53.4 Body Temperature Control

Question 1

What are some normal variations in body temperature?

Answer 1

*Circadian rhythms
*Menstrual cycle.

Question 2

What is the normal range of body temperature?

Answer 2

36oC-37.5oC

Question 3

How does body temp change in the circadian rhythm?

Answer 3

• Variations of around 1oC
• Lowest temp from 3-6am
• Highest temp from 3-6pm

Question 4

How does body temp change in the menstrual cycle?

Answer 4

• Rise of ~0.5 - 1 oC seen after ovulation

Question 5

What are the two compartments of the body in temp regulation?

Answer 5

• Core
• Peripheral shell

Question 6

What is the difference between the core and peripheral temperatures?

Answer 6

• Important for core temp to stay constant - for optimal enzymatic reactions
• Whereas peripheral temp is highly variable - changes depdendent on environmental conditions. Can drop as low as 21oC in fingers and toes

Question 7

Why is (core) body temperature so tightly regulated?

Answer 7

• To provide optimal conditions for cellular enzymatic reactions
  
  • When body temperature exceeds 42*C, cellular proteins are damaged
  • If the temperature drops, these enzymes and proteins become less efficient/effective
  • Energy expenditure falls 13% for every 1*C drop in core temperature (as drop in body temperature results in decreased enzymatic rates)

Question 8

What type of organ is brown adipose tissue?

Answer 8

Thermogenic

Question 9

How does brown adipose tissue underlie thermogenesis?

Answer 9

• Rich in mitochondria and uncoupling protein 1
• UCP-1 dissipates the proton gradient across the inner mitochondrial membrane
• Energy released as heat

Question 10

When is brown adipose tissue thermogenesis important?

Answer 10

For neonates that cannot shiver yet

Question 11

What is the definition of hyperthermia?

Answer 11

Sustained high body temp above 40.6oC

Question 12

What is the definition of hypothermia?

Answer 12

Sustained low temp below 35oC

Question 13

What are some causes of hyperthermia?

Answer 13

• Febrile crisis
• Malignant hyperthermia (e.g. suxamethonium)
• Overheating

Question 14

What are consequences of hyperthermia?

Answer 14

• Heat exhaustion, heat stroke
• Fibrinolysis –> disseminated intravascular coagulation (DIC), haemorrhage
• Damage to membranes –> rhabdomyolosis (muscle cells release contents), myocardial necrosis
• High brain temp –> CNS disturbances
• Loss of consciousness, death

Question 15

What are some causes of hypothermia?

Answer 15

More to do with environment

• Cold water
• Cold weather

Question 16

What are consequences of hypothermia?

Answer 16

• Death and decay of tissue (gangrene) - interrupted blood flow
• Freezing of body tissue (frostbite)

Question 17

What are the peripheral injuries caused by extreme heat and cold respectively?

Answer 17

• Frostbite
  -Burns

Question 18

What are the four processes that cause heat loss?

Answer 18

• Convection
• Evaporation
• Conduction
• Radiation

Question 19

What is conduction?

Answer 19

Transfer of heat through direct contact with objects

Question 20

What is convection?

Answer 20

Removal of heat from body by air currents

Question 21

What is evaporation?

Answer 21

Heat loss through conversion of water in sweat as a liquid to a gas (requires energy to overcome latent heat of vaporisation, which is released to environment)

Question 22

What is radiation?

Answer 22

Heat loss through emission of infrared rays

Question 23

Which process accounts for least heat loss?

Answer 23

Conduction

Question 24

Which process accounts for most heat loss?

Answer 24

Radiation

Question 25

How do neurons in the anterior/preoptic hypothalamus respond to cold? What are the effects?

Answer 25

- Fall in temp detected by cold-sensitive neurons --> TSH released to anterior pit, stimulates thyroxine release to increase metabolism - Vasoconstriction in skin and piloerection (goose bumps) - Shivering - Behavioural response - seek warmth, more clothes

Question 26

How do neurons in the anterior/preoptic hypothalamus respond to hot temp? What are the effects?

Answer 26

- Rise in temp detected by warm-sensitive neurons --> reduces TSH, metabolism slowed - Blood shunted to periphery to dissipate heat - Sweating - Behavioural response - seek shade

Question 27

How are changes in temperature sensed?

Answer 27

* Primarily through nervous feedback systems from the periphery to the CNS * Centre in the brain: temperature regulating centers in the hypothalamus

Question 28

Where is temperature centrally controlled?

Answer 28

Hypothalamus

Question 29

What is the 'thermostat' for the body?

Answer 29

* The preoptic and anterior hypothalamus operates as the body's thermostat * This part of the brain has greater sensitivity to heat than to cold

Question 30

Where are the peripheral temperature sensors located and what are they more sensitive to?

Answer 30

* Spinal cord, abdominal viscera and great veins (**deep body sensors**) * Skin * Greater sensitivity to cold than hot

Question 31

How can heat loss be increased without the presence of a thermal gradient?

Answer 31

Through sweat production and its subsequent evaporation facilitates heat loss from the body, particularly in hot conditions

Question 32

How can sweat production be increased? What properties does this secretion have?

Answer 32

* Sweat production can be increased by cholinergic sympathetic nerve fibres * The glands can also be stimulated by adrenaline/noradrenaline, e.g. during exercise to increase heat dissipation * This elicits the secretion of a primary fluid into the sweat duct * This fluid has a composition similar to that of plasma, just without plasma proteins * As regulation/reabsorption occurs along the tubule, if the demand for secretion increases and therefore the transit time decreases, then the time over which this secretion can be modified is rapidly decreased

Question 33

Are the following more sensitive to hot or cold stimuli? 1. Preoptic and Anterior Hypothalamus 2. Peripheral temperature receptors

Answer 33

1. Hot 2. Cold

Question 34

Which part of the hypothalamus controls temperature?

Answer 34

Anterior hypothalamus and preoptic area (sometimes grouped together)

Question 35

What is the thermoneutral zone and its values?

Answer 35

- The temp range over which there is no 'active' heat generation or loss - 25-30oC

Question 36

How does light clothing affect the thermoneutral zone?

Answer 36

- Reduces loss of heat from body - Reduces thermoneutral zone range to 18-22oC

Question 37

What are some adaptations seen in cold acclimatisation?

Answer 37

- Increased thermogenesis with less shivering - Blunted cutaneous vasoconstriction response - Behavioural: clothing, shelters, posture, activity - piloerection

Question 38

What are some adaptations seen in exercise heat acclimatisation?

Answer 38

- Improved sweating - Improved skin blood flow - Lowered body temp - Reduced cardiovascular strain - Enhanced cellular protection

Question 39

How do NSAIDs and paracetamol act as antipyretics (reduce fever)?

Answer 39

Reduce production of prostaglandins So they cannot signal to hypothalamus to raise set-point

Question 40

How does fever occur following infection?

Answer 40

- Body releases pyrogens - e.g. cytokines, prostaglandins, thromboxane - Pyrogens induce COX2 to convert arachidonic acid to prostaglandin E2 (PGE2) - PGE2 binds to E2 receptors in the anterior hypothalamus - raises set-point - Elevated set-point means body works harder to achieve a higher internal temperature - Sweating, vasodilation, hot skin

Question 41

What is the definition of the set-point for temperature control?

Answer 41

The point above or below which there are intense changes that bring about heat loss or production

Question 42

What is the mechanism by which PGE2 binding to its receptors raises the set-point?

Answer 42

PGE2 binds to GqPCR in warm neurons --> increases cAMP sythesis and release cAMP acts as a neurotransmitter --> activates thermostatic neurons Increases set point

Question 43

Which drugs can act as antipyretics?

Answer 43

- NSAIDs - Paracetamol (Acetaminophen)