So the filtrate has finished DCT. About 5% of sodium is left. Zero glucose. The fluid is dilute. It enters the collecting duct — and here, for the first time, the nephron STOPS and WAITS for hormonal instructions.
Everything upstream — PCT, LOH, DCT — was mostly on autopilot. Transporters running constitutively. But the CD does nothing until it hears from its two masters: Aldosterone and AVP.
The CD has three cell types:
1. Principal cells — these listen to BOTH aldosterone and AVP
2. α-Intercalated cells — these secrete H⁺ (acid) into urine
3. β-Intercalated cells — these secrete HCO₃⁻ (rarely relevant, skip for exam)
Where does aldosterone come from?
Adrenal cortex, specifically the zona glomerulosa — the outermost layer. Remember: the adrenal cortex goes GFR from outside to inside — Glomerulosa (mineralocorticoid = salt), Fasciculata (glucocorticoid = sugar), Reticularis (androgen = sex). "Deeper you go, sweeter it gets" — salt, sugar, sex.
What triggers aldosterone release?
Two main triggers:
1. Angiotensin II — from RAAS (primary trigger, I'll cover this in Part 4)
2. Hyperkalemia — direct trigger, independent of RAAS. K⁺ rises in blood → adrenal senses it → releases aldosterone → CD secretes K⁺. This is the safety valve — even if RAAS is broken, hyperK⁺ can still trigger aldo to dump K⁺.
3. ACTH — minor, permissive role. Not a main driver.
What does aldosterone DO at the principal cell?
It binds the mineralocorticoid receptor (MR) — this is intracellular, nuclear. Goes to nucleus, turns on gene transcription, upregulates four things:
1. ENaC — epithelial Na⁺ channel on the apical (lumen-facing) side. Na⁺ flows IN from lumen into cell.
2. ROMK — K⁺ channel on the apical side. K⁺ flows OUT from cell into lumen. (Remember: ROMK = Renal Outer Medullary K⁺ channel. Not ROMTK.)
3. Na⁺/K⁺-ATPase — on the basolateral (blood-facing) side. Pumps Na⁺ to blood, K⁺ into cell. This creates the gradient that makes ENaC and ROMK work.
4. H⁺-ATPase — minor. Secretes some H⁺ into lumen. Contributes to alkalosis when aldo is excess.
Net effect of aldosterone: Na⁺ RETAINED, K⁺ EXCRETED, H⁺ EXCRETED → volume goes up, K⁺ goes down, metabolic alkalosis.
TOO MUCH = Conn's syndrome (primary hyperaldosteronism):
Adrenal adenoma → pumps out aldo autonomously → ENaC always ON → Na⁺ retention → resistant hypertension + hypoK⁺ + metabolic alkalosis. Renin is SUPPRESSED (body is volume-expanded, no need for RAAS). Screening test: aldosterone-to-renin ratio (ARR) >30. Indian context: underdiagnosed cause of resistant HTN. Any patient on 3+ antihypertensives who doesn't respond — check ARR.
TOO LITTLE = RTA Type 4 (hypoaldosteronism):
No aldo → ENaC doesn't open → Na⁺ not reabsorbed → ROMK doesn't open → K⁺ NOT secreted → HYPERK⁺. Also H⁺-ATPase underactive → less acid secretion → mild metabolic acidosis (but HCO₃⁻ usually stays >17).
#1 cause: DM nephropathy. Diabetes damages JG cells on AFFERENT arteriole → less renin → less Ang II → less aldo = "hyporeninemic hypoaldosteronism." Also caused by ACEi/ARB (↓Ang II → ↓aldo), spironolactone (blocks MR directly), NSAIDs (↓renin).
Aldosterone escape (bonus concept):
In CHF and cirrhosis, RAAS is chronically active → aldo keeps telling CD to retain Na⁺. But after a few days, ANP and BNP rise and cause "escape" — Na⁺ retention plateaus. BUT K⁺ wasting does NOT escape — it continues. This is why CHF patients develop hypoK⁺ over time. And this is why spironolactone works in CHF (RALES trial — 30% ↓ mortality) — it blocks the aldo that keeps wasting K⁺ and causing fibrosis even after Na⁺ escape.
Same molecule, three names: Arginine Vasopressin (AVP) = Antidiuretic Hormone (ADH) = Vasopressin. 9 amino acids.
Where does it come from?
Made in the supraoptic nucleus of hypothalamus (mainly). Also paraventricular nucleus. Travels down axons → stored in posterior pituitary. Released when needed.
What triggers AVP release?
1. Osmotic trigger (primary): Serum osmolality rises above ~285 mOsm/kg → osmoreceptors in hypothalamus fire → AVP released. Response is steep above 290. This is exquisitely sensitive — 1-2% change in osmolality triggers response.
2. Non-osmotic triggers: ↓Blood volume (baroreceptors in carotid and aorta detect low stretch → signal AVP release — this OVERRIDES the osmostat, meaning a volume-depleted patient will retain water even if already hypo-osmolar), pain, nausea, stress, drugs, and critically — Angiotensin II stimulates AVP release. This links RAAS to water retention: CHF → ↑RAAS → ↑Ang II → ↑AVP → water retained → dilutional hypoNa⁺. This is WHY CHF patients are hyponatremic.
Thirst: Thirst center activates at ~295 mOsm/kg — HIGHER than AVP threshold (285). So AVP is first defense, thirst is backup. Elderly with impaired thirst sensation → vulnerable to hypernatremia.
What does AVP DO? Three receptors:
V1a — on vascular smooth muscle → Gq → IP3/Ca²⁺ → vasoconstriction. This is the "vasopressin" action. Clinical use: vasopressin drip in septic shock (V1a vasoconstriction when catecholamines are failing).
V2 — on CD principal cell (basolateral) → Gs → cAMP → PKA → Aquaporin-2 (AQP2) inserted into apical membrane → water channels open → water floods back from lumen into cell → into blood → concentrated urine.
V1b (V3) — on anterior pituitary → ACTH release. Stress response link. Minor.
The key action at CD: V2 → AQP2 → water reabsorbed. No AVP = no AQP2 = water stays in lumen = dilute polyuria.
TOO MUCH AVP = SIADH:
Inappropriate AVP → AQP2 always inserted → water pours back in → serum diluted → euvolemic dilutional hyponatremia.
Diagnosis: serum Na⁺ <135 + serum osm <275 + urine osm >100 (inappropriately concentrated — kidneys SHOULD be making dilute urine in hypo-osmolar state, but AVP forces concentration) + urine Na⁺ >40 + EUVOLEMIC (no edema, no dehydration).
Causes: SCLC (ectopic AVP production — #1 malignant cause), pneumonia, CNS (meningitis, SAH, stroke), drugs (SSRIs, carbamazepine, cyclophosphamide, ecstasy).
Treatment:
— First-line: fluid restriction
— Refractory: Tolvaptan — V2 receptor antagonist. "Vaptan = aquaretic" (excretes water without Na⁺, unlike diuretics).
— Severe/seizures: 3% hypertonic saline
— CRITICAL: correct ≤8-10 mEq/L per 24 hours. Faster = osmotic demyelination syndrome (ODS) = central pontine myelinolysis → locked-in syndrome. Highest risk: chronic hypoNa⁺ + malnourished + alcoholic.
TOO LITTLE AVP = Central Diabetes Insipidus:
Posterior pituitary damaged → no AVP → AQP2 never inserted → water NOT reabsorbed → massive dilute polyuria (>3L/day, urine osm <300, very dilute). Patient drinks enormous water to compensate (polydipsia).
Causes: pituitary surgery (#1), craniopharyngioma, Sheehan syndrome (postpartum pituitary necrosis), autoimmune, idiopathic.
How to prove it's central:
Step 1: Water deprivation test — urine does NOT concentrate (confirms DI exists).
Step 2: Give desmopressin (DDAVP) — synthetic V2 agonist. If urine NOW concentrates → kidney was fine all along, it just had no AVP → Central DI confirmed.
Treatment: desmopressin (intranasal or oral), lifelong.
KIDNEY IGNORES AVP = Nephrogenic Diabetes Insipidus:
V2 receptor or AQP2 is defective → AVP arrives but kidney doesn't respond → same polyuria/polydipsia.
Desmopressin test: urine STILL does NOT concentrate. This is the key distinction — you gave exogenous AVP and kidney still doesn't respond → problem is in the kidney.
Causes: Lithium (#1 drug cause — lithium enters CD principal cell via ENaC → accumulates → disrupts AQP2 signaling), hypercalcemia, chronic hypokalemia, sickle cell (vasa recta damage).
Treatment: remove cause + thiazide (paradoxical — volume depletion → PCT reabsorbs more upstream → less fluid reaches CD → less polyuria) + amiloride (specifically for lithium — blocks ENaC → blocks lithium entry into cell).
The trigger: Renal hypoperfusion → three things sensed:
1. Baroreceptors on AFFERENT arteriole sense ↓stretch
2. Macula densa senses ↓Na⁺/Cl⁻ in tubular fluid
3. Sympathetic nervous system activation (β1 receptors on JG cells)
→ JG cells (which are on the AFFERENT arteriole wall — not efferent) release RENIN.
The cascade:
Renin (enzyme) cleaves angiotensinogen (made by liver, always circulating in blood) → Angiotensin I (10 amino acids, inactive, just a precursor)
→ ACE (angiotensin-converting enzyme, mainly in lung endothelium) cleaves 2 amino acids off → Angiotensin II (8 amino acids, extremely active)
Ang II does 6 things (talk through these as a list):
1. Vasoconstriction → raises BP directly
2. Aldosterone release from adrenal zona glomerulosa → Na⁺ retention at CD
3. Efferent arteriole constriction → maintains GFR when BP is low
4. AVP/ADH release → water retention (this is the RAAS-AVP link)
5. Thirst stimulation → drink more water
6. Sympathetic activation → heart rate up, more vasoconstriction
One more thing ACE does: ACE also degrades bradykinin. Bradykinin is a vasodilator and causes cough reflex. Normally ACE clears it. When you block ACE with an ACEi → bradykinin accumulates → dry cough (15-20%) and rarely angioedema.
ARBs block the AT1 receptor (downstream of Ang II), NOT ACE → bradykinin is still degraded normally → no cough. This is why ARBs are the switch when ACEi cough occurs.
Walk down the cascade and pin each drug:
Renin → Ang I → ✖ ACEi (enalapril, ramipril) blocks HERE → ACE cannot convert → less Ang II. Side effects: cough + angioedema (bradykinin), hyperK⁺ (↓aldo), acute ↓GFR (efferent dilation), first-dose hypotension.
Ang II → AT1 receptor → ✖ ARB (losartan, valsartan, telmisartan) blocks HERE → Ang II cannot act. No cough (bradykinin unaffected). Otherwise similar: hyperK⁺, ↓GFR. Special: losartan is the only ARB that also lowers uric acid (inhibits URAT1 at PCT).
Aldosterone → MR at CD → ✖ Spironolactone/Eplerenone blocks HERE → Na⁺ not reabsorbed, K⁺ not secreted → hyperK⁺. But: mortality benefit in CHF (RALES) and helps cirrhotic ascites. Side effect of spironolactone: gynecomastia (also binds androgen receptors). Eplerenone is selective MR — no gynecomastia.
The efferent arteriole rule (you missed this):
ACEi/ARB → less Ang II → efferent DILATES → intraglomerular pressure drops → GFR drops acutely. But this pressure drop is RENOPROTECTIVE long-term (less hyperfiltration damage to glomerulus in DM).
Creatinine rise ≤30% = KEEP GOING. This is the drug working.
Creatinine rise >30% = STOP. Think bilateral renal artery stenosis (both kidneys depend on Ang II to maintain GFR → remove it → both collapse).
Triple whammy: ACEi (efferent open) + diuretic (volume depleted) + NSAID (afferent constricted) = no blood in, no pressure built = AKI. Common Indian scenario: DM + HTN on ramipril + hydrochlorothiazide, takes diclofenac for knee pain.
The problem ARNi solves:
In heart failure, RAAS is overactive → vasoconstriction + Na⁺ retention + fibrosis. The body's counter-system is the Natriuretic Peptide System (NPS):
— Atrial stretch → ANP (atrial natriuretic peptide)
— Ventricular stretch → BNP (B-type natriuretic peptide)
— ANP/BNP do the EXACT OPPOSITE of RAAS: vasodilation + natriuresis + ↓aldosterone + ↓ADH
But: An enzyme called neprilysin rapidly degrades ANP and BNP → clears them → RAAS wins. The counter-system is defeated before it can help.
Solution: Block neprilysin → ANP/BNP SURVIVE → counter-RAAS effect enhanced.
But you can't just block neprilysin alone: Neprilysin also degrades Ang II. Block neprilysin → Ang II ALSO survives → RAAS paradoxically activated. So you MUST combine neprilysin inhibition with RAAS blockade.
ARNi = two drugs in one pill:
— Sacubitril = neprilysin inhibitor (prodrug → sacubitrilat). Blocks neprilysin → ANP/BNP survive → more natriuresis + vasodilation + less fibrosis.
— Valsartan = ARB. Blocks AT1 receptor → less vasoconstriction + less aldo. Also handles the Ang II that accumulates from neprilysin inhibition.
Trial: PARADIGM-HF (2014):
Sacubitril/valsartan vs enalapril in HFrEF (EF ≤40%), NYHA II-IV. Result: 20% reduction in CV death + HF hospitalization. Trial stopped early for overwhelming benefit. This is why ARNi replaced ACEi as first-line in HFrEF guidelines.
Indication: HFrEF (EF ≤40%), NYHA class II-IV. Replaces ACEi/ARB. Some signal in HFpEF (PARAGON-HF — trend toward benefit but not statistically significant).
Side effects:
— Hypotension (dual vasodilation — NPS + ARB). Indian: low-BMI patients need careful dose titration.
— HyperK⁺ (less aldosterone from the valsartan component)
— Angioedema (if ACEi washout not respected)
— GFR decline (same efferent dilation logic as ACEi)
Filtrate reaches CD → waits for orders → Aldosterone from zona glomerulosa (triggered by Ang II + hyperK⁺) → binds MR → ENaC opens (Na⁺ in), ROMK opens (K⁺ out), H⁺ pumped out → Na⁺ retained, K⁺ excreted. Too much aldo = Conn's (HTN + hypoK⁺). Too little = RTA4 (hyperK⁺, DM #1 cause, commonest RTA).
AVP from supraoptic nucleus → posterior pituitary → V2 at CD → AQP2 inserted → water reabsorbed. Too much = SIADH (euvolemic hypoNa⁺, correct ≤8-10/day or ODS). No AVP = central DI (desmopressin works). Kidney ignores AVP = nephrogenic DI (desmopressin fails, lithium #1, treat with thiazide + amiloride).
α-Intercalated cells secrete H⁺. When they fail = RTA Type 1 → urine pH >5.5 + hypoK⁺ + stones. Sjögren's, amphotericin B.
RAAS: JG cells (AFFERENT) → renin → Ang I → ACE (lung) → Ang II → 6 effects. ACEi blocks ACE (cough, hyperK⁺, Cr bump). ARB blocks AT1 (no cough). Spironolactone blocks MR. Cr ≤30% = keep. >30% = stop. Triple whammy = ACEi + diuretic + NSAID.
ARNi: Sacubitril (neprilysin inhibitor → saves ANP/BNP) + valsartan (ARB). PARADIGM-HF = 20% ↓death. 36-hour ACEi washout or angioedema. For HFrEF EF ≤40%.