Temperature regulation: fundamental aspects Flashcards
What is core body temperature?
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.
How is core body temp maintained?
Within a narrow range, in contrast with skin temperature
What is the normal range of core body temp at rest?
36 - 37.5 ºC
What happens if the core body temp goes above and below 37 ºC?
Above = hyperthermia Below = hypothermia
What other processes is a range of temperature associated with?
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
How does Tc measured orally and rectally differ?
Temperature measured orally are about 0.5 ºC lower than rectally
What is the range of atmospheric pressure the Tc can remain stable in?
Between 20 and 30 ºC
How can Tc be measured?
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’
What generates heat within the body?
Metabolism - energy is required to perform all the chemical reactions of the body at rest
What makes up the basal metabolic rate?
The sum of the sleeping metabolic and arousal
= 60% of daily energy usage
What does the % daily usage consist of?
- Sleeping metabolic rate
- Arousal
- Basal metabolic rate (60%)
- Thermic effect of food (7%)
- Nonexercise activity (8%)
- Purposeful physical activity (25%); can greatly affect DU
What is the basal metabolic rate?
This minimum level of energy required to live and generates heat
What is the MET?
Number of times your BMR is increased by e.g. increase BMR by 10 if running at 10 METs, but Tc doesn’t increase
During exercise, how is the majority of heat generated?
Skeletal muscle contraction
What’s the % of energy used for skeletal muscle contraction?
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.
How is temp regulated to avoid increasing too much?
The body has active control of heat loss
Why does heat loss = heat production?
To maintain a steady temp
What are some modes of heat transfer?
1) Radiation (60%) - infrared wavelength; heat waves
2) Convection - air currents
3) Conduction; to air (15%), to objects (3%)
4) Evaporation (22%)
What is radiation?
- 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
What is convection?
- 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
What happens in forced convection?
A strong air (e.g. wind chill effect) or liquid flow can increase heat loss markedly.
What is conduction?
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
What is the thermal conductivity of water?
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
What can happen in cold water immersion 10 ºC?
Can lead to death in 2 hours
Summarise the first three modes of heat transfer
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
What is evaporation?
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)
How much energy does evaporation of 1 litre of water need?
2,400 kJ of energy
What is the daily insensible water loss
About 800 ml, thus accounts for ~20 Watt
What does sweating increase?
Heat loss to about 20 times basal metabolic rate (2,000 Watt) with loss of up to 3 litres of water per hour
What is the basic closed loop: modern cruise control?
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
What are some generic errors in the feedback loop?
- 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
What are some components in normal negative feedback in thermoregulation?
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
What is the negative feedback loop?
Restores to homeostasis
What is the hypothalamus?
Lies in the walls of the 3rd ventricle just about the pituitary gland
What are the preoptic and posterior areas of the hypothalamus?
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.
What increases with exercise intensity?
Cardiac output - blood is reditributed, with skeletal muscle receiving a large portion
What is the difference in cardiac output during rest and heavy exercise?
Rest = ~6 L/min Exercise = ~18 L/min
3x increase
What is the difference in skin blood flow during rest and heavy exercise?
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.
What is the thermaoneutral zone (TZ)?
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
What is skin deep thermoregulation?
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
What is ateriovenous anastomosis?
????
What is horripilation?
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
What is the countercurrent exchange mechanism?
- 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.
What is the role of sweat glands?
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
How do sweat glands function at low and high sweating rates?
Low:
Much of the salt and water are reabsorbed
Sweat is rich in urea, lactate and potassium
High:
More salt than water is reabsorbed
What can alter the set point (or range) of temperature?
Skin temperature alters the set point and can anticipate the change in core temp
What happens when skin temperature increases and decreases?
Increases:
Sweating starts at a lower hypothalamic temp (sweat point is reduced)
Decreases:
Sweating starts at a higher hypothalamic temperature (set point is raised)
What happens when the skin is warm and cold?
Warm:
Shivering stats at a lower core temp (set point is reduced)
Cold:
Shivering starts at a higher core temp (set point is raised)
