Temperature Regulation

Dr Elizabeth J

Prof & HOD

Dept of Physiology

AIMS &RC , Bangalore

4 / 09 /14

Scheme• Introduction• General Considerations• Mechanisms of heat loss & heat gain• Control of heat production, by radiation,

conduction & evaporation• Skin &Temperature regulating reflexes• Thermoneutral Zone• Control mechanisms of temperature regulation• Fever & Hyperthermia• Heat exhaustion, heat stroke & hypothermia

Introduction• Homeothermic- Humans capable of maintaining

their body temperatures within narrow limits.• Poikilothermic- (Cold blooded) • Biochemical reactions do not fluctuate due to the

constant & high temperatures.• 410 C (1060F) – 430C convulsions are seen• Nerve malfunction & protein denaturation seen

with higher temperature.

General Considerations

Temperature can be expressed as 0C or 0F.

C = ( F - 32) x 5/9 and F = (C x 9/5) + 32

Normal is 370C or 98.60F

• Measured under tongue, axilla or rectum• Oral temp is 0.50C less than core body temperature (rectal temp).

• Internal temp varies with activity pattern and changes in ext temp.

• Circadian fluctuation of about 10C - lowest at night and highest during the day.

• Women show higher temp during second half of menstrual cycle

General Considerations

Considering temp as a measure of heat concentration

Total heat content gained / lost by the body

is determined by NET DIFFERENCE

in heat produced and lost

STEADY STATE- heat production must equal heat loss

Mechanisms of heat loss / heat gain

• Radiation- Surface of all objects emit heat in form of Electromagnetic radiation.

Rate of emission α Temperature

• Conduction- Loss/ gain by transfer of thermal energy during collisions between adjacent molecules– Heat is conducted from Molecule to Molecule

Mechanisms of heat loss / heat gain

• Convection- Conductive heat loss or gain is aided by movement of air / water next to body.-Cool air replacing warm air.

• Evaporation- Water evaporates from skin & membrane lining the respiratory tract MAJOR

PROCESS for Loss of body heat. • 600Kcal/L reqd to transfer water from fluid to

gaseous state.- Heat reqd to evaporate water from the surface is conducted

from the surface cooling it.

• Skin is not a perfect insulator• Temp of outer surface of skin is between external temp &

core temp. (net conduction = 0)• Skin acts as insulator by change in blood flow.• The heat from within is brought out by the blood and lost

to the outside.• Vasoconstrictor sympathetic nerves firing rates

– in response to cold &

– in response to heat

SKIN & ITS ROLE

Temperature – Regulating reflexes

• Thermoreceptors detect changes in the balance between heat loss & production due to metabolic rate (exercise).

• Two types– Peripheral on Skin– Central in hypothalamus (integrating center), spinal cord,

abdominal organs.

• Output from hypothalamus is sent to effectors via

– sympathetic nerves to sweat glands,– skin arterioles & adrenal medulla.– Motor neuron to skeletal muscles.

• Core temp is maintained relatively constantly.• Peripheral thermoreceptors help identify heat & cold.

Thermoneutral zone(TNZ)

• All the mechanisms are Graded and not All or None responses.

• 250C to 300 C or 750F to 860F is known as a thermoneutral zone.

• At temps lower than this TNZ max vasoconstriction cannot prevent heat loss from exceeding heat production & this requires the body to increase heat production.

• At temps above this TNZ max vasodilation cannot eliminate the heat as fast as it is produced & this requires another heat loss mechanism ‘SWEATING’

Control Mechanisms of Temperature Regulation

• Nervous Mechanisms – Thermoregulatory centers

• Direct Action• Reflex Mechanisms• Efferent Nerves

– Endocrine and Temperature Control• Adrenal Medulla• Adrenal Cortex• Thyroid

• Behavioral & Voluntary control

Scheme• Introduction• General Considerations• Mechanisms of heat loss & heat gain• Control of heat production, by radiation,

conduction & evaporation• Skin &Temperature regulating reflexes• Thermoneutral Zone• Control mechanisms of temperature regulation• Fever & Hyperthermia

• Heat exhaustion, heat stroke & hypothermia

Hypothalamus control• Warming of anterior hypothalamus (Heat loss)

– Vasodilatation– Sweating– Hyperpnoea– Injury abolishes the heat loss responses to hot environment.

• Stimulation of Posterior hypothalamus (Heat production)– Vasoconstriction– Injury abolishes responses to cold & interferes with the

responses to heat.

• Preoptic region of Ant. Hypothalamus is regarded as the thermostat.

• SET POINT is maintained by this region

Hypothalamic temperature Regulation (contd)

• Direct action:

When environmental temp – Is high, warm blood flowing through hypothalamus

causes HEAT LOSS responses.

– Is low, cool blood causes HEAT PRODUCTION / CONSERVATION responses.

Hypothalamic temperature Regulation (contd)• Reflex Mechanisms

– Sensitive thermoreceptors • in the skin carry information via cutaneous nerves

and hypothalamus– Efferent Nerves

• Autonomic

– Sympathetic adrenergic vasomotor nerves (cutaneous vasoconstriction & vasodilatation)

– Sympathetic cholinergic nerves to sweat glands• Somatic

– Nerves to skeletal muscle ( tone, activity, shivering)

– Nerves to respiratory muscles

Endocrine control• Adrenal medulla-

– immediate adrenaline release (Calorigenic)

– Exposure to cold leads to• Cutaneous vasoconstriction the heat loss• metabolic rate & heat production

• Adrenal Cortex– BMR is low in adrenal cortical insufficiency– Patients do not tolerate cold well & body temp is subnormal.

• Thyroid– Calorigenic– Permissive action on adrenaline calorigenesis– Hyperthyroidism-skin is warm

Behavioral and Voluntary Control

• Animals move from warm to cold regions

• Curling up of body in cold conditions

• Clothing in woolens in winter and thin cotton clothing in summer.

• Fans, air conditioners, heaters & central heating

Control of heat production

Response to exposure to cold

Shivering thermogenesis- This contains rhythmical

oscillatory muscle contractions.- No external work is involved, all

the energy liberated by the metabolic machinery appears as internal heat.

Non shivering Thermogenesis- Increase in (metabolic rate) heat

production not due to muscular activity.

- Increased epinephrine, sympathetic activity to adipose tissue & contribution of thyroid hormone.

Temperature regulating mechanisms

Skin temperature

Cerebral cortex

Core temperature

Peripheral thermoreceptors

Hypothalamus

Adrenal medulla Sweat glands Skin arterioles Skeletal muscles

Central thermoreceptors

Epinephrine

Via sympathetic nerves“Involuntary” motor responses

Via Motor nerves

“ Voluntary” motor responses

Effector mechanisms in Temperature Regulation

STIMULATED BY COLD

Desired Effect Mechanism

Decreased Heat Loss

Vasoconstriction of skin vessels

Reduction of surface area (curling up)

Behavioral responses( warm clothes)

Increased Heat Production

Increased muscle tone

Shivering & increased voluntary activity

Epinephrine secretion (minimal)

Increased appetite

Effector mechanisms in Temperature Regulation

STIMULATED BY HEAT

Desired effect Mechanism

Increased Heat Loss

Vasodilation of skin vessels

Sweating

Behavioral response

Decreased Heat Production

Decreased muscle tone

Decreased secretion of epinephrine (minimal)

Decreased Appetite

Fever & Hyperthermia

• Fever is an elevation of body temperature due to a “resetting of the thermostat”

• > 990F• Bacterial / Viral Infections, trauma,lesions of CNS,

exposure to high temperatures & drug induced.• Increased heat production by shivering (rigor) &

increased metabolism • Diminished heat loss by vasoconstriction• Skin is warm & flushed• Subsides by sweating

INFECTION

LIVER

MACROPHAGES

IL-1, IL-6

Firing of neural receptors

MULTIPLE ORGANSMACROPHAGES

IL-1, IL6 HYPOTHALAMUS

TEMP SET POINT

Skeletal Muscles

Shivering Curling up

SKIN ARTERIOLES

VASOCONSTRICTION

HEAT PRODUCTION HEAT LOSS

Heat production greater than heat loss

Heat retention

BODY TEMPERATURE

Heat Exhaustion

• Heat waves in a humid atmosphere• Increased sweating with water and salt loss.• Headache, dizziness, cramps, vomiting and

rise in temperature.• Skin is cold & clammy• Fall in BP, circulatory collapse & stupor.• Treated by fluids & electrolytes

Heat Stroke

• Serious Condition,high environmental temp• Overheating of body, impaired sweating• Headache, restlessness & mental confusion• Hyperpyrexia (410C or 1060F)• Delirium, convulsions,CV collapse & COMA• Death results if untreated• Temp to be brought down to 1020C with ice

packs

Hypothermia

• Reduction in temp < 350C• At 270C the metabolism is greatly reduced

(<280C unable to correct by itself)

• HR,BP & RR are decreased• Unconscious state• Exposure to low temps• Cardiac surgery where heart is stopped