1

Thermoregulation

Human Physiology

Dr James Betts

Lecture Outline:

• Homeostasis and Thermoregulation

• Temperature Measurement and Heat Balance

• Passive Heat Transfer

• Active Control Mechanisms

• Variance in Internal Body Temperature

• Increased Metabolism and Extreme Environments.

‘‘The The milieu milieu interieurinterieur never varies. never varies.

All the vital mechanisms....have All the vital mechanisms....have

only one object, that of preservingonly one object, that of preserving

constant the conditions of life inconstant the conditions of life in

the internal environment.the internal environment.’’

Claude Bernard,1878Claude Bernard,1878

Internal Body Temperature

• Humans are homeothermic (i.e. body temperature is

maintained independent of environmental temperature)

• Body temperature often described as either:

– ‘Core’ (typically 36.1-37.8˚C)

– ‘Shell’ (ideally 33˚C but up to 42˚C in contracting muscle)

• Specific Sites of Assessment:

– Brain; Aorta; Oesophagus; Aural Cavity; Rectum;

Intestine (Gant et al. November 2006 MSSE).

Heat Balance• Constant internal temperature requires a balance

between heat gain and heat loss

• Even in a thermoneutral environment, basal

metabolism produces 1 kcal⋅kg-1⋅h-1

• The specific heat of human tissue only requires

0.83 kcal⋅kg-1 to raise internal temperature by 1˚C

• Therefore, without heat loss processes, internal

temperature would elevate by 1˚C⋅h-1 even at rest

38˚C

36˚C

34˚C

32˚C

30˚C

28˚C

26˚C

24˚C

22˚C

40˚C

42˚C

‘Core’Temperature -Normal Range

-Intense Shivering and Impaired Coordination

-Violent Shivering; Impaired Thought/Speech

-Decreased Shivering; Erratic Movements; Incoherent

-Muscular Rigidity; Semiconscious

-Unconscious; Cardiac Arrhythmia

-Possible Death due to Cardiac Arrest

Thermoregulation

Absent

-Good Luck!

-Risk of Heat Stroke

-Risk of Death

2

Passive Heat Transfer

• Body heat content usually exceeds 1500 kcal

• Metabolic heat production ≈70-1500 kcal⋅h-1, so

heat transfer with the environment is essential

• However, this operates both ways and overall heat

balance can be expressed as:

Metabolic Rate ± Radiation ± Convection ± Conduction – Evaporation.

Heat Transfer

Heat Gain

Metabolic Heat

Production

Therm

al Radiation

Conduction

Conve

ction

Solar R

adiatio

n

Heat Loss

Convection

Radiation

Sweat Evapo

ration

Respiratory

Evaporation

Conduction

Heat loss at rest in ambient

environmental conditions:

-60% via radiation

-12% via convective air currents

-3% via conduction (e.g. feet to floor)

-25% via evaporation (lungs and skin)

During exercise:

-Up to 80% via evaporation of sweat

from the skin (dependent on humidity).

Active Control Mechanisms

3

The Central Controller

CNS

EvaporationRadiation

ConvectionConduction

CNS

Hypothalamic Thermoreceptors

Peripheral Input

Central Input

Subcutaneous Thermoreceptors

Sweat Command

Shiver Command

Vasomotor CommandCirculatory

Heat

Transfer

Minimal

Fixed Tissue

Conductance

Heat Loss to the Environment

Cutaneous/Subcutaneous Thermoreceptors

-Cold and warm receptors

-‘Cold’ respond to 10 - 38˚C

-‘Warm’ respond to 28 - 45˚C

-10 x more cold sensors than

warm sensors

-Cold sensors are also closer to

the skin surface.

Sensory Homunculus

Functions of the Hypothalamus• Temperature

• Blood pressure

• Heart rate

• Blood osmolarity

• Water intake

• Food intake

• Sleep-wake

• etc…

MAINTENANCE

OF

HOMEOSTASIS

4

Heat Gain Mechanisms

-Venous blood redirected via vena comitantes

-Shivering

-Piloerection

-Upregulated Metabolism

⇓⇓⇓⇓ Ambient/Body Temperature

-Cutaneous Vasoconstriction

Sympathetic Stimulation

-Behavioural Responses.

Heat Loss Mechanisms

-Sweating

-Panting

-Downregulated Metabolism

⇑⇑⇑⇑ Ambient/Body Temperature

-Cutaneous Vasodilation

Sympathetic Stimulation

-Behavioural Responses.

Variance in Internal Body Temperature

-Range of ≈≈≈≈1˚C over 24 hours

-Lowest 4-6 am

Circadian Variation

-Highest 6-8 pm

Endocrine status can alter

circadian variation:

-e.g. Estrogen and progesterone

release on day 14 of the

menstrual cycle cause internal

temperature to be 0.5-0.75˚C

higher at baseline.

Variation with Age

• Infants have a limited thermoregulatory response

– Partly due to the immaturity of the CNS but also due to

a relatively large surface area to body mass ratio

• The elderly are also vulnerable to variations in

environmental temperature

In the cold- Inadequate catecholamine release contributes

to a poor vasoconstrictor reflex

In the heat- Compromised cardiac output results in

impaired peripheral blood flow.

Exercise in Hot Environments

• Active muscle and skin compete for blood supply

• Internal temperature increases

• Sweating increases

• Blood (plasma) volume is reduced

• Stroke volume is reduced

• Compensatory rise in heart rate (Cardiovascular Drift).

5

% Body Mass Loss1 2 3 4 5

Increase in Oesophageal Temp (oC)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Montain et al. (1992)

r = 0.98

Time (min)20 60 80 120

Blood Volume (% baseline)

-8

-6

-4

-2

0

No Fluid Intake

Fluid Intake

Hamilton et al. (1999)

P <0.05*

*

***

Time (min)20 60 80 120

Stroke Volume (ml.beat-1)

120

130

140

150

160

No Fluid Intake

Fluid Intake

Hamilton et al. (1999)

P <0.05*

**

Time (min)20 60 80 120

Heart Rate (beats.min-1)

130

135

140

145

150

155

160

No Fluid Intake

Fluid Intake

Hamilton et al. (1999)

P <0.05*

*

*

Time (min)20 60 80 120

Cardiac Output (l.min-1)

19

20

21

22

23

No Fluid Intake

Fluid Intake

Hamilton et al. (1999)

P <0.05*

**

% Body Mass Loss1 2 3 4 5

Change in Forearm Blood Flow (ml. 100 ml-1.min-1)

-2

-1

0

1

2

3

Montain et al. (1992)

r = 0.99

6

Exercise in Cold Environments

• Peripheral vasoconstriction results in increased

muscle fatigability and reduced skin sensitivity

• Reduced temperature of nervous tissue reduces the

conduction velocity of nerve impulses

• Thermal conductivity of water is 26 x that of air

• Skin temperatures between -3.7 and -4.8˚C result in

the formation of intracellular ice crystals.

• The human body operates within a narrow range

of physiological temperatures

• There is a constant transfer of heat between the

body and its environment

• Heat balance is actively controlled via a complex

system of negative feedback

• Extreme conditions of thermal stress can exceed

the body’s thermoregulatory capacity.

Summary

J.Betts@bath.ac.uk