Hydration and heat illness

Question 1

Fluid content of body

Answer 1

45-70% total body mass
 Muscle: 70-75% water
 Fat: 10-15% water
 Intra cellular compartment
 Extracellular compartment

Question 2

“Average” 75 kg man contains approx_____ of water

Answer 2

45 L (60%)

Question 3

Osmotic pressure

Answer 3

-Determines water content of all compartments
-Water will move from a region of lower [solute] to
a region of higher [solute]
-Blood pressure and osmotic effects determine
the rate at which water leaves the circulation to
enter the tissues

Question 4

Sweating

Answer 4

• 2 - 4 million ecrine sweat glands
• Density & sensitivity varies over body surface
• Back most sensitive with lowest threshold

Question 5

Electrolyte losses with sweating

Answer 5

• Mainly sodium and chloride

- Heat acclimation reduces electrolyte concentration in sweat

Question 6

Effects of dehydration

Answer 6

• Impaired thermoregulation

- Reduced muscle perfusion

Question 7

Hypohydration & performance:

Endurance

Answer 7

• Dehydration by <2% in a temperate environment doesn’t decrease performance in exercise lasting <90 min
• Dehydration by 2-7% decreases performance especially in hot environments

Question 8

Hypohydration & performance:

Strength, power & high intensity exercise

Answer 8

• Dehydration by 2-3% has no effect on performance, possibly because decreased mass makes exercise “easier”
• Dehydration by 3-4% decreases strength, power & high intensity endurance

Question 9

Hypohydration & performance:

Mental function

Answer 9

Dehydration by >3% may influence cognitive function, mood and mental readiness

Question 10

Hyponatremia

Answer 10

Abnormally low plasma sodium concentration

Caused by excessive ingestion of sodiumfree fluid during prolonged exercise

Question 11

Hyponatremia Sequence of events

Answer 11

• Disruption of osmotic balance across blood-brain barrier
• Influx of water to brain
• Swelling
• Neurological disturbances (confusion, seizures, coma)
• Death from rupture of brainstem