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Water Physiology
Related Subjects: |Water Physiology |Sodium Physiology |Potassium Physiology |Aldosterone Physiology |Atrial Natriuretic Peptide (ANP) |Brain Natriuretic Peptide (BNP)
💧 Water Physiology: Water makes up ~60% of adult body weight. It is essential for cellular function, temperature regulation 🌡️, and biochemical processes ⚙️. Small changes in water balance can lead to profound effects on circulation, kidneys, and brain function 🧠.
🥤 Daily Water Intake
- Total: ~2500 mL/day.
- Sources:
- Gastrointestinal tract: ~2.2 L (food + drink).
- Metabolism: ~0.3 L (oxidation of nutrients).
🚽 Daily Water Output
- Total: ~2500 mL/day (normally matches intake).
- Routes:
- Insensible loss & sweating: ~0.9 L/day (lungs + skin).
- GI tract: ~0.1 L/day (faeces).
- Kidneys: ~1.5 L/day (main site of regulation).
🌊 Osmosis
- Definition: Diffusion of water across a semipermeable membrane, from low solute → high solute.
- Water reabsorption in kidney follows solute reabsorption, ensuring balance.
📊 Osmolarity of Body Fluids
- Normal plasma osmolality: ~300 mOsm/kg.
- Maintained within a tight range by:
- 💧 Kidneys → adjust reabsorption/secretion.
- 🧠 Hypothalamic osmoreceptors → trigger thirst & ADH release.
🌀 Water Reabsorption in Nephron
- Proximal tubule: ~70% reabsorbed (passive, alongside solutes).
- Distal tubule: ~20% reabsorbed (fine-tuning).
- Collecting duct: ~10% reabsorbed - regulated by ADH.
🔑 Key Concepts
- 🧠 ADH: ↑ Collecting duct permeability → more water retained.
- 🥤 Thirst mechanism: Activated when plasma osmolality rises.
- ⚖️ Homeostasis: Balance of intake & output ensures optimal cellular function.
⚠️ Clinical Relevance
- Dehydration: ↑ Osmolality → thirst + ADH release → concentrated urine.
- Overhydration: ↓ ADH → dilute urine, water diuresis.
- Disorders of water balance:
- Diabetes insipidus → lack of ADH action → excessive dilute urine.
- SIADH → inappropriate ADH release → water retention & hyponatraemia.
- Heart failure → perceived low volume triggers ADH despite fluid overload.
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