Renal Tubular Physiology — Water Handling

• Total body water = 60% body weight (approx. 42L in 70kg male)
  • Intracellular fluid = 2/3 Total body water (approx. 28L)
  • Extracellular fluid = 1/3 Total body water (approx. 14L)
    • Plasma compartment = 1/4 Extracellular fluid (approx. 3L)
• Renal blood flow
  • 1.25l/min
• Filtration fraction
  • 0.2
• Glomerular filtration rate
  • 180l/day
  • >4x total body water
    • Therefore renal water reabsorption important in H₂O homeostasis

Water reabsorption

Mechanisms:

• difference in osmolality lumen : interstitial fluid. This created by reabsorption solutes (Na, Cl, glucose, AA)
• solute reabsorb actively ⇒ ↓luminal osmolality & ↑interstitial osmolality ⇒ gradient to drive water reabsorb across cells +/- tight junctions

Locations of Water Reabsorption:

PCT

• Highly permeable to water
• 65% H₂O reabsorption → follows Na⁺ reabsorption via osmosis

LoH

• Thin descending limb
  • Highly permeable to water
  • 20% H₂O reabsorbed
  • Follows ions via osmosis
• Thick ascending limb
  • Impermeable to water
• Countercurrent mechanism to increase papillary osmolarity to ~1400mosm/l
  • Important for further H₂O reabsorption

DCT/CD

• Variable permeability
  • Controlled by aldosterone and vasopressin

If decreased total body water

• Decreased renal blood flow (beyond autoregulatory capabilities)
• Decreased glomerular filtration
• Decreased Na reaching DCT macula dense
• Release of Renin from Juxtaglomerular apparatus → Angiotensin II → Aldosterone

Angiotensin II

• Direct upregulation of proximal tubular Na reabsorption
• Efferent arteriolar vasoconstriction (as well as weak afferent arteriolar constriction) → increased filtration
• Increased thirst sensation
• Stimulates aldosterone release
• Stimulates vasopressin release
• Potentiates post-ganglionic sympathetic stimulation

Aldosterone

• Upregulates CD Na/K ATPase and inserts ENaC channels to increase Na reabsorption
  • H₂O follows Na via osmosis

Vasopressin (ADH)

• Stimulated by Angiotensin II, Osmoreceptors and low-volume baroreceptors
• Causes insertion of aquaporin 2 via V₂ receptor stimulation
• Increased permeability of CD to H₂O
  • H₂O reabsorbed due to high interstitial osmolarity created by counter-current mechanism and reabsorption of urea

If increased total body water

• Atrial stretch → production of natriuretic peptides (ANP, BNP)
• Leads to decreased reabsorption of Na in PCT
• Leads to decreased reabsorption of H₂O

Sakurai 2016

Answer by Sakurai