L30. Thermoregulation Flashcards
(20 cards)
Core temperature?
- Humans are endotherms (warm blooded)
- Core temperature is maintained around 37 degrees through:
- Behaviour modification; cortex (awareness, choice, action)
- Homeostatic mechanisms; hypothalamus (autonomic control of thermoregulation)
Thermoneutral zone: 21-30 degrees (depending on clothing)
This is the range of environmental temperatures where:
- The body’s basal rate of heat production is equal to the rate of heat loss to the environment
- Core body temperature can be maintained with minimal metabolic regulation at rest
Circadian rhythm of core temperature?
Core temperature follows a circadian rhythm with:
- A ~1 degree range
- High at ~4pm and a low at ~4am
Heat balance?
37 degrees is in the middle of your resting zone: 36.5 - 37.5 degrees
If we stray from this too far, bad things can happen:
- An increase above resting zone can send you hyperthermic
- A decrease below resting zone can send you hypothermic
Mechanisms balancing heat loss and heat gain?
Three common methods of heat transfer: loss or gain
1. Conduction
2. Convection
3. Radiation
One mechanism of heat loss only:
1. Evaporation
One mechanism of heat gain only:
1. Metabolism
Conduction?
Transmission of heat through physical contact (requires a gradient)
Convection?
Transmission of heat through the movement of fluid or air
Radiation?
Transmission of heat through electromagnetic waves
Evaporation?
Loss of heat through conversion of water (sweat) to gas
- Energy (heat) imparted into moisture, which then evaporates from the skin
- Sweat glands secrete sweat onto the skin (~600mL/day at rest in neutral environment)
- Requires a temperature and moisture gradient for water to evaporate, speed of evaporation is determined by the size of the gradient: humidity
- The evaporation of sweat cools the skin (not sweating itself)
Metabolism?
Gain of heat through oxidation of nutrients to make ATP, and hydrolysis of ATP
Heat is produced as a byproduct during both processes
–> Resting (basal) metabolism generates heat:
* At rest, 70% from internal organs
E.g. liver, kidney, GI tract, brain
–> Exercise:
* Increases ATP production and use increasing heat production
–> Shivering thermogenesis:
* Cold-induced involuntary muscle contractions
* Increases ATP production and breakdown
* Increased production of heat as a byproduct of these processes
–> Non-shivering thermogenesis:
* Main mechanism of heat gain for infants under 1 year
* Metabolism in brown fat cells generates more heat than shivering
* Hypothalamus activates the sympathetic NS when core body temperature drops causing release of noradrenaline and thyroid hormones causing:
1. Activation of uncoupling proteins in the mitochondria of brown adipose tissue
2. Increased heat production of fatty acids (and some glucose) due to making heat rather than ATP
Hypothermia?
HEAT LOSS
- Core body temperature < 35 degrees
- Imbalance between heat loss and heat gain
- Hypothermia slows all physiological functions:
* Decreased metabolic rate
* Reduced ventilation and heart rate
* Electrolyte disturbances (low potassium, acidosis)
* Slower neural conduction
Exercise and hypothermia?
Exercise affects the perception of a decrease in core body temperature
- You don’t feel “as cold” as you would with the same drop in core body temperature when not exercising
- Shivering begins after a larger drop in core body temperature during exercise
- Sweating leads to greater heat loss in the cold
Overall, exercising in cold temperatures can increase the risk of hypothermia
Thermoregulation during exercise?
- Heat production: is INCREASED by exercise
- Heat loss: sweating increases and skin blood vessels vasodilate
- Core body temperature: increases in relation to exercise intensity (and some effect of environmental temperature)
- There is a point you reach when heat production = heat loss and as a results your core body temperature stabilises
- Heat production increases immediately at the start of exercise leading to an increase in core body temperature –> the increase in core body temperature stimulates the hypothalamus to activate heat loss mechanisms:
- Sweating: increased heat loss via evaporation
- Vasodilation of skin arterioles: increased heat loss via convection/radiation
- During “steady-state” exercise heat loss mechanisms will increase until heat production and heat loss are balanced, and core body temperature stabilises
Temperature tolerance?
Heat tolerance limit:
- When heat loss mechanisms can’t keep up with heat production and core body temperature climbs to dangerous levels leading to fatigue and/or heat stroke
- Exercise training can increase the heat tolerance limit
Temperature acclimation?
Initially in a hot environment:
- Thermoregulatory mechanisms struggle and core body temperature increases rapidly
* Skin vasodilation reduces skeletal muscle blood flow
* Sweating causes dehydration
After acclimation:
- Improved thermoregulatory mechanisms
* Vasodilation of skin arterioles; increased plasma volume, increased maximal skin blood flow
* Sweating; begins earlier in exercise, increased rate of sweat production, decreases sodium content of sweat (aldosterone)
Hyperthermia - heat exhaustion?
- Doesn’t necessarily require a large increase in core body temperature
- Depends on temperature tolerance
- Symptoms are largely attributed to hypotension from:
Decreased MABP = decreased CO x decreased TPR
Decreased CO = HR x decreased SV - Vasodilation of skin arterioles decreases total peripheral resistance
- Sweating causes dehydration, and loss of plasma volume, causing a decrease in preload and stroke volume
Treatment: stop exercising, cool down, rehydrate
Hyperthermia - heat stroke?
- Core body temperature > 40 degrees
- Extreme imbalance between heat loss and heat gain
- Heat loss: in hot and humid conditions heat loss systems can be overwhelmed
- Core body temperature: continues to increase
Medical emergency:
- Symptoms; fainting, seizures, delirium and prolonged unconsciousness
- Treatment; medical attention to help internal cooling and fluid replacement
Fever?
- Regulation of core body temperature still occurs but the set point changes and can fluctuate up and down
Onset of fever:
- Core body temperature set-point increases
- Feel suddenly ‘too cold’ = chills
- Heat gain mechanisms activated: vasoconstriction of skin arterioles and shivering to increase temperature until it reaches the new set-point
Fever ‘breaks’:
- Core body temperature set-point decreases
- Feel suddenly ‘too hot’ = sweats
- Heat loss mechanisms activated: sweating and vasodilation of skin arterioles
Fever from infection?
Macrophages release endogenous pyrogens (chemical messengers) which act via:
- Directly on the hypothalamus (integration centre)
- By stimulating the vagus nerve, which acts on the hypothalamus
- Both cause release of prostaglandins within the hypothalamus; which increases the core body temperature set point
Essentially !! (Macrophages (when we have an infection) make chemicals which are going to tell our hypothalamus to increase our core body temp set point)
Effectors:
- Skeletal muscle
* Voluntary response: feel cold so put more clothes/blankets on
* Involuntary response: shivering
- Skin arterioles
* Vasoconstriction to reduce blood flow to the skin
= increase in the core body temperature to match new set point
Exercising in very cold weather increases the risk of developing hypothermia BECAUSE exercise increases heat production
A. If both statements are true, and the second causes the first
B. If both statements are true, but the second does not cause the first
C. If the first is true and the second is false
D. If the first is false and the second is true
E. Both statements are false
B
Which statement about thermoregulation is INCORRECT?
A. At the start of exercise heat production exceeds heat loss
B. Heat stroke can occur when core body temperature exceeds 40 degrees
C. The symptoms of heat exhaustion are largely a result of hypotension
D. After temperature acclimation sweating decreases
D
