Temperature regulation: fundamental aspects

Question 1

What is core body temperature?

Answer 1

The temperature of the body around the main organs (37 ºC).

Rather than a single core body temperature, healthy individuals show a range of normal temperatures.

Question 2

How is core body temp maintained?

Answer 2

Within a narrow range, in contrast with skin temperature

Question 3

What is the normal range of core body temp at rest?

Answer 3

36 - 37.5 ºC

Question 4

What happens if the core body temp goes above and below 37 ºC?

Answer 4

Above = hyperthermia
Below = hypothermia

Question 5

What other processes is a range of temperature associated with?

Answer 5

1) Circadian rhythm - changes during 24 hr period; during daytime, Tc fluctuates but during sleep Tc decreases
2) Menstrual cycle - changes during days of month

Question 6

How does Tc measured orally and rectally differ?

Answer 6

Temperature measured orally are about 0.5 ºC lower than rectally

Question 7

What is the range of atmospheric pressure the Tc can remain stable in?

Answer 7

Between 20 and 30 ºC

Question 8

How can Tc be measured?

Answer 8

1) Oral: simple, non-invasive, but can underestimate Tc
2) Aural: fast, can be uncomfortable and underestimate Tc
3) Rectal: continuous, slow, close to Tc
4) Oesophageal: continuous, slow, close to Tc (affected by food and drink)
5) GI tract: ‘temperature pill’ radio transmitter continuous, varies along tract. Has to be ‘retrieved’

Question 9

What generates heat within the body?

Answer 9

Metabolism - energy is required to perform all the chemical reactions of the body at rest

Question 10

What makes up the basal metabolic rate?

Answer 10

The sum of the sleeping metabolic and arousal

= 60% of daily energy usage

Question 11

What does the % daily usage consist of?

Answer 11

• Sleeping metabolic rate
• Arousal
• Basal metabolic rate (60%)
• Thermic effect of food (7%)
• Nonexercise activity (8%)
• Purposeful physical activity (25%); can greatly affect DU

Question 12

What is the basal metabolic rate?

Answer 12

This minimum level of energy required to live and generates heat

Question 13

What is the MET?

Answer 13

Number of times your BMR is increased by e.g. increase BMR by 10 if running at 10 METs, but Tc doesn’t increase

Question 14

During exercise, how is the majority of heat generated?

Answer 14

Skeletal muscle contraction

Question 15

What’s the % of energy used for skeletal muscle contraction?

Answer 15

Up to about 25%* is converted to mechanical work (e.g. walking, swimming). The rest of the energy is lost as heat.

*varies between animals, amphibians, etc.

Question 16

How is temp regulated to avoid increasing too much?

Answer 16

The body has active control of heat loss

Question 17

Why does heat loss = heat production?

Answer 17

To maintain a steady temp

Question 18

What are some modes of heat transfer?

Answer 18

1) Radiation (60%) - infrared wavelength; heat waves
2) Convection - air currents
3) Conduction; to air (15%), to objects (3%)
4) Evaporation (22%)

Question 19

What is radiation?

Answer 19

• Is in the infrared wavelength, longer than those of the visible spectrum
• Meaured using infrared camera
• Temp is very hot where red (closer to Tc) and very cold where dark

Question 20

What is convection?

Answer 20

• A gravitationally-induced heat transport, driven by the expansion of air or fluid on heating
• The hot expanded air has lower density, so will rise to the top of colder, and therefore denser air
• E.g. oven with heating source at the bottom and heat travels upwards

Question 21

What happens in forced convection?

Answer 21

A strong air (e.g. wind chill effect) or liquid flow can increase heat loss markedly.

Question 22

What is conduction?

Answer 22

Little heat is lost by conduction normally, as still air (in clothing) is a poor conductor - layer of clothing traps some air between clothes and body and air doesn’t conduct heat well

E.g. material to conduct heat (candle) which heats up metal stick; heat transfers from tip to hand of individual

Question 23

What is the thermal conductivity of water?

Answer 23

25x that of air

e.g. animal that lives in water - needs thick layer of fat instead of fur to isolate organs from cold
animal that live on land - thick layer of fur protects temp better

Question 24

What can happen in cold water immersion 10 ºC?

Answer 24

Can lead to death in 2 hours

Question 25

Summarise the first three modes of heat transfer

Answer 25

1) Convection = gravitational gradient effect - heat travels upwards 2) Conduction = need some material that conducts the heat from the heat source, away from the heat source 3) Radiation = occurs in any direction - heat that travels with waves

Question 26

What is evaporation?

Answer 26

Water evaporates insensibly from the skin and lungs throughout the day; this heat loss cannot be controlled for purposes of temperature regulation. E.g. breath in room air that gets heated up as it travels down the resp tract, breathe out = air comes out humidified (form of heat loss)

Question 27

How much energy does evaporation of 1 litre of water need?

Answer 27

2,400 kJ of energy

Question 28

What is the daily insensible water loss

Answer 28

About 800 ml, thus accounts for ~20 Watt

Question 29

What does sweating increase?

Answer 29

Heat loss to about 20 times basal metabolic rate (2,000 Watt) with loss of up to 3 litres of water per hour

Question 30

What is the basic closed loop: modern cruise control?

Answer 30

Analogy: car maintains speed with CC - Going uphill = actual speed initially declines - Sensors in car which measure reduction in speed (measure difference between actual and desired speed) - These inform the control centre (accelerator) to give more fuel to the engine - Engine develops a greater torque - This makes the wheels spin more rapidly so that the car maintains a speed close to the desired one

Question 31

What are some generic errors in the feedback loop?

Answer 31

- Alterations in setpoint (includes too much gain) - Inaccurate sensitivity of receptors and effectors and can have limits which beyond, don't work normally - Delays can occur in each step e.g. from receptor to control centre

Question 32

What are some components in normal negative feedback in thermoregulation?

Answer 32

1) Receptors (measures) - Hypothalamus - Skin - Spinal cord - Abdominal viscera - Great veins 2) Control centre (compares) - Hypothalamus 3) Effectors (responds) - Skin blood vessels - Muscles - Hair - Sweat glands - Brown adipose tissue

Question 33

What is the negative feedback loop?

Answer 33

Restores to homeostasis

Question 34

What is the hypothalamus?

Answer 34

Lies in the walls of the 3rd ventricle just about the pituitary gland

Question 35

What are the preoptic and posterior areas of the hypothalamus?

Answer 35

Preoptic area = contains cells which respond to local change in temp (most respond to warming) Posterior area = contains cells which respond to distant change in temp (e.g. skin). This looks like a central controller.

Question 36

What increases with exercise intensity?

Answer 36

Cardiac output - blood is reditributed, with skeletal muscle receiving a large portion

Question 37

What is the difference in cardiac output during rest and heavy exercise?

Answer 37

``` Rest = ~6 L/min Exercise = ~18 L/min ``` 3x increase

Question 38

What is the difference in skin blood flow during rest and heavy exercise?

Answer 38

``` Rest = 500 mL Exercise = 1900 mL ``` 4x increase = can lose heat by warm blood travelling close to the skin surface, so that it can dissipate heat by the skin Skin blood flow can also increase for thermoregulation.

Question 39

What is the thermaoneutral zone (TZ)?

Answer 39

Defined as the range of ambient temperatures in which humans employ vasomotor control alone to maintain a constant core body temp; no swearing or shivering occurs

Question 40

What is skin deep thermoregulation?

Answer 40

Skin = largest ‘specialised organ’ in the body (~8 kg) - arteriovenous anastomosis is present only in apical areas - blood flow in the skin can range from barely above 0 to more than 30% of CO - skin receptors can adapt markedly

Question 41

What is ateriovenous anastomosis?

Answer 41

????

Question 42

What is horripilation?

Answer 42

The erection of hairs on the skin due to cold, fear, or excitement Fur and feathers thickness is important as an air-trapping mechanism in animals, but gives only goose-pimples in man

Question 43

What is the countercurrent exchange mechanism?

Answer 43

- arteries transport warm blood deep in the arms and legs - deep veins are positioned alongside arteries In the cold, blood returns through deep veins, heat transferring from the arteries to the veins and is conserved via a countercurrent exchange mechanism. In situations where we want to lose heat, the deep veins vasoconstrict and more blood flows through the superficial veins.

Question 44

What is the role of sweat glands?

Answer 44

A primary protein-free secretion is formed by the glandular portion Water and salt are reabsorbed while passing grouch the duct, depending on the rate of sweating

Question 45

How do sweat glands function at low and high sweating rates?

Answer 45

Low: Much of the salt and water are reabsorbed Sweat is rich in urea, lactate and potassium High: More salt than water is reabsorbed

Question 46

What can alter the set point (or range) of temperature?

Answer 46

Skin temperature alters the set point and can anticipate the change in core temp

Question 47

What happens when skin temperature increases and decreases?

Answer 47

Increases: Sweating starts at a lower hypothalamic temp (sweat point is reduced) Decreases: Sweating starts at a higher hypothalamic temperature (set point is raised)

Question 48

What happens when the skin is warm and cold?

Answer 48

Warm: Shivering stats at a lower core temp (set point is reduced) Cold: Shivering starts at a higher core temp (set point is raised)