Quantitative Water and Sodium Reabsorption by Nephron Segment
The nephron reabsorbs approximately 60-70% of filtered sodium and water in the proximal tubule, 20-30% of sodium (with no water) in the thick ascending limb, 5-10% in the distal tubule, and the final 3-5% in the collecting duct, with hormonal regulation primarily affecting the distal segments through aldosterone, ADH, angiotensin II, and ANP.
Baseline Reabsorption Without Hormonal Influence
Proximal Tubule
- Sodium reabsorption: 60-70% of filtered load 1, 2
- Water reabsorption: 60-70% of filtered load (coupled with sodium) 1
- Bicarbonate reabsorption: ~90% of filtered load 1
- Mechanism: Na+/H+ exchange on luminal membrane, driven by basolateral Na+/K+-ATPase 1
- This segment maintains glomerulotubular balance—absorption is proportional to filtered load
Thick Ascending Limb of Loop of Henle
- Sodium reabsorption: 20-30% of filtered load 1, 2
- Water reabsorption: 0% (water-impermeable segment) 1, 2
- Mechanism: Na+-K+-2Cl- cotransporter (NKCC2) driven by basolateral Na+/K+-ATPase 3, 1
- This is the primary site of action for loop diuretics 4, 5
Distal Convoluted Tubule
- Sodium reabsorption: 5-10% of filtered load 1, 2
- Water reabsorption: Variable, depends on ADH presence
- Mechanism: Na+-Cl- cotransporter (NCC) driven by basolateral Na+/K+-ATPase 1
- This is the site of action for thiazide diuretics 4, 5
Collecting Duct
- Sodium reabsorption: 3-5% of filtered load 1, 2
- Water reabsorption: 10-15% of filtered load (ADH-dependent) 1
- Mechanism: Epithelial sodium channels (ENaC) for sodium; aquaporin-2 channels for water 6, 1
Hormonal Regulation and Quantitative Changes
Aldosterone Effects
Primary site: Collecting duct (principal cells) 7, 8
- Increases sodium reabsorption in the collecting duct by upregulating ENaC channels 3, 8
- Increases Na+/K+-ATPase activity in basolateral membrane 7
- Effect is slow (requires 24-72 hours) because it involves gene transcription 7, 8
- In cirrhosis with ascites, hyperaldosteronism is the primary driver of increased distal tubular sodium reabsorption 7
- Can increase sodium retention sufficiently to cause fluid overload when unopposed 7
Antidiuretic Hormone (ADH/Vasopressin) Effects
Primary site: Collecting duct 6, 1, 9
- Increases water reabsorption by inserting aquaporin-2 channels into the apical membrane of collecting duct cells 6
- Allows reabsorption of 10-15% of filtered water when maximally stimulated 1
- Also increases sodium reabsorption via ENaC activation in the collecting duct 9
- This dual action (water + sodium reabsorption) is critical for maintaining the corticomedullary osmotic gradient 9
- Without ADH, urine osmolality remains <200 mOsm/kg; with maximal ADH, can exceed 1200 mOsm/kg 6
Angiotensin II Effects
Primary sites: Proximal tubule and efferent arteriole 3, 10, 11
- Stimulates sodium and water reabsorption in the proximal tubule 11, 12
- Causes preferential efferent arteriolar vasoconstriction, increasing filtration fraction 3
- Reduces renal blood flow but maintains or increases GFR 3
- Stimulates aldosterone release from adrenal cortex 10, 13
- ANP at physiological concentrations inhibits angiotensin II-stimulated proximal reabsorption 11
Atrial Natriuretic Peptide (ANP) Effects
Primary site: Collecting duct, with proximal tubule modulation 11, 14
- Decreases sodium reabsorption in the collecting duct 11
- Antagonizes angiotensin II effects in the proximal tubule at physiological (picomolar) concentrations 11
- At higher concentrations, can completely abolish angiotensin II-stimulated proximal reabsorption 11
- Does not directly affect basal proximal tubular reabsorption in the absence of angiotensin II 11
- Increases sodium excretion without necessarily changing GFR 11, 14
Integrated Hormonal Response to Volume Loading
When saline is administered (as studied in 14):
- Plasma renin concentration decreases by 33%
- Plasma angiotensin II decreases by 50%
- Plasma aldosterone decreases by 67%
- Sodium excretion increases from ~15 μmol/min to 55-70 μmol/min
- These changes occur independent of beta-1 adrenergic receptor activity 14
- Central venous pressure increases by ~2 mmHg, with sodium excretion increasing at ~7 μmol/min per mmHg CVP rise 14
Clinical Context: Diuretic Resistance Mechanisms
The 2025 heart failure guidelines 3 emphasize that distal tubular hypertrophy and hyperplasia develop in chronic volume overload states, leading to:
- Compensatory increases in NaCl reabsorption through the NCC cotransporter in the distal tubule 3
- Enhanced sodium reabsorption in the collecting duct via aldosterone-mediated ENaC upregulation 3
- This represents the majority of diuretic resistance mechanisms 3
Net Urinary Excretion Range
Under normal physiological conditions with intact hormonal regulation 1:
- Sodium excretion: 0.1% to 3% of filtered load
- Water excretion: 0.3% to 15% of filtered load
This means that from approximately 25,000 mEq of sodium filtered daily, urinary excretion can range from 25 mEq to 750 mEq depending on hormonal milieu and volume status.
Key Clinical Pitfalls
Avoid assuming linear dose-response relationships: Aldosterone effects require 72+ hours to manifest fully 7, while loop diuretics act within hours 4, 5. Titrating aldosterone antagonists more frequently than every 7 days is physiologically inappropriate 7.
Recognize the primacy of aldosterone in volume overload states: In cirrhosis and heart failure, aldosterone-mediated distal sodium reabsorption is the dominant mechanism 7, 15. Loop diuretics alone will fail without aldosterone antagonism because hyperaldosteronism can completely override their natriuretic effect 7.
Understand that ADH affects both water AND sodium: The traditional teaching focuses on ADH's water-reabsorbing effects, but its stimulation of ENaC-mediated sodium reabsorption in the collecting duct is equally important for maintaining the osmotic gradient necessary for water reabsorption 9.