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Chapter 27, Lecture 2

Daily Water Balance

  • Normal intake ≈ 2{,}500\ \text{mL\,day}^{-1} ≈ normal output (homeostasis)

Sources of Water Gain

  • Metabolic water: \approx 200\ \text{mL} (cellular respiration, dehydration reactions)
  • Food moisture: \approx 700\ \text{mL}
  • Ingested fluids: \approx 1{,}600\ \text{mL} (major variable component)

Routes of Water Loss

  • Kidneys (urine): \approx 1{,}500\ \text{mL}
  • Skin (evaporation/sweat): \approx 600\ \text{mL}
  • Lungs (expired water vapour): \approx 300\ \text{mL}
  • GI tract (faeces): \approx 100\ \text{mL}
  • Menstruation: small additional loss (variable)
  • Losses rise with exercise, fever, diarrhoea, vomiting etc.

Thirst Mechanism ─ Regulation of Water Gain

Triggers (↑ = stimulate thirst centre in hypothalamus):

  • ↑ Blood osmolarity → osmoreceptors fire
  • ↓ Blood volume / ↓ Blood pressure → baroreceptor & atrial stretch input; activates RAAS
  • Dry mouth (↓ salivary flow)
    Effects:
  • Conscious urge to drink → ↑ water intake → ↓ osmolarity, ↑ volume/pressure (negative feedback)
  • Behaviour is required; lack of access or willingness → risk of dehydration (common in infants, elderly, confused)

Hormonal Control of Water & Na⁺ Balance (Regulation of Loss)

Antidiuretic Hormone (ADH)

Stimuli:
• ↑ plasma osmolarity (primary)
• ↓ blood volume/pressure
• Pain, stress, nausea
Actions:
• Released from posterior pituitary
• ↑ aquaporins in late distal tubule & collecting duct → ↑ H₂O reabsorption
• Produces small, concentrated urine
Net result: ↓ water loss, ↓ osmolarity, ↑ blood volume/pressure
Note: Alcohol inhibits ADH → diuresis.

Aldosterone (RAAS)

Stimuli:
• ↓ Na⁺ (hyponatraemia) sensed by macula densa
• ↓ blood pressure → renin release → \text{Ang II}
Actions:
• ↑ Na⁺ reabsorption in principal cells (late distal tubule & collecting duct)
• Obligatory water follows Na⁺ (if ADH present)
Net result: ↓ Na⁺ loss, ↓ water loss, ↑ blood volume/pressure

Atrial Natriuretic Peptide (ANP)

Stimulus:
• ↑ blood volume → atrial stretch
Actions:
• Inhibits aldosterone & Na⁺ reabsorption → natriuresis
• Water follows Na⁺ into urine
Net result: ↑ Na⁺ & water loss, ↓ blood volume/pressure

Integrated Summary

  • Water follows solute (mainly Na⁺) by osmosis; controlling Na⁺ movement controls water movement.
  • Dehydration (water loss > gain) engages thirst centre + ADH + Aldosterone; over-hydration (volume excess) promotes ANP release.
  • Negative feedback loops restore:
    • Osmolarity to \approx 285{-}295\ \text{mOsm\,kg}^{-1}
    • Blood volume/pressure to normal range.
  • Only behavioural intake adds water; renal, skin, lung & GI routes can only conserve or excrete.