Tubular Function & Water Balance – Comprehensive Lecture Notes

A. Parts of the Tubule

• Two nephron categories (classified by glomerular position)
  • Cortical nephrons – $\approx 75\%$ of total
  • Sub-classes: superficial cortical & mid-cortical
  • Short loops of Henle; no thin ascending limb
  • Juxtamedullary nephrons – $\approx 25\%$ of total
  • Long loops of Henle
  • Possess a thin ascending limb
  • Mid-cortical nephrons may have either short or long loops
• Functional significance
  • Long loops ➔ crucial for generating high medullary osmotic gradient (counter-current multiplication)
  • Short loops ➔ chiefly involved in bulk solute reabsorption, not urine concentration

B. Thin Descending Limb (dLH) of Henle

• Permeability profile
  • Highly permeable to water (AQP-1 always present)
  • Impermeable / only slightly permeable to $\text{Na}^+$, $\text{Cl}^-$, urea
  • No active transporters (movement is passive)
• Consequences while filtrate descends
  • Water exits → tubular fluid volume falls, osmolality rises
  • End-luminal osmolality
  • Cortical nephrons: $600\;\text{mOsm}\,\text{kg}^{-1}$ (electrolytes $560$, urea $40$)
  • Juxtamedullary: $1200\;\text{mOsm}\,\text{kg}^{-1}$ (electrolytes $1120$, urea $80$)

C. Thin Ascending Limb (aLH) of Henle

• Permeability profile
  • Water-impermeable
  • Highly permeable to $\text{Na}^+$ and $\text{Cl}^-$ (passive efflux down gradient)
  • Moderately permeable to urea (passive influx)
  • Lacks active solute pumps
• Net effects during ascent
  • $\text{NaCl}$ diffuses out, urea diffuses in
  • Tubular fluid becomes more dilute; volume fairly constant
  • Exit osmolality $\approx 500\;\text{mOsm}\,\text{kg}^{-1}$ (electrolytes $400$, urea $100$)

D. Thick Ascending Limb (TAL) of Henle

• Key transport processes
  • Active $\text{Na}^+$–$\text{K}^+$–$2\,\text{Cl}^-$ cotransport (NKCC2)
  • Lumen-positive voltage drives paracellular $\text{Ca}^{2+}$ & $\text{Mg}^{2+}$ reabsorption ("leaky" tight junctions)
  • $\text{HCO}_3^-$ reabsorbed via $\text{H}^+$ secretion
• Impermeabilities
  • Water-impermeable
  • Low urea permeability
• Dilution achieved; TAL is called the "diluting segment"
• Osmolality at TAL exit
  • Juxtamedullary: $200\;\text{mOsm}\,\text{kg}^{-1}$ (electrolytes $100$, urea $100$)
  • Cortical: $140\;\text{mOsm}\,\text{kg}^{-1}$ (electrolytes $100$, urea $40$)
  • Average: $150\;\text{mOsm}\,\text{kg}^{-1}$

E. Distal Convoluted Tubule (DCT) & Connecting Tubule (CNT)

• Functional similarity; CNT uniquely hormone-sensitive (e.g.
  parathyroid hormone)
• Properties
  • Water-impermeable (no AQP2 unless ADH acts later)
  • Very tight epithelia; low urea permeability
  • Active $\text{NaCl}$ reabsorption via NCC (thiazide-sensitive); CLC-KB mediates $\text{Cl}^-$ exit
• Reabsorbs $8{-}10\%$ of filtered $\text{Na}^+$
• Luminal osmolality at DCT end $\approx 100\;\text{mOsm}\,\text{kg}^{-1}$ (electrolytes $50$; urea $50$)

F. Collecting Duct System (Cortical → Medullary → Papillary)

• Two major cell populations
  1. Principal (light) cells
  • $\text{Na}^+$ reabsorption (ENaC) & $\text{K}^+$ secretion (ROMK)
  • Respond to aldosterone (↑ ENaC, ↑ $\text{Na}^+$–$\text{K}^+$ ATPase) and ADH (↑ water permeability)
  1. Intercalated (dark) cells
  • $\alpha$-IC: reabsorb $\text{K}^+$, secrete $\text{H}^+$ via H$^+$–K$^+$-ATPase & V-type H$^+$-ATPase
  • $\beta$-IC: secrete $\text{HCO}_3^-$ (anion exchanger 1) under alkaline load
• Cortex: principal $70{-}80\%$, intercalated $20{-}30\%$; medulla >$90\%$ principal; papilla $100\%$ principal

Water & Urea Handling in Collecting Ducts

• Without ADH ➔ water-impermeable; urine remains dilute
• With ADH ➔ insertion of AQP2 channels ➔ high water permeability → equilibration with interstitium
• Segments & urea permeability
  • Cortical & medullary: essentially urea-tight
  • Papillary: ADH-stimulated UTA1/UTA3 allow urea diffusion → "urea recycling"
• Final urine composition extremes
  • Maximal ADH: osmolality $\approx 1200\;\text{mOsm}\,\text{kg}^{-1}$ (non-urea $600$; urea $600$); flow $0.2{-}0.6\,\text{mL min}^{-1}$
  • Absent ADH: osmolality $60{-}70\;\text{mOsm}\,\text{kg}^{-1}$ (non-urea $20$; urea $50$); flow $\approx 15\,\text{mL min}^{-1}$

G. Volume & Osmolality Profile Along Nephron

• Filtrate volume progressively falls from $100\%$ at Bowman’s space to <$1\%$ at papilla
• Osmolality trajectory
  • Rises steeply in dLH (water loss)
  • Falls sharply in TAL & DCT (salt removal)
  • Variable in collecting duct depending on ADH

H. Whole-Body Water Balance

Daily Inputs

• Food: $800{-}1000\,\text{mL day}^{-1}$
• Metabolic (oxidation): $300{-}400\,\text{mL day}^{-1}$
• Drinks: $1000{-}2000\,\text{mL day}^{-1}$ (range <$1000$ to >$20000$)
• Total: $2100{-}3400\,\text{mL day}^{-1}$

Daily Outputs

• Insensible (skin + lungs): $800{-}1000\,\text{mL day}^{-1}$
• Sweat: $\approx 200\,\text{mL day}^{-1}$
• Faeces: $100{-}200\,\text{mL day}^{-1}$
• Urine: $500{-}20000\,\text{mL day}^{-1}$
  • Obligatory renal loss $\approx 500{-}600\,\text{mL day}^{-1}$ (solute clearance requirement)
• Total matches inputs for zero balance

Regulatory Elements

1. Thirst (hypothalamus + cerebral cortex)
2. Kidneys
   • Produce dilute urine when excess water (↓ ADH)
   • Produce concentrated urine when water-restricted (↑ ADH)
3. ADH modulation – pivotal for renal water excretion

I. Generating Dilute vs Concentrated Urine

Dilute (Water Diuresis)

• Remove solute beyond thin aLH while keeping duct water-tight (no ADH)
• Medullary interstitial gradient partially "washed out" to $\approx 600\;\text{mOsm}\,\text{kg}^{-1}$

Concentrated (Antidiuresis)

• Requirements
  1. High medullary osmolality (outer & inner medulla)
  2. ADH to render collecting duct water-permeable
• Water exits collecting duct until luminal osmolality = interstitium (up to $1200\;\text{mOsm}\,\text{kg}^{-1}$)

J. Counter-Current Mechanisms

1. Active Counter-Current Multiplication (outer medulla)
   • Powered by TAL active $\text{NaCl}$ transport → establishes $\approx 200\,\text{mOsm}\,\text{kg}^{-1}$ horizontal gradient → vertical gradient via fluid flow
2. Passive Counter-Current Multiplication (inner medulla)
   • Driven by differential permeabilities: thin limbs (salt vs urea) + urea recycling from papillary CD
3. Counter-Current Exchange
   • Vasa recta act as heat-exchangers for solute/water → preserve gradient while supplying blood

Medullary Mass Balance (example values)

• Water in = $100 + 36 + 6 = 142\,\text{mL min}^{-1}$
• Water out = $117 + 24 + 1 = 142\,\text{mL min}^{-1}$
• Solute in = $28.5 + 10.3 + 1.7 = 40.5\,\text{mOsm min}^{-1}$
• Solute out = $36.9 + 2.4 + 1.2 = 40.5\,\text{mOsm min}^{-1}$

Factors Limiting/Enhancing Concentration Ability

• Loop length; % long loops
• Urea availability (protein intake, liver function)
• Flow rates in LH & CD (fast flow washes out)
• Vasa recta blood flow (↑ flow dissipates gradient)
• Loop diuretics (NKCC2 blockade) ↓ gradient
• Medullary structure pathology (e.g. sickle-cell disease, pyelonephritis)

K. Antidiuretic Hormone (ADH, AVP)

• Nonapeptide; synthesized as pre-prohormone in supraoptic & paraventricular hypothalamic nuclei
• Axonal transport to posterior pituitary; cleavage en route; released upon action potentials

Control of ADH Release

1. Osmotic
   • Osmoreceptors in lamina terminalis shrink when $\uparrow$ plasma osmolality
   • Even $+1\%$ rise triggers several-fold ADH increase; conversely, $-1{-}2\%$ drop suppresses ADH to near-zero
2. Haemodynamic
   • $\downarrow$ Blood volume/pressure (≈$15\%$) sensed by low/high pressure baroreceptors → potent ADH stimulus (volume cues override tonicity)
3. Other stimulants
   • Cold, surgery, anaesthesia, haemorrhage, pain, emotional stress, nausea/vomiting
   • Angiotensin II
   • Drugs: narcotics, tricyclic antidepressants, nicotine
4. Inhibitors
   • Atrial natriuretic peptide (ANP)
   • Alcohol, narcotic antagonists (e.g. naloxone)

Renal Actions of ADH

• Principal cells: binds V2-receptor → cAMP → PKA → AQP2 insertion into apical membrane; also stimulates UTA1/UTA3 in papillary CD
• Net effect: ↑ water reabsorption, ↑ urea recycling → concentrates urine, conserves water

L. Integrated Concepts / Clinical Correlations

• Diabetes insipidus (central or nephrogenic) ➔ failure to concentrate urine → high output hypo-osmolar urine, polydipsia
• Syndrome of inappropriate ADH (SIADH) ➔ persistent ADH despite hypo-osmolarity → water retention, hyponatraemia
• Loop/thiazide diuretics target NKCC2 and NCC respectively, blunting counter-current mechanisms and diluting ability
• High-protein diet (↑ urea) augments medullary gradient & concentrating capacity
• Infants have shorter loops, lower gradient ➔ limited ability to concentrate urine → risk of dehydration