Exercise Thermoregulation, Fluid Balance, and Rehydration

Chapter 10

Part 2

Rehydration

Achieving Euhydration before exercising in a hot environment protects against heat stress because it:• Delays dehydration• Increases sweating during exercise• Diminishes the rise in core temperature

• Recommendation• 400-600 ml (13-20 oz) 20 min prior to exercise

Adequacy of Rehydration

Body weight changes Extent of water loss from exercise Adequacy of rehydration during and after

exercise or athletic competition. 16 oz water loss/1 lb weight loss

Adequacy of Rehydration

Urine and hydration:• Dark yellow urine with a strong odor =

inadequate hydration • Large volume, light color, without a strong

odor = adequate hydration

Rehydration

• Factors affecting consumption– Flavor, sweetness– Temperature – Color– Carbonation – Viscosity

Sodium and Rehydration

Sodium added to a rehydration beverage Provides more complete rehydration. Why?

Maintains plasma osmolality:• Sustains the thirst drive• Leads to greater fluid ingestion

• Thus,• More rapidly restore lost plasma volume during

rehydration

Sodium during Rehydration

Hyponatremia

A sustained low plasma sodium Osmotic imbalance across the blood–brain

barrier Rapid water influx into the brain. Swelling - encephalopathy

Symptoms (mild to severe) Confusion, convulsions, collapse, and coma

Hyponatremia

Hyponatremia

Occurrence

• Most cases (> 8 hrs)– Ultra marathon– Prolonged triathlon events

• Plasma sodium concentrations (Noakes)– 7-10 hours, 4 cases – 115-125 mmol/L– Fluid intake – 6-12 L

Treatment

• IV rehydration with saline– Glucose

• Oral rehydration as effective

Acclimatization

• Improving heat tolerance Physiologic adaptations of body

• Repeated exposure to heat stress • Requires exercise in a hot environment Exercising in the heat for 1 hour or more

• Daily for 9 to 14 days.

Acclimatization

• Cardiovascular changes by– Plasma volume expansion, first 1-3 days

• Sweating mechanisms – Longer, up to 10 days or more.

• Sweat earlier and at a greater rate• Sweat is more dilute• This causes skin temp to drop

– Increases the thermal gradient from the core to the skin

Heat Acclimation

• Results in:• Core temperature reduced• Heart rate at any given workload is reduced• Stroke volume increases• Due to expansion of plasma volume

• Extent of Heat acclimation • Training status • Duration of exposure• Rate of internal heat production

Age Differences in Acclimatization

Older individuals have:• A decreased sensitivity of thermoreceptors • Limited sweat gland output • Altered structure and function of the skin and

its vasculature • A decreased recovery from dehydration

Other Factors

Children More sweat glands, less sweat Longer acclimatization

Gender• Men sweat more.• Women show heat tolerance similar to men.

Body fat Insulation

Heat Illnesses

• Heat Edema– Unacclimated person

• Profuse sweating• Red face• Sodium retention• Swelling feet, legs

– Not a problem

Heat Illnesses

• Heat Syncope– Fainting – Lightheadedness– End of exercise

• Should stop exercising and rehydrate

Heat Illnesses

Heat cramps• Involuntary muscle spasms that occur after

intense physical activity• Usually high sweat rates • Electrolyte imbalance

• Stop exercise and rehydrate

Heat Illnesses

Heat exhaustion• Most common heat illness• High sweat rate• Syncope, cramps• Headache, blurred vision, • Fast heart rate and breathing• Mild confusion, agitation, poor coordination

• Stop exercise, rehydrate and get to a cool place

Heat Illnesses

Heat stroke• Most serious and requires immediate medical attention• Second leading cause of death in athletes• Delirium, memory loss• Seizures, coma, confusion, weakness• Symptoms-dry skin, no sweating

• Get person to hospital• If not,

• Ice baths (have to bring down temperature)• Cold water enema