L23: Thermoregulation

Question 1

Ways of heat gain and heat loss in body

Answer 1

Gain:
Basal Metabolism
Activity
Thermogenesis
Heat transfer from environment

Loss:
Heat transfer to environment

Question 2

Ways of transferring heat from skin

Answer 2

1. Conduction
2. Convection (velocity of air movement)
3. Radiation (Infrared)
4. Evaporation of water (humidity)

Question 3

Neural feedback mechanism in thermoregulation

Answer 3

Sensor
Skin: Peripheral thermoreceptor (cold receptors > warmth receptors; face and hands)
Core: Central thermoreceptor
1. Hypothalamus receptor:
- Anterior hypothalamic-preoptic area: heat sensitive>cold sensitive
- Posterior hypothalamus: cold sensitive
2. Deep tissues e.g. abdominal viscera, spinal cord, great veins: mainly cold receptors

Peripheral and Deep tissue: preventing hypothermia (more cold receptor)

Controller
Temperature-regulating centre (Posterior hypothalamus: thermal set point)

Effector
Physiological + Behavioural response

Question 4

Mechanisms of physiological and behavioural response in thermoregulation

Answer 4

Physiological

1. Vasoconstriction / vasodilatation (sympathetic stimulation)
2. Piloerection
3. Sweating
4. Shivering
5. Epinephrine / Norepinephrine
6. Thyroxine / Thyroid-stimulating hormone / Thyrotropin-releasing hormone (slow)
7. Chemical thermogenesis by catcholamine / brown fat-mediated

Behavioural

1. Clothing
2. Voluntary activity
3. Appetite
4. Choice of surrounding
5. Curling

Question 5

Response to heat

Answer 5

Detected by thermoregulatory centre

1. Physiological response
   - increase heat loss (vasodilatation of cutaneous arterioles by inhibiting sympathetic input)
   - decrease heat production
2. Behavioural response
   - decrease voluntary activity, reduction in clothing

Question 6

Response to cold

Answer 6

Detected by thermoregulatory centre

1. Physiological response
   - decrease heat loss (vasoconstriction of cutaneous arterioles by stimulating sympathetic input (core to skin) + piloerection (skin to surroundings))
   - increase heat production (shivering + non-shivering thermogenesis)
2. Behavioural response
   - increase voluntary activity, more clothing

Question 7

Mechanism of shivering

Answer 7

Activated by cold centre, inhibited by heat centre

Primary motor centre (dorsomedial portion of posterior hypothalamus)
—> Bilateral tracts down brain stem
—> Lateral column of spinal cord
—> Anterior motor neurone

Question 9

Process of pyrexia

Answer 9

Pyrogens
—> Pyrogenic cytokines
—> Prostaglandins (PGE2) from hypothalamus
—> elevated thermal set point
—> increase heat production, decrease heat loss
—> chills and rigors
—> vasoconstriction, piloerection, epinephrine secretion, shivering

Question 10

Heat exhaustion vs Heat stroke

Answer 10

Heat exhaustion: less severe, dehydration —> weakness, nausea, fainting

Heat stroke:
increased body temp to limit —> thermoregulatory centre depressed —> reduce sweating —> continue increase in body temp (41-42oC)
1. Salt and water depletion
2. Decreased blood flow to organs e.g. brain, kidneys, visceral region
3. CNS symptoms: fatigue, confusion, unconsciousness

Question 11

Hypothermia

Answer 11

Below 35oC (lethal below 32oC)

Question 12

Acclimatisation to heat

Answer 12

1. Rate of sweating increase
2. Plasma volume increase
3. Aldosterone increase —> NaCl conc in sweat and urine decrease

Question 13

Fever vs Hyperthermia

Answer 13

Fever: elevation of thermal set point

Hyperthermia: increase of body temp above thermal set point