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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.
| The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms. |
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