WK 9+10 Exercise and thermal stress & Fluid balance

Question 1

What are the avenues of heat loss?

Answer 1

Conduction: The direct transfer of heat to objects in direct contact with the skin (clothes, air molecules, water molecules, etc.)

Convection: When heated, air (or water) molecules rise and are replaced by cooler air (or water): convection currents. Moving through the air, or wind moving past the skin, increases convective heat losses (‘forced convection’)

Radiation: The transfer of heat from the skin, via electromagnetic waves, to surrounding objects at a lower temperature (walls, ceiling, etc.).

Evaporation: The latent heat of evaporation of water is 2.4 kJ.g-1, so every mL of fluid that evaporates from the skin surface (or the respiratory tract) removes this amount of heat energy. At rest, the majority of evaporative heat losses are due to the saturation with water vapour of air in the respiratory tract (along with a small amount of ‘insensible’ perspiration). During exercise, thermoregulatory sweating becomes the most important avenue of heat loss.

Question 2

The cooling effect

Answer 2

To allow this to happen sweat must evaporate: the secretion of sweat onto the skin does not, in itself, produce cooling. High ambient humidity (and vapour impermeable clothing) inhibit the evaporation of sweat.

Question 3

What is the heat balance equation?

Answer 3

When the body is in heat balance:
M-W±C±K±R-E = 0

Where:
M = metabolic energy production
W = external work done (the proportion of M accounted for by the mechanical energy of exercise)
C, K & R = heat losses / gains through conduction, convection and radiation respectively
E = heat lost through evaporation (both from the skin surface and via the respiratory tract)

Question 4

What does the exposure of heat or cold stress initiate?

Answer 4

thermoregulatory mechanisms that regenerate and conserve heat at low ambient temperatures and dissipate heat at high temperatures.

Question 5

How does the hypothalamus regulate temperature?

Answer 5

the coordinating centre imitates adjustments in response to input from thermal receptors in the skin and changes temperature of the blood that perfuses the hypothalamic region.

Question 6

How is heat conserved in the cold?

Answer 6

• from vascular adjustments that shunt blood from the cooler peripheral to the warmer deep tissues of the body’s core.

Question 7

What happens if the vascular mechanisms are ineffective during cold stress?

Answer 7

shivering provides metabolic heat. Prolonged cold stress stimulates release of hormones that elevate resting metabolism.

Question 8

What four factors contribute to eat dissipation?

Answer 8

radiation, conduction, convection and evaporation

Question 9

What is the major physiologic defence against overheating at his ambient temperatures and intense exercise?

Answer 9

evaporation

Question 10

What happens to evaporation in warm, humid environments? and what is the effect?

Answer 10

evaporation diminishes dramatically, making a person vulnerable to dehydration and spiralling core temperature

Question 11

How do you evaluate the environments potential heat challenge?

Answer 11

practical heat-stress indices use ambient temperature, radiant heat, and relative humidity

Question 12

What 3 factors influence sweat vaporisation from the skin or pulmonary surfaces?

Answer 12

1. surface exposure
2. ambient air temperature
3. relative humidity and convective air currents

Question 13

Vigorous exercise in cold climates

Answer 13

generates metabolic heat to maintain core temperature in cold air environments, even if the person wears little clothing.

Question 14

What is the clo-index?

Answer 14

the thermal resistance from clothing - the insulator capacity of air trapped between skin and clothing including the clothing’s insulation value.

Question 15

What is the effect of wearing several layers of light clothing?

Answer 15

traps a zone of air against the skin; this provides more effective insulation from cold than a single layer of clothing.

Question 16

What is the effect of wet clothing?

Answer 16

loses its insulating properties; greatly facilitates heat flow from the body.

Question 17

What is the ideal warm-weather clothing?

Answer 17

lightweight, loose fitting, and light coloured. Even with these characteristics, heat loss slows until the clothing becomes wet and allows evaporative clothing.

Question 18

What is the negative effect of football kits?

Answer 18

they impose a barrier to heat dissipation because they effectively shield about 50% of the body’s surface from the beneficial effects of evaporative cooling.

Question 19

Core temperature during exercise and how it regulates temperature…

Answer 19

core temperature increases during exercise; the relative stress of exercise determines the magnitude of the increase. A well-regulated temperature increase creates a more favourable environment for physiologic and metabolic functions.

Question 20

what is the effect of excessive sweating?

Answer 20

comprises fluid reserves causing dehydration.

Question 21

What is the effect of sweating without fluid replacement?

Answer 21

decreases plasma volume, which leads to circulatory dysfunction and precipitous rise in core temperature.

Question 22

What is the consequences of exercising in hot humid conditions?

Answer 22

poses a thermoregulatory challenge because large loss of sweat in high humidity contributes little to evaporative cooling.

Question 23

What is the effect of fluid loss more than 4% of body weight?

Answer 23

impedes heat dissipation, comprises cardiovascular function and diminishes exercise capacity.

Question 24

Who is the effect of adequate fluid replacement?

Answer 24

maintains plasma volume so circulation and sweating progress optimally.

Question 25

what is the ideal fluid replacement during exercise?

Answer 25

matching fluid intake (a process monitored by changes in body weight).

Question 26

How much water can the small intestine absorb each hour?

Answer 26

1000mL

Question 27

Why is it best to drink rehydration beverage fluids rather than plain water?

Answer 27

the small amount of electrolytes in the rehydration fluid facilitates fluid replacement.

Question 28

What is the benefit of adding a small amount of salt to the replacement fluid during prolonged exercise in the heat?

Answer 28

facilitates soda and fluid replenishment

Question 29

What is the effect of repeated heat stress?

Answer 29

initiates thermoregulatory adjustments that improve exercise capacity and reduce discomfort on heat exposure.

Such heat acclimatisation triggers favourable cardiac output distribution while increasing sweating capacity.

ten days of heat exposure promotes full acclimatisation.

Question 30

Men and women and thermoregulation.

Answer 30

women and men show equivalent thermoregulation during exercise when controlled for levels of fitness and acclimatisation.

women produce less sweat than men when exercising at the same core temperature.

Question 31

Why does oral temperature after exercise inaccurately measure core temperature?

Answer 31

because of evaporative cooling in the mouth and airways with high levels of pulmonary ventilation during exercise and recovery.

Question 32

how many times faster does water conduct heat than air?

Answer 32

25 times. Immersion in water of only 28 to 30 degrees provides considerable thermal stress that imitates rapid thermoregulatory adjustments.

Question 33

How much does shivering increase the metabolic rate?

Answer 33

3 to 6 METS.

Question 34

What is the effect of subcutaneous fat?

Answer 34

provides excellent insulation against cold stress.

It greatly enhances the effectiveness of vasomotor adjustments so individuals with excess body fat retain a large % of metabolic heat.

Question 35

What does the wind-chill index determine?

Answer 35

the wind’s cooling effect on exposed tissue.

ambient temperature and wind influence the coldness of an environment.

Question 36

The respiratory tract during cold-weather exercise.

Answer 36

considerable water loss occurs from the respiratory passages during exercise on a cold day, but inspired air temperature generally does not pose a danger to respiratory tract tissues.