A.2.1 Water and Electrolyte Balance

Adapted from class notes.

Water

• Essential for life on Earth

• Water works as a solvent to transport substances throughout the body and cells

• Helps us maintain a constant body temperature

• Allows joints to have reduced friction

• Is a place for reactions to happen

In the Body

• Around 50-70% of body mass is water

  • Greater fat percentage means lower water percentage

  • Lean mass is 60-80% water

• In a 70kg person:

  • ~2-3 is intracellular fluid (water in cells)

    • Higher potassium ion (K)

  • ~1/3 is extracellular fluid

    • Higher sodium ion (Na)

    • 80% is interstitial fluid (fluid between tissues)

    • 20% is blood plasma

Balance

• Water loss of about 2.5L/day is normal, and replaced through food, drinking, and metabolic porcesses

• Water loss:

  • Evaporation of sweat

  • Evaporation from breathing

  • Excretion of urine

  • Excretion of faeces

Negative Feedback

• Hypothalamus, pituitary, and kidneys regulate water and electrolyte balance

• Antidiuretic hormone (ADH)

  • Released by pituitary, acts on kidneys

  • Kidneys reabsorb more water from urine (decreases urine production)

• Hypothalamus also activates thirst

• Hypertonicity (low water/high solute)

  • Thirst causes intake of more water

  • ADH causes water retention

• Isotonicity (normal water/solute)

• Hypotonicity (high water/low solute)

  • Thirst decreases, slowing water intake

  • ADH broken down/decreases for less water retention

Kidneys

• The kidneys work to produce urine, which functions to remove waste (such as urea)

• Nephron: functional unit of the kidney

  • Water, electrolye, waste diffuse from blood to top area

  • Fluid moves down, then up, then down, where urine is collected and fed to the bladder

  • As the fluid moves down, the body attempts to retain water, as it diffuses out

  • As the fluid moves up, the body attempts to retain salt, as it diffuses out

  • Result is concentrated waste (urea and other)

  • ADH acts on the walls of the collecting duct to increase water reabsorption

Hydration

• Thirst is a method of knowing hydration level, but is not perfect

• Urine Analysis:

  • Color/darker= = dehydrated

  • Specific gravity (higher = dehydrated)

  • Freezing point with osmometer (more depressed below 0C = dehydrated)

Necessity

• Athletes need more fluid. Sweat loss due to evaporation to dissipate heat from muscle contraction

• If the environment is:

  • Hotter = More sweat needed because of smaller difference between air and skin

  • Colder = Less sweat needed because greater difference between air and skin

  • Humid = Body will sweat more because evaporation is slower

  • Dry = Body will not cool as fast because evaporation is faster and body cools quickly

  • Windy = Water removed from body faster, sweat less

Electrolytes

• Electrolytes are ions that can carry an electric charge in water (typically salts)

• As water moves, electrolyte concentration changes

  • Catabolic (break down) uses water, so water moves into cells passively (leaving high salt behind)

  • High blood pressure causes water to be forced out of blood

Hydration Errors

• Hypernatremia and hyponatremia are issues with hydration levels (specifically Na)

• Hypernatremia is a high concentration of sodium (low water/dehydration)

• Hyponatremia is a low concentration of sodium (high water/overhydration)

• Drinking only water can lead to hyponatremia in endurance sports

  • Sweating loses water and salt

  • Water replaces only water, not salrt

  • Sports drinks with electrolytes replace both

• Signs of hyponatremia: bloating, headache, nausea. Can be fatal

• More common in:

  • Long compeition/training

  • Swimming > running > cycling (different sports)

  • Biological females

  • High temperatures

Cardiovascular Drift

• Occurs because of prolonged exercise

• Cardiac output = (heart rate times stroke volume)/1000

• Steady state exercise requires the same cardiac output (transportingO2, CO2, sugar)

• In prolonged exercise, sweating increasing reduces blood volume

• Decreased blood volume equates to decreased stroke volume

• If stroke volume goes down, heart rate must increase to maintain cardiac output