Renal Regulation of Sodium & Potassium

The kidney is the master organ of sodium and potassium homeostasis, sensing "effective circulating volume" and adjusting tubular handling segment by segment. This note integrates the physiology of aldosterone, ENaC, ROMK, natriuretic peptides, and the channelopathies (Bartter vs Gitelman) that NEET PG loves to test.

Orientation: where sodium and potassium are handled

Filtered sodium (~180 L/day GFR × 140 mEq/L ≈ 25,000 mEq) is reabsorbed segmentally; less than 1% is finally excreted. Potassium is freely filtered, reabsorbed almost completely in the proximal tubule and loop, and then secreted in the distal nephron — making urinary K⁺ essentially a function of distal secretion, not filtration.

Segment	Fraction of Na⁺ reabsorbed	Main transporter	Regulation
Proximal convoluted tubule (PCT)	~65–67%	NHE3 (Na⁺/H⁺ exchanger), Na-glucose/amino acid cotransporters	Angiotensin II, sympathetic
Thick ascending limb (TAL)	~25%	NKCC2 (Na-K-2Cl)	Loop diuretics block; site of Bartter
Distal convoluted tubule (DCT)	~5%	NCC (Na-Cl cotransporter)	Thiazides block; site of Gitelman
Collecting duct — principal cells	~2–3%	ENaC (sodium), ROMK (potassium secretion)	Aldosterone, ADH

High-yield: The collecting duct reabsorbs only a small fraction of sodium, but it is the finely regulated fraction — this is where aldosterone acts and where the body makes its final decision on Na⁺ and K⁺ balance.

Aldosterone and the principal cell

Aldosterone is a mineralocorticoid synthesised in the zona glomerulosa of the adrenal cortex. Its two principal stimuli are angiotensin II (volume depletion) and hyperkalaemia; ACTH plays only a minor, transient role.

Mechanism — the classic flow:

Aldosterone (lipophilic) crosses the principal cell membrane → binds the cytoplasmic mineralocorticoid receptor (MR) → MR–aldosterone complex translocates to nucleus → transcribes SGK1 and serum/glucocorticoid kinase products → increases apical ENaC number/activity and basolateral Na⁺/K⁺-ATPase → Na⁺ enters the cell down the ENaC gradient → lumen becomes electronegative → this favourable gradient drives K⁺ secretion via ROMK and H⁺ secretion via the α-intercalated cell H⁺-ATPase.

This is why aldosterone excess produces the triad of sodium retention, hypokalaemia, and metabolic alkalosis.

High-yield: Aldosterone increases Na⁺ reabsorption and simultaneously drives K⁺ and H⁺ loss — the net result is hypertension (or oedema), hypokalaemia, and metabolic alkalosis.

The 11β-HSD2 "guardian enzyme"

Cortisol binds the MR with equal affinity to aldosterone, yet circulates at ~1000× higher concentration. The enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) in principal cells converts cortisol to inactive cortisone, protecting the MR for aldosterone.

When 11β-HSD2 is inhibited or deficient, cortisol floods the MR → apparent mineralocorticoid excess (AME). Causes:

• Liquorice (glycyrrhetinic acid) ingestion — classic exam vignette.
• Hereditary AME (loss-of-function mutation).
• Ectopic ACTH / severe Cushing's overwhelming the enzyme.

High-yield: Liquorice → 11β-HSD2 inhibition → cortisol acts on MR → hypertension + hypokalaemia + low renin + low aldosterone. The dissociation (low renin AND low aldosterone) distinguishes AME from primary hyperaldosteronism.

Effective circulating volume (ECV) sensing

The kidney does not measure total body sodium directly; it senses effective arterial blood volume — the part of the ECF that perfuses tissues. Sensors:

1. Carotid sinus & aortic arch baroreceptors — high-pressure arterial sensing.
2. Afferent arteriole (renal baroreceptor) — controls renin release.
3. Macula densa — senses NaCl delivery to the distal tubule (tubuloglomerular feedback).
4. Cardiac atria/ventricles — stretch releases natriuretic peptides.

Low ECV (haemorrhage, heart failure, cirrhosis, nephrosis) → activates RAAS + sympathetic + ADH → Na⁺ and water retention. This explains why oedematous states (cirrhosis, CHF) show avid sodium retention despite total body sodium overload — the effective volume is low.

RAAS cascade — quick recall

Renin (from juxtaglomerular cells) → cleaves angiotensinogen to Angiotensin I → ACE (lung) converts to Angiotensin II → vasoconstriction + aldosterone release + proximal Na⁺ reabsorption + ADH/thirst.

Renin release is stimulated by: ↓afferent arteriolar pressure, ↓NaCl at macula densa, and β1-sympathetic stimulation.

Natriuretic peptides — the RAAS antagonists

Peptide	Source	Trigger	Receptor / 2nd messenger
ANP (atrial)	Atrial myocytes	Atrial stretch (volume↑)	NPR-A → cGMP
BNP (B-type)	Ventricular myocytes	Ventricular stretch/strain	NPR-A → cGMP
CNP	Endothelium	Local	NPR-B → cGMP

Actions of ANP/BNP (oppose RAAS):

• ↑ GFR (afferent dilation, efferent constriction) → ↑ filtered Na⁺.
• Inhibit Na⁺ reabsorption in the inner medullary collecting duct.
• Suppress renin and aldosterone release.
• Vasodilation, ↓ ADH, ↓ sympathetic tone.

Degraded by neprLysIn (neutral endopeptidase) — the target of sacubitril (ARNI: sacubitril/valsartan in heart failure). BNP/NT-proBNP are diagnostic markers of heart failure.

High-yield: Natriuretic peptides work via cGMP, promote natriuresis, and inhibit renin/aldosterone — they are the body's natural counter to volume overload. Sacubitril potentiates them by blocking their breakdown.

Determinants of potassium balance

Potassium is the major intracellular cation (~150 mEq/L inside cells; ~4 mEq/L in plasma). Two systems regulate it:

1. Internal balance (transcellular shift) — minute-to-minute:

Drives K⁺ INTO cells (↓ serum K⁺)	Drives K⁺ OUT of cells (↑ serum K⁺)
Insulin	Insulin deficiency / hyperglycaemia (DKA)
β2-agonists (salbutamol, adrenaline)	β-blockers
Alkalosis	Acidosis (esp. mineral acid)
Aldosterone (minor)	Cell lysis (tumour lysis, rhabdomyolysis, haemolysis)
	Digoxin (Na⁺/K⁺-ATPase block), succinylcholine, hyperosmolality

2. External balance (renal excretion) — hours to days: depends on distal K⁺ secretion via ROMK, governed by:

• Aldosterone (↑ secretion).
• Distal Na⁺ delivery and tubular flow (high flow → washes away luminal K⁺ → ↑ secretion; explains diuretic-induced hypokalaemia).
• Plasma K⁺ itself (hyperkalaemia stimulates both aldosterone and direct secretion).
• Acid–base status (alkalosis promotes kaliuresis).

High-yield: In DKA, total body potassium is depleted despite a normal/high serum K⁺ (acidosis + insulin deficiency + hyperosmolality shift K⁺ out). Insulin therapy unmasks profound hypokalaemia — always add potassium.

Mnemonic for hyperkalaemia ECG progression — "Tall T, then trouble": peaked T waves → flattened/wide P → prolonged PR → widened QRS → sine wave → asystole.

Hyperaldosteronism — electrolyte patterns

Feature	Primary hyperaldosteronism (Conn)	Secondary hyperaldosteronism
Cause	Adrenal adenoma / bilateral hyperplasia	Renal artery stenosis, CHF, cirrhosis, diuretics
Renin	Low (suppressed)	High
Aldosterone	High	High
Aldosterone : renin ratio (ARR)	High (>20–30, screening test)	Normal/low
BP	Hypertension	Variable (often oedema in CHF/cirrhosis)
K⁺	Low (may be normal)	Variable
Acid–base	Metabolic alkalosis	Variable

High-yield: The aldosterone-to-renin ratio (ARR) is the screening test for primary hyperaldosteronism. Confirm with a saline suppression / salt-loading test (failure to suppress aldosterone confirms autonomy). Adrenal venous sampling differentiates unilateral adenoma from bilateral hyperplasia.

Treatment: unilateral adenoma → laparoscopic adrenalectomy; bilateral hyperplasia → spironolactone or eplerenone (MR antagonists). Eplerenone is more selective and causes less gynaecomastia.

Bartter vs Gitelman syndrome — a NEET PG favourite

Both are autosomal recessive salt-wasting tubulopathies producing hypokalaemic metabolic alkalosis with normal/low BP and high renin/aldosterone (i.e., they mimic chronic loop/thiazide diuretic use). The differentiator is the defective segment and the calcium/magnesium profile.

Feature	Bartter syndrome	Gitelman syndrome
Defective segment	Thick ascending limb (TAL)	Distal convoluted tubule (DCT)
Transporter affected	NKCC2 / ROMK / ClC-Kb	NCC (Na-Cl cotransporter)
"Acts like" which diuretic	Loop diuretic (furosemide)	Thiazide
Urinary calcium	High (hypercalciuria) → nephrocalcinosis	Low (hypocalciuria)
Serum magnesium	Normal / mildly low	Low (hypomagnesaemia — hallmark)
Age of presentation	Neonatal / early childhood, often severe	Adolescence / adulthood, milder
Clinical clues	Polyhydramnios, failure to thrive, polyuria	Tetany, cramps, fatigue, chondrocalcinosis

Memory hook: Bartter = Big (severe, early, high calcium, loop-like). Gitelman = Gentle, Grown-up, low calcium, low Magnesium, thiazide-like.

High-yield: Urinary calcium is the single best discriminator: Bartter = hypercalciuria, Gitelman = hypocalciuria. Hypomagnesaemia strongly favours Gitelman.

Both share: hypokalaemia, metabolic alkalosis, hyperreninaemia, hyperaldosteronism, normal or low blood pressure (this normal BP distinguishes them from Liddle and hyperaldosteronism, which are hypertensive).

Liddle syndrome — the hypertensive mimic

Liddle syndrome is an autosomal dominant gain-of-function mutation of ENaC → constitutive sodium reabsorption independent of aldosterone.

• Profile: hypertension + hypokalaemia + metabolic alkalosis with LOW renin AND LOW aldosterone.
• Treatment: amiloride / triamterene (ENaC blockers), NOT spironolactone (the defect is downstream of the MR).

High-yield: Liddle (low renin, low aldosterone, hypertensive, responds to amiloride) vs Conn (low renin, high aldosterone, responds to spironolactone). The aldosterone level and the choice of diuretic separate them.

A unified approach to hypokalaemic metabolic alkalosis

When you see hypokalaemia + metabolic alkalosis, use blood pressure and renin/aldosterone:

1. Hypertensive? → think Conn's, Liddle, AME (liquorice), Cushing's, renovascular HTN.
   • High aldosterone, low renin → Conn's.
   • Low aldosterone, low renin → Liddle / AME.
   • High aldosterone, high renin → renovascular / renin-secreting tumour.
2. Normotensive / hypotensive? → think Bartter, Gitelman, diuretic abuse, vomiting.
   • Check urine chloride: low in vomiting (chloride-responsive), high in Bartter/Gitelman/active diuretic use.
   • Then use urine calcium / serum Mg²⁺ to split Bartter vs Gitelman.

Complications

• Chronic hypokalaemia: muscle weakness, ileus, rhabdomyolysis, arrhythmias (U waves, prolonged QT, torsades), hypokalaemic nephropathy (impaired concentrating ability, polyuria), worsened digoxin toxicity.
• Chronic hyperaldosteronism: LV hypertrophy, vascular fibrosis, stroke risk beyond what BP predicts (aldosterone has direct cardiac/vascular toxicity — rationale for MR antagonists in heart failure).
• Bartter: nephrocalcinosis, growth retardation, chronic kidney disease.
• Gitelman: tetany, chondrocalcinosis, ventricular arrhythmia from combined hypokalaemia + hypomagnesaemia.

Key differentials at a glance

Disorder	BP	Renin	Aldosterone	K⁺	Distinguishing clue
Conn's (primary hyperaldo)	↑	↓	↑	↓	High ARR
Secondary hyperaldo	variable	↑	↑	variable	Underlying cause (RAS, CHF)
Liddle	↑	↓	↓	↓	Responds to amiloride
AME / liquorice	↑	↓	↓	↓	11β-HSD2 block
Bartter	normal/↓	↑	↑	↓	Hypercalciuria, loop-like
Gitelman	normal/↓	↑	↑	↓	Hypocalciuria + hypomagnesaemia
Vomiting	normal/↓	↑	↑	↓	Low urine chloride

Recently asked / exam angle

• Bartter vs Gitelman discrimination via urinary calcium — the most repeated PG one-liner: Bartter = hypercalciuria, Gitelman = hypocalciuria; Gitelman additionally has hypomagnesaemia.
• "Acts like which diuretic?" Bartter ≈ furosemide (loop), Gitelman ≈ thiazide.
• Liddle syndrome mechanism (ENaC gain-of-function) and the amiloride vs spironolactone treatment distinction.
• Liquorice/AME: low renin AND low aldosterone with hypertension + hypokalaemia → 11β-HSD2 inhibition.
• DKA potassium paradox — normal serum K⁺ with total-body depletion; add K⁺ with insulin.
• ARR as screening, saline suppression as confirmation for primary hyperaldosteronism.
• Natriuretic peptides act via cGMP and are degraded by neprilysin (sacubitril mechanism).
• Site of aldosterone action = principal cell of cortical collecting duct; transporters ENaC (Na⁺ in) and ROMK (K⁺ out).
• 11β-HSD2 as the enzyme protecting the MR from cortisol.
• Spironolactone vs eplerenone — eplerenone selective, less gynaecomastia.

Rapid revision

1. PCT reabsorbs ~67% of filtered Na⁺ (NHE3); TAL ~25% (NKCC2); DCT ~5% (NCC); collecting duct ~2–3% (ENaC) — the last is the aldosterone-regulated fraction.
2. Aldosterone acts on principal cells: ↑ENaC + ↑Na⁺/K⁺-ATPase → Na⁺ retention, K⁺ and H⁺ loss → hypokalaemic metabolic alkalosis.
3. 11β-HSD2 converts cortisol to inactive cortisone, guarding the MR; its inhibition (liquorice) causes apparent mineralocorticoid excess.
4. Conn's = low renin, high aldosterone; Liddle & AME = low renin, low aldosterone — all hypertensive with hypokalaemia.
5. Screen primary hyperaldosteronism with the aldosterone-renin ratio; confirm by saline/salt-loading suppression.
6. Natriuretic peptides (ANP/BNP) oppose RAAS via cGMP, are broken down by neprilysin (sacubitril target), and BNP marks heart failure.
7. Effective circulating volume — not total body Na⁺ — drives renal sodium handling; explains avid retention in CHF and cirrhosis.
8. Bartter = TAL defect, loop-diuretic-like, hypercalciuria, severe/neonatal; Gitelman = DCT defect, thiazide-like, hypocalciuria + hypomagnesaemia, milder.
9. Both Bartter and Gitelman are normotensive with hypokalaemic metabolic alkalosis and high renin/aldosterone.
10. Liddle (ENaC gain-of-function) responds to amiloride/triamterene, not spironolactone.
11. Distal K⁺ secretion via ROMK rises with aldosterone, high distal Na⁺ delivery, and high tubular flow.
12. In DKA, serum K⁺ overestimates total body K⁺; insulin shifts K⁺ intracellularly — replace potassium early.