of the hypothalamic temperature control center.

they contribute? The answer is that they alter the set-

slightly to body temperature regulation. But how do

(spinal cord and abdominal viscera), also contribute

signals from the peripheral areas of the body, espe-

hypothalamic-preoptic area. However, temperature

The critical temperature set-point in the hypothalamus

Set-Point for Core Temperature Control

Skin Temperature Can Slightly Alter the

instance, has a feedback gain of less than 2).

(the baroreceptor arterial pressure control system, for

27), which is

control averages about 27 (28/1.0 – 1.0

environmental temperature. Therefore, the feedback

Body Temperature, Temperature Regulation, and Fever

Chapter 73

897

gain of the total mechanism for body temperature

an extremely high gain for a biological control system

above which sweating begins and below which shiver-
ing begins is determined mainly by the degree of activ-
ity of the heat temperature receptors in the anterior

cially from the skin and certain deep body tissues

that the set-point increases as the skin temperature

36.7°C when the skin temperature was higher than
33°C to a set-point of 37.4°C when the skin tempera-

hypothalamic temperature than when the skin tem-

sweating be inhibited when the skin temperature is

temperature and sweating could cause far too much
loss of body heat.

it drives the hypothalamic centers to the shivering
threshold even when the hypothalamic temperature

can understand the value of the control system,
because a cold skin temperature would soon lead to a
deeply depressed body temperature unless heat pro-

Aside from the subconscious mechanisms for

temperature-control mechanism that is even more

which

36.8 37.0 37.2 37.4

36.0 36.2 36.4 36.6

Evaporative heat loss (calories/second)

Internal head temperature (

Insensible

evaporation

Skin temperature

Set-point

Sweating

(Courtesy Dr. T. H. Benzinger.)

ture determines the set-point level at which sweating begins.

evaporative heat loss from the body. Note that the skin tempera-

Effect of changes in the internal head temperature on the rate of

Figure 73–8

36.6

37.6

36.8 37.0 37.2 37.4

Basal heat
production

Skin temperature (20

)

Heat production (calories/second)

Internal head temperature (

Set-point

Shivering

(Courtesy Dr. T. H. Benzinger.)

determines the set-point level at which shivering begins.

heat production by the body. Note that the skin temperature

Effect of changes in the internal head temperature on the rate of

Figure 73–9

by large granular killer lymphocytes.

macrophages,

phagocytized by the blood leukocytes, by tissue

are present in the tissues or in the blood, they are

When bacteria or breakdown products of bacteria

endotoxins

This is true of many of the bacterial pyrogens, espe-

several hours of latency before causing their effects.

some pyrogens, when injected into the hypothalamus,

Mechanism of Action of Pyrogens in Causing Fever—Role of

in Figure 73–11.

body temperature also approaches this level, as shown

few hours after the set-point has been increased, the

conservation and increased heat production. Within a

body temperature are brought into play, including heat

higher than normal, all the mechanisms for raising the

during disease conditions. When the set-point of the

gens.

pyro-

rise. Substances that cause this effect are called

toxins released from bacterial cell membranes, can

certain other substances, especially lipopolysaccharide

Many proteins, breakdown products of proteins, and

Effect of Pyrogens

Resetting the Hypothalamic Temperature-

terminate in heatstroke.

brain tumors, and environmental conditions that may

sented in Figure 73–10. They include bacterial diseases,

temperature-regulating centers. Some causes of fever

usual range of normal, can be caused by abnormalities

Fever, which means a body temperature above the

Temperature Regulation

Abnormalities of Body

that is, by behavioral control of clothing and by moving

must be regulated principally by the patient’s psychic

In people with this condition, body temperature

control of body temperature.

abdominal receptors still exist. These reflexes are

reflexes originating in the skin, spinal cord, and intra-

body. This is true even though the local temperature

above the sympathetic outflow from the cord, regulation

Regulation of Internal Body Temperature Is Impaired by Cutting

the body.

local signal. Such reflexes can help prevent excessive

ture controller, so that their overall effect is propor-

is, in addition, controlled by the central brain tempera-

and the sweat glands. The

sation of sweating. These reactions are caused by local

occur. Conversely, placing the foot

local sweating

leaves it there for a short time,

When a person places a foot under a hot lamp and

Local Skin Temperature Re

environments.

physiologists have acknowledged in the past. Indeed,

in freezing weather. This is a much more powerful

adjustments to re-establish comfort, such as moving

fore, the person makes appropriate environmental

receptors elicit the feeling of cold discomfort. There-

versely, whenever the body becomes too cold, signals

person a psychic sensation of being overheated. Con-

body temperature becomes too high, signals from the

can be explained as follows: Whenever the internal

Metabolism and Temperature Regulation

898

Unit XIII

temperature-controlling areas in the brain give the

from the skin and probably also from some deep body

into a heated room or wearing well-insulated clothing

system of body temperature control than most

this is the only really effective mechanism to prevent
body heat control breakdown in severely cold

flexes

local vasodilation and

mild
in cold water causes local vasoconstriction and local ces-

effects of temperature directly on the blood vessels and
also by local cord reflexes conducted from skin recep-
tors to the spinal cord and back to the same skin area

intensity of these local effects

tional to the hypothalamic heat control signal times the

heat exchange from locally cooled or heated portions of

the Spinal Cord.

After cutting the spinal cord in the neck

of body temperature becomes extremely poor because
the hypothalamus can no longer control either skin
blood flow or the degree of sweating anywhere in the

extremely weak in comparison with hypothalamic

response to cold and hot sensations in the head region—

into an appropriate warm or cold environment.

Fever

in the brain itself or by toxic substances that affect the

(and also of subnormal body temperatures) are pre-

Regulating Center in Febrile Diseases—

cause the set-point of the hypothalamic thermostat to

Pyrogens released from toxic bacteria or those

released from degenerating body tissues cause fever

hypothalamic temperature-regulating center becomes

Interleukin-1.

Experiments in animals have shown that

can act directly and immediately on the hypothalamic
temperature-regulating center to increase its set-point.
Other pyrogens function indirectly and may require

cially the

from gram-negative bacteria.

and

All these cells digest the bacterial products and then

114

110

106

102

Temperature

efficient in

Temperature

Heatstroke
Brain lesions
Fever therapy

Upper limit
of survival?

regulation

seriously

impaired

regulation

febrile disease,

health, and work

regulation

impaired

regulation

lost

Febrile disease
and hard exercise

Lower limit
of survival?

Usual range
of normal

DuBois EF: Fever. Springfield, IL: Charles C Thomas, 1948.)

Body temperatures under different conditions. (Redrawn from

Figure 73–10

fatal. For this reason, many authorities recommend

sible for many of the effects. In fact, even a few minutes

to the body tissues, especially the brain, and is respon-

The hyperpyrexia itself is also exceedingly damaging

circulatory shock

decreased. These symptoms are often exacerbated by a

by vomiting, sometimes delirium, and eventually loss of

dizziness, abdominal distress sometimes accompanied

The symptoms include

heatstroke.

temperature, into the range of 105° to 108°F, the person

When the body temperature rises beyond a critical

as 85° to 90°F.

heavy work, the critical

rises above about 94°F. If the person is performing

fied or if the body is in water, the body temperature

at 130°F. Conversely, if the air is 100 per cent humidi-

flowing to promote rapid evaporation from the body, a

The upper limit of air temperature that one can stand

begin falling.

assumed that the patient’s temperature would soon

awaited, because once this occurred, the doctor

advent of antibiotics, the crisis was always anxiously

more appropriately, the “flush.” In the days before the

of events in a febrile state is known as the “crisis” or,

because of vasodilation everywhere.This sudden change

hypothalamic-preoptic area, which causes intense

to regulate the temperature to 98.6°F. This situation

is still 103°F, but the hypothalamus is attempting

in Figure 73–11. In this instance, the body temperature

value—perhaps even back to the normal level, as shown

temperature is removed, the set-point of the hypothal-

manner, but at the high temperature set-point level.

thalamic temperature controller is present, the body

but instead feels neither cold nor hot. As long as the

of 103°F. Then the person no longer experiences chills

and the person shivers. Chills can continue until the

body temperature may already be above normal. Also,

chills and feels extremely cold, even though his or her

ture occur. During this period, the person experiences

point of the hypothalamic temperature controller, the

increasing the temperature set-point to a level of 103°F.

Figure 73–11 demonstrates the effect of suddenly

tion), the body temperature usually takes several hours

tissue destruction, pyrogenic substances, or dehydra-

When the set-point of the hypothalamic

hypothalamus by a brain tumor.

ture control. Another condition that frequently causes

hypothermia, occurs, demonstrating both the potency

fever almost always occurs; rarely, the opposite effect,

operates in the region of the hypothalamus, severe

When a brain surgeon

antipyretics.

prostaglandins from arachidonic acid. Drugs such as

fever, because aspirin impedes the formation of

abrogated or at least reduced. In fact, this may be the

tion is blocked by drugs, the fever is either completely

elicit the fever reaction. When prostaglandin forma-

a similar substance, which acts in the hypothalamus to

, or

one of the prostaglandins, mainly prostaglandin E

fever is only a few nanograms.

cytes, can cause fever. The amount of interleukin-1 that

leukocytes, tissue macrophages, and killer lympho-

from bacteria, acting in concert with the blood

charide

one ten-millionth of a gram of endotoxin lipopolysac-

noticeable amount in only 8 to 10 minutes.

fever, sometimes increasing the body temperature a

mus, immediately activates the processes to produce

fluids. The interleukin-1, on reaching the hypothala-

endogenous pyrogen

cyte pyrogen

Body Temperature, Temperature Regulation, and Fever

Chapter 73

899

release the substance interleukin-1—also called leuko-

—into the body

As little as

is formed in response to lipopolysaccharide to cause

Several experiments have suggested that inter-

leukin-1 causes fever by first inducing the formation of

explanation for the manner in which aspirin reduces

aspirin that reduce fever are called

Fever Caused by Brain Lesions.

of the hypothalamic mechanisms for body tempera-
ture control and the ease with which abnormalities of
the hypothalamus can alter the set-point of tempera-

prolonged high temperature is compression of the

Characteristics of Febrile Conditions
Chills.

temperature-control center is suddenly changed from

the normal level to higher than normal (as a result of

to reach the new temperature set-point.

Because the blood temperature is now less than the set-

usual responses that cause elevation of body tempera-

the skin becomes cold because of vasoconstriction,

body temperature reaches the hypothalamic set-point

factor that is causing the higher set-point of the hypo-

temperature is regulated more or less in the normal

Crisis, or “Flush.”

If the factor that is causing the high

amic temperature controller will be reduced to a lower

is analogous to excessive heating of the anterior

sweating and the sudden development of hot skin

Heatstroke

depends almost entirely on whether the air is dry or wet.
If the air is dry and sufficient convection air currents are

person can withstand several hours of air temperature

begins to rise whenever the environmental temperature

environmental temperature

above which heatstroke is likely to occur may be as low

is likely to develop

consciousness if the body temperature is not soon

degree of

brought on by excessive loss

of fluid and electrolytes in the sweat.

of very high body temperature can sometimes be

immediate treatment of heatstroke by placing the

1. Vasoconstriction

Vasodilation
Sweating

2. Piloerection
3. Epinephrine
secretion
4. Shivering

Body temperature (

Time in hours

Chills:

Crisis

Setting of the thermostat
Actual body temperature

105

104

103

102

101

100

Set-point
suddenly

raised to

high value

Set-point
suddenly
reduced to
low value

controller.

Effects of changing the set-point of the hypothalamic temperature

Figure 73–11

tissue thermogenesis. News Physiol Sci 19:67, 2004.

Morrison SF: Central pathways controlling brown adipose

fevers, and possible therapies. Trends Mol Med 8:550,

McDermott MF: Genetic clues to understanding periodic

knockout mice. J Appl Physiol 92:2648, 2002.

Leon LR: Cytokine regulation of fever: studies using gene

917:121, 2000.

of fever and endogenous antipyresis. Ann N Y Acad Sci

Kozak W, Kluger MJ, Tesfaigzi J, et al: Molecular mechanisms

ulation. J Appl Physiol 95:2598, 2003.

Kenney WL, Munce TA: Aging and human temperature reg-

Physiol Sci 19:11, 2004.

Katschinski DM: On heat and cells and proteins. News

tor of fever: synthesis and catabolism. Front Biosci 9:1977,

Ivanov AI, Romanovsky AA: Prostaglandin E

Surg 187:363, 2004.

outcome in elective surgery and trauma patients. Am J

Hildebrand F, Giannoudis PV, van Griensven M, et al:

temic inflammation: roles of purines. Front Biosci 9:1011,

Gourine AV, Dale N, Gourine VN, Spyer KM: Fever in sys-

ulation and biological aging. Physiol Rev 78:339, 1998.

Florez-Duquet M, McDonald RB: Cold-induced thermoreg-

fever. Front Biosci 9:1433, 2004.

Conti B, Tabarean I, Andrei C, Bartfai T: Cytokines and

tivity to temperature. Ann N Y Acad Sci 856:108, 1998.

Boulant JA: Hypothalamic neurons. Mechanisms of sensi-

inflammation: the role of complement. Front Biosci 9:915,

Blatteis CM, Li S, Li Z, et al: Signaling the brain in systemic

111:304, 2001.

action and clinical use in fever suppression. Am J Med

Aronoff DM, Neilson EG: Antipyretics: mechanisms of

procedure.

so that the body’s cells can survive 30 minutes to more

to this extent does not cause tissue damage, but it does

stopped artificially for many minutes at a time. Cooling

water or alcohol on the body. Such artificial cooling has

cooling blankets until the temperature falls. The tem-

cold all the time.

blood to the skin. This mechanism is far less developed

occurs, often manifested by a flush of the skin. This

because of the cold itself, and sudden vasodilation

temperature of tissues falls almost to freezing, the

When the

Frostbite at Almost Freezing Temperatures.

Cold-Induced Vasodilation Is a Final Protection Against

and the frostbitten areas must be removed surgically.

local tissue damage. Often gangrene follows thawing,

crystals in the cells, permanent damage usually results,

ears and in the digits of the hands and feet. If the freeze

This occurs especially in the lobes of the

frostbite.

temperatures, surface areas can freeze; the freezing is

When the body is exposed to extremely low

prevents shivering.

followed by coma), which depresses the activity of the

in body temperature. Also, sleepiness develops (later

when the body temperature falls below about 94°F. Part

regulate temperature is lost; it is greatly impaired even

below about 85°F, the ability of the hypothalamus to

in Figure 73–10, once the body temperature has fallen

Loss of Temperature Regulation at Low Temperatures.

heat, the person’s life can often be saved.

77°F. If warmed rapidly by the application of external

heart standstill or heart fibrillation. By that time, the

Unless treated immediately, a person exposed to ice

Exposure of the Body to

terone by the adrenal glands.

loss of salt in the sweat and urine to almost none; the

sweating, an increase in plasma volume, and diminished

and humid conditions in 1 to 3 weeks.

approaches 100 per cent. A person exposed to heat for

gold mines of South Africa, where the temperature

to acclimatize people to extreme heat. Examples of

sometimes not until several days after the heatstroke.

or more of these organs eventually causes death, but

replaced. Also, damage to the liver, kidneys, and other

Once neuronal cells are destroyed, they can never be

throughout the entire body, but especially in the brain.

The pathological find-

Harmful Effects of High Temperature.

temperature.

the rate of heat production, others have suggested that

uncontrollable shivering, with a considerable increase in

person in a cold water bath. Because this often induces

Metabolism and Temperature Regulation

900

Unit XIII

sponge or spray cooling of the skin is likely to be
more effective for rapidly decreasing the body core

ings in a person who dies of hyperpyrexia are local
hemorrhages and parenchymatous degeneration of cells

organs can often be severe enough that failure of one

Acclimatization to Heat.

It can be extremely important

people requiring acclimatization are soldiers on duty
in the tropics and miners working in the 2-mile-deep

approaches body temperature and the humidity

several hours each day while performing a reasonably
heavy workload will develop increased tolerance to hot

Among the most important physiological changes

that occur during this acclimatization process are an
approximately twofold increase in the maximum rate of

last two effects result from increased secretion of aldos-

Extreme Cold

water for 20 to 30 minutes ordinarily dies because of

internal body temperature will have fallen to about

As noted

of the reason for this diminished temperature regula-
tion is that the rate of chemical heat production in each
cell is depressed almost twofold for each 10°F decrease

central nervous system heat control mechanisms and

Frostbite.

called

has been sufficient to cause extensive formation of ice

such as permanent circulatory impairment as well as

smooth muscle in the vascular wall becomes paralyzed

mechanism helps prevent frostbite by delivering warm

in humans than in most lower animals that live in the

Artificial Hypothermia.

It is easy to decrease the tempera-

ture of a person by first administering a strong sedative
to depress the reactivity of the hypothalamic tempera-
ture controller and then cooling the person with ice or

perature can then be maintained below 90°F for several
days to a week or more by continual sprinkling of cool

been used during heart surgery so that the heart can be

slow the heart and greatly depresses cell metabolism,

than 1 hour without blood flow during the surgical

References

2004.

Pathophysiologic changes and effects of hypothermia on

as a media-

2004.

2002.

metabolism. Curr Opin Clin Nutr Metab Care 6:469, 2003.

van Marken Lichtenbelt WD, Daanen HA: Cold-induced

fever. Ann N Y Acad Sci 994:246, 2003.

Tatro JB, Sinha PS: The central melanocortin system and

856:90, 1998.

Saper CB: Neurobiological basis of fever. Ann N Y Acad Sci

Biosci 9:816, 2004.

antipyretics: neuropeptides and glucocorticoids. Front

Roth J, Zeisberger E, Vybiral S, Jansky L: Endogenous

ture sensation. Nat Rev Neurosci 4:529, 2003.

moTRP channels and beyond: mechanisms of tempera-

Patapoutian A, Peier AM, Story GM, Viswanath V: Ther-

hypothermia for acute stroke. Lancet Neurol 2:410, 2003.

Olsen TS, Weber UJ, Kammersgaard LP: Therapeutic

Body Temperature, Temperature Regulation, and Fever

Chapter 73

901