Medicine – Physiology of the Renal Tubule

The renal tubule modifies the glomerular filtrate by selectively reabsorbing substances that the body needs and secreting substances that must be eliminated. Different nephron segments have distinct transport functions, and many diuretics act at specific tubular sites.

The major functional regions are the proximal tubule, loop of Henle, distal convoluted tubule and collecting duct.

1. Proximal Tubule

The proximal convoluted tubule – PCT performs the largest proportion of tubular reabsorption.

The original figure of:

50% sodium reabsorbed

is somewhat low.

In modern physiology, approximately:

65–70% of filtered sodium and water

are reabsorbed in the proximal tubule.

Water follows sodium almost proportionately, so proximal tubular reabsorption is largely:

Iso-osmotic.

2. Sodium Reabsorption in the Proximal Tubule

Sodium enters proximal tubular cells through several transport systems, including:

Na⁺/H⁺ exchange.

Na⁺-glucose cotransport.

Na⁺-amino acid cotransport.

The basolateral:

Na⁺/K⁺-ATPase

then pumps sodium from the tubular cell into the interstitium.

Therefore:

PCT → REABSORBS ABOUT TWO-THIRDS OF FILTERED Na⁺ AND WATER.

3. Bicarbonate Reabsorption

The proximal tubule reabsorbs most filtered:

Bicarbonate – HCO₃⁻.

Approximately:

80–90%

of filtered bicarbonate is reclaimed here.

This process depends importantly on:

Hydrogen ion secretion

and

Carbonic anhydrase.

4. Mechanism of Bicarbonate Reabsorption

Tubular cells secrete:

H⁺

into the lumen, largely through the:

Na⁺/H⁺ exchanger.

Hydrogen combines with filtered bicarbonate:

H⁺ + HCO₃⁻ → H₂CO₃.

Carbonic anhydrase facilitates conversion to:

CO₂ + H₂O.

CO₂ enters the tubular cell, where bicarbonate is regenerated and transported back into blood.

Therefore:

PCT = MAJOR SITE OF BICARBONATE RECLAMATION.

5. Carbonic Anhydrase Inhibitors

Because bicarbonate reabsorption depends on carbonic anhydrase, drugs such as:

Acetazolamide

reduce proximal bicarbonate reabsorption.

This causes:

Bicarbonaturia.

Alkaline urine initially.

Metabolic acidosis.

Acetazolamide is therefore a:

Proximal tubular diuretic.

6. Glucose and Amino Acid Reabsorption

The proximal tubule normally reabsorbs almost all filtered:

Glucose

and

Amino acids.

Glucose reabsorption occurs through sodium-glucose cotransporters, particularly:

SGLT2

in the early proximal tubule.

7. SGLT2 Inhibitors

Drugs such as:

Dapagliflozin

and

Empagliflozin

inhibit SGLT2.

This reduces proximal glucose and sodium reabsorption and causes:

Glycosuria

with mild:

Natriuresis and osmotic diuresis.

These drugs are important in modern treatment of:

Type 2 diabetes, CKD and heart failure.

8. Phosphate Reabsorption

The proximal tubule is also the major site of:

Phosphate reabsorption.

Filtered phosphate is normally reabsorbed through:

Sodium-phosphate cotransporters.

9. Effect of PTH on Phosphate

The original notes correctly associate phosphate handling with:

Parathyroid hormone – PTH.

However, PTH does not increase phosphate reabsorption.

Instead, PTH:

DECREASES proximal tubular phosphate reabsorption.

Therefore:

PTH → PHOSPHATURIA → ↑ URINARY PHOSPHATE EXCRETION.

This is an important correction.

10. Urate Handling

The proximal tubule has a major role in handling:

Urate.

Urate undergoes a complex combination of:

Filtration.

Reabsorption.

Secretion.

Post-secretory reabsorption.

Therefore the final urinary urate concentration reflects several proximal tubular transport processes rather than simple secretion alone.

11. Creatinine Secretion

Most creatinine is eliminated by:

Glomerular filtration.

However, a small amount is also:

Secreted by the proximal tubule.

This is why creatinine clearance slightly:

Overestimates true GFR.

12. Drugs Affecting Creatinine Secretion

Certain medications inhibit proximal tubular creatinine secretion.

Important examples include:

Trimethoprim.

Cimetidine.

These may produce:

A modest increase in serum creatinine without a true fall in GFR.

13. Other Proximal Tubule Functions

The proximal tubule also reabsorbs much of the filtered:

Potassium.

Calcium.

Phosphate.

Urea.

It also reabsorbs nearly all filtered:

Small proteins and peptides

through endocytic mechanisms.

Therefore proximal tubular dysfunction can produce:

Glucosuria without hyperglycaemia.

Phosphaturia.

Bicarbonaturia.

Aminoaciduria.

Tubular proteinuria.

14. Fanconi Syndrome

Generalized dysfunction of the proximal tubule is called:

Fanconi syndrome.

It can cause urinary loss of:

Glucose.

Phosphate.

Bicarbonate.

Amino acids.

Uric acid.

This may result in:

Proximal type 2 renal tubular acidosis.

15. Loop of Henle

The loop of Henle is essential for generating the:

Medullary concentration gradient.

This gradient allows the kidney to produce concentrated urine when:

ADH is present.

16. Descending Limb

The thin descending limb is highly permeable to:

Water.

However, it is relatively less permeable to electrolytes.

As tubular fluid descends into the increasingly hypertonic medulla:

Water leaves the tubule.

The tubular fluid therefore becomes:

More concentrated.

17. Thick Ascending Limb

The thick ascending limb behaves very differently.

It is essentially:

Impermeable to water.

But it actively reabsorbs:

Na⁺, K⁺ and Cl⁻.

18. NKCC2 Cotransporter

The major transporter in the thick ascending limb is:

Na⁺-K⁺-2Cl⁻ cotransporter – NKCC2.

This reabsorbs:

1 Na⁺ + 1 K⁺ + 2 Cl⁻

from the tubular lumen.

19. Sodium Reabsorption in the Loop

The original notes state:

40% sodium reabsorption.

This is higher than the modern estimate for the loop itself.

Approximately:

20–25% of filtered sodium

is reabsorbed in the:

Thick ascending limb.

Therefore:

PCT ≈ 65–70%.

THICK ASCENDING LIMB ≈ 20–25%.

DISTAL TUBULE ≈ 5%.

The remainder is fine-tuned in the distal nephron and collecting duct.

20. Diluting Segment

Because the thick ascending limb removes solute without allowing water to follow, it dilutes the tubular fluid.

It is therefore called a:

Diluting segment.

At the same time, NaCl accumulation in the medullary interstitium contributes to the:

Corticomedullary osmotic gradient.

21. Countercurrent Multiplication

The interaction between:

Descending limb water permeability

and

Ascending limb active NaCl transport

creates:

Countercurrent multiplication.

This establishes a progressively hyperosmotic environment toward the:

Inner medulla.

22. Medullary Concentration Gradient

The medullary gradient is produced mainly by:

NaCl reabsorption from the thick ascending limb

and

Urea recycling in the inner medulla.

This gradient is essential for:

ADH-dependent water reabsorption in the collecting duct.

23. Loop Diuretics

The original notes correctly identify:

Furosemide

as a loop diuretic.

Other examples include:

Bumetanide.

Torsemide.

24. Mechanism of Loop Diuretics

Loop diuretics inhibit:

NKCC2

in the thick ascending limb.

This reduces:

NaCl reabsorption.

As a result, more sodium remains in the tubular lumen and water follows.

Therefore:

LOOP DIURETIC → NKCC2 BLOCKADE → POWERFUL NATRIURESIS AND DIURESIS.

25. Calcium and Magnesium in the Loop

The lumen-positive electrical potential in the thick ascending limb promotes paracellular reabsorption of:

Calcium

and

Magnesium.

Loop diuretics reduce this potential.

Therefore they increase urinary excretion of:

Ca²⁺ and Mg²⁺.

A useful memory point is:

LOOPS LOSE CALCIUM.

26. Distal Convoluted Tubule

The distal convoluted tubule performs further fine control of:

Sodium, chloride and calcium handling.

Approximately:

5% of filtered sodium

is reabsorbed here.

This part of the original notes is therefore broadly correct.

27. Sodium-Chloride Cotransporter

The major sodium transporter in the early distal convoluted tubule is:

Na⁺-Cl⁻ cotransporter – NCC.

This transporter is inhibited by:

Thiazide diuretics.

28. Thiazide Diuretics

Examples include:

Hydrochlorothiazide.

Bendroflumethiazide.

Chlortalidone/chlorthalidone.

Indapamide is thiazide-like.

These drugs inhibit:

NCC

and therefore reduce:

NaCl reabsorption.

29. Calcium and Thiazides

Thiazides have an important effect on calcium:

They increase renal calcium reabsorption.

Therefore urinary calcium decreases.

A useful contrast is:

LOOP DIURETICS → ↑ URINARY Ca²⁺.

THIAZIDES → ↓ URINARY Ca²⁺.

30. PTH in the Distal Tubule

PTH promotes:

Calcium reabsorption

in the distal nephron.

Thus the distal tubule contributes importantly to:

Fine regulation of calcium balance.

31. Where Does Spironolactone Act?

The original notes place:

Spironolactone

under the distal tubule.

This requires refinement.

Spironolactone acts primarily on:

Mineralocorticoid receptors

in principal cells of the:

Late distal tubule and cortical collecting duct.

Therefore it is more accurate to place it in the:

Aldosterone-sensitive distal nephron.

32. Spironolactone Mechanism

Spironolactone antagonises:

Aldosterone receptors.

This reduces expression and activity of sodium transport mechanisms including:

ENaC

and the:

Na⁺/K⁺-ATPase.

Therefore:

Less sodium is reabsorbed

and

less potassium is secreted.

33. Potassium-Sparing Effect

Because spironolactone decreases potassium secretion, it is classified as a:

Potassium-sparing diuretic.

A major adverse effect is therefore:

Hyperkalaemia.

34. Collecting Duct

The collecting duct provides the final regulation of:

Water.

Sodium.

Potassium.

Hydrogen ions.

It is strongly influenced by:

ADH

and

Aldosterone.

35. Sodium Reabsorption in the Collecting Duct

Only a relatively small proportion of the originally filtered sodium reaches this region.

Approximately:

A few percent

of filtered sodium is reabsorbed in the late distal nephron and collecting system.

Although quantitatively small, this segment is physiologically important because it allows:

Precise hormonal regulation of sodium balance.

36. ENaC

Principal cells reabsorb sodium through:

Epithelial sodium channels – ENaC.

These channels are stimulated by:

Aldosterone.

Therefore:

ALDOSTERONE → ↑ ENaC ACTIVITY → ↑ Na⁺ REABSORPTION.

37. Potassium Secretion

Principal cells also secrete:

Potassium.

Aldosterone increases potassium secretion.

Therefore:

ALDOSTERONE → Na⁺ RETENTION + K⁺ LOSS.

38. Amiloride

The potassium-sparing diuretic:

Amiloride

acts directly by blocking:

ENaC.

This differs from spironolactone, which blocks:

The aldosterone receptor.

39. Hydrogen Ion Secretion

The collecting duct plays a major role in final urinary:

Acidification.

Specialised cells called:

α-intercalated cells

secrete:

Hydrogen ions.

40. Alpha-Intercalated Cells

α-intercalated cells use pumps including:

H⁺-ATPase

to secrete hydrogen into the tubular lumen.

At the same time, bicarbonate is returned to:

The blood.

Therefore these cells help defend against:

Metabolic acidosis.

41. Minimum Urine pH

Through distal hydrogen secretion, normal kidneys can reduce urinary pH to approximately:

4.5.

Failure of distal acid secretion occurs in:

Distal type 1 renal tubular acidosis.

42. ADH Action

The original notes correctly identify the collecting duct as the major site of:

ADH action.

ADH binds:

V₂ receptors

on collecting-duct principal cells.

43. Aquaporin-2

V₂ receptor stimulation leads to insertion of:

Aquaporin-2 water channels

into the apical membrane.

Water can then move out of the collecting duct into the hyperosmotic medullary interstitium.

Therefore:

ADH → AQUAPORIN-2 → ↑ WATER REABSORPTION → CONCENTRATED URINE.

44. What Happens Without ADH?

Without ADH, the collecting duct remains relatively:

Impermeable to water.

Therefore large amounts of dilute urine are excreted.

This is the physiological basis of:

Diabetes insipidus.

45. What Happens With Excess ADH?

Excessive ADH causes excessive water retention.

This occurs in:

SIADH.

The result is:

Dilutional hyponatraemia.

46. Segment-by-Segment Note Form

Proximal tubule:

Reabsorbs approximately 65–70% Na⁺ and water.

Reabsorbs approximately 80–90% bicarbonate.

Reabsorbs nearly all glucose and amino acids.

Major phosphate reabsorption site.

PTH decreases phosphate reabsorption.

Small amount of creatinine secretion.

Complex urate reabsorption and secretion.

Acetazolamide acts here.

SGLT2 inhibitors act here.

Loop of Henle:

Descending limb → water reabsorption.

Thick ascending limb → Na⁺/K⁺/2Cl⁻ reabsorption through NKCC2.

Thick ascending limb impermeable to water.

Approximately 20–25% Na⁺ reabsorbed.

Generates medullary concentration gradient.

Loop diuretics such as furosemide act here.

Distal convoluted tubule:

Approximately 5% NaCl reabsorbed.

Na⁺-Cl⁻ cotransporter – NCC.

Thiazide diuretics act here.

Increases calcium reabsorption.

PTH promotes distal calcium reabsorption.

Late distal tubule / collecting duct:

Fine control of Na⁺ and K⁺.

Aldosterone stimulates sodium reabsorption and potassium secretion.

Spironolactone blocks mineralocorticoid receptors.

Amiloride blocks ENaC.

Intercalated cells regulate acid–base balance.

ADH controls water permeability through aquaporin-2.

47. Important Corrections to the Original Notes

The original:

“50% sodium reabsorbed in the proximal tubule”

is better approximated as:

ABOUT 65–70%.

The original:

“40% sodium reabsorbed in the loop of Henle”

is too high for modern standard physiology.

The thick ascending limb reabsorbs approximately:

20–25%.

The statement:

“Phosphate reabsorption (PTH)”

could be misleading.

PTH actually:

DECREASES PROXIMAL PHOSPHATE REABSORPTION → INCREASES PHOSPHATE EXCRETION.

Spironolactone should not be thought of as acting mainly on the early distal convoluted tubule.

It acts at:

MINERALOCORTICOID RECEPTORS IN THE LATE DISTAL TUBULE AND COLLECTING DUCT.

48. Diuretic Sites of Action

A useful nephron sequence is:

PROXIMAL TUBULE → ACETAZOLAMIDE + SGLT2 INHIBITORS.

↓

THICK ASCENDING LOOP → LOOP DIURETICS.

↓

DISTAL CONVOLUTED TUBULE → THIAZIDES.

↓

COLLECTING DUCT/LATE DISTAL NEPHRON → SPIRONOLACTONE + AMILORIDE.

Key Clinical Pattern

Remember the nephron from proximal to distal:

PCT → BULK REABSORPTION.

LOOP → BUILDS MEDULLARY GRADIENT.

DCT → FINE-TUNES NaCl AND CALCIUM.

COLLECTING DUCT → HORMONAL FINE CONTROL OF Na⁺, K⁺, H⁺ AND WATER.

And remember the key transporters:

PCT → SGLT2 + Na⁺/H⁺ exchange.

THICK ASCENDING LOOP → NKCC2.

DCT → NCC.

COLLECTING DUCT → ENaC + AQUAPORIN-2.

Finally, the high-yield diuretic sequence is:

ACETAZOLAMIDE → PCT.

FUROSEMIDE → LOOP/NKCC2.

THIAZIDE → DCT/NCC.

SPIRONOLACTONE → ALDOSTERONE RECEPTOR.

AMILORIDE → ENaC.