Regulating Water Balance

Electrolyte Balance

Electrolytes (e.g., sodium, potassium) dissolve in blood and sweat.

• Regulate fluid balance and muscle contraction.

Osmotic Balance

If blood solute levels are too high (high osmolarity), osmosis draws water out of cells → leads to dehydration.

Overhydration without electrolyte replenishment decreases blood osmolarity → hyponatremia.

• Causes water to move into cells, leading to swelling and serious health risks.

Hypernatremia (High sodium concentration > 145 mmol)

• Less water relative to sodium → leads to cell dehydration.

• Occurs after prolonged physical activity if fluids are not properly balanced.

Hyponatremia (Low sodium concentration < 135 mmol)

• Early symptoms:

  • Bloating, nausea, vomiting, headache, brain swelling.

• Risks increase in endurance sports, depending on:

  • Duration of performance

  • Type of sport

  • Biological sex

  • Environmental conditions (temperature, country of competition)

Why Athletes Need More Fluids

• Exercise & hot environments increase sweat loss, disrupting water balance.

• Electrolytes lost in sweat must be replenished to maintain balance.

Fluid loss control:

• Athletes should not lose more than 2% of body mass due to dehydration.

Cardiovascular Drift:

• Heart rate increases, and stroke volume decreases over time during prolonged exercise.

• More noticeable in hot environments.

• As sweat rate increases, blood pressure declines.

Measuring Water & Electrolyte Balance

• Body mass – Monitored before and after exercise.

• Urine colour –

  • Dark urine = dehydration.

  • Pale urine = normal hydration.

• Osmosis measurements:

  • Berkeley-Hartley Method.

• Osmometer:

  • Measures solute concentration in plasma or urine using freezing point depression or vapor pressure.

  • Pros: Highly accurate and reliable.

  • Cons: Requires specialized equipment and trained personnel.

• Hydrometer:

  • Measures urine solute concentration by comparing density to water.

  • Pros: Simple, portable.

  • Cons: Affected by recent fluid intake and diet.

Role of the Kidneys in Water Regulation

• Kidneys control water and electrolyte retention/loss.

• Water & electrolytes filter from the blood into the glomerulus.

• Loop of Henle:

  • Water is absorbed out of the tubule, concentrating and reducing the volume of the fluid.

  • Sodium chloride is actively transported out, reducing volume even more

• Collecting Duct:

  • ADH increases permeability, allowing water reabsorption.

  • Final regulation of water before excretion as urine.