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Understanding Water Balance and Kidney Function

Jan 31, 2025

Water Balance and Kidney Function

Importance of Water

  • Essential for life.
  • Intake through drinking and eating.
  • Loss through:
    • Breathing (water vapor)
    • Skin (evaporation, sweating)
    • Urine
    • Faeces

Body Fluid Regulation

  • Body maintains constant concentration of internal body fluids.
  • Kidneys adjust urine production based on water gained or lost.

Case Examples

  • Lisa

    • Sweating profusely and no fluid intake since yesterday.
    • Extremely dehydrated.
    • Producing very little, dark urine.
  • Joanne

    • Consumed lots of liquid.
    • Producing plentiful, pale urine.
    • Fully hydrated, losing excess water.

Kidney Function

  • Regulates urine production to maintain blood volume and concentration.
  • Blood flows in through renal artery, out through renal vein.
  • Nephrons: Millions in kidney, responsible for filtering blood.
    • Fluid content of blood filtered and passed into nephron tube.
    • Right amount of water reabsorbed to adjust blood volume.
    • Remaining waste exits as urine.

Response to Drinking Water

  • Lisa and Joanne's Experiment: Both drink a liter of water and give urine samples every half hour.

    • Lisa's urine gradually becomes paler, but less volume than Joanne.
    • Joanne quickly loses excess water.
  • Mechanism

    • Large water intake increases body fluid.
    • Excess fluid appears in blood.
    • Kidney reabsorbs little water, most exits as urine.
    • Normalizes blood volume.

Dehydration and Kidney Response

  • During dehydration, kidneys adjust reabsorption to retain water and maintain blood volume.