Renal Artery Stenosis & RAAS

A 64-year-old man with a history of smoking and peripheral vascular disease presents with resistant hypertension despite maximal doses of three antihypertensives. Renal angiography reveals 90% stenosis of the right renal artery. A biopsy of the juxtaglomerular apparatus would most likely show:

A. Efferent arteriole dilation

B. Juxtaglomerular cell hyperplasia

C. Decreased renin secretion

D. Macula densa atrophy

E. Mesangial cell proliferation

THE PATTERN

One Clogged Pipe, Whole-Body Chaos

Why a problem in ONE kidney artery raises blood pressure EVERYWHERE

Here's the setup. Your kidney doesn't just filter blood · it monitors blood pressure. There's a cluster of specialized cells wrapping around the afferent arterioleThe vessel that brings blood INTO the glomerulus. "Afferent" = Arriving. The JGA cells sit here like bouncers checking the pressure before letting blood through. called the juxtaglomerular apparatus (JGA)A pressure-sensing station where the afferent arteriole meets the distal convoluted tubule. Contains JG cells (sense pressure) and macula densa (sense sodium). Together they control renin release.. Their one job: sense pressure.

When that renal artery gets squeezed to 90% closed, the kidney downstream sees drastically less blood flow. The JGA's pressure sensors scream: "We're losing perfusion!" Their response? Dump renin. 🔑Renal Artery Stenosis = Renin Always Spiking

Renin is the match that lights the whole RAAS fire. And when the stenosis is permanent, the match never goes out. The JGA cells don't just work harder · they multiply. That's JGA hyperplasia: more cells making more renin, day after day, month after month.

The kidney thinks it's in shock. It's not · it's just downstream of a clog. But it can't tell the difference. So it keeps screaming for more pressure, forever.

THE CASCADE

RAAS: The Domino Chain

Tap to watch the cascade fire, step by step

🧊

Renal artery stenosis → decreased perfusion pressure at the afferent arteriole. JGA cells detect low stretch.

↓

R

Renin released from JG cells into the bloodstream. This is the rate-limiting step · everything downstream depends on this.

↓

L

Angiotensinogen (made by the liver, always floating around) gets cleaved by renin → Angiotensin I RENIN

↓

🫁

ACE (in lung endothelium) converts Angiotensin I → Angiotensin II ACE
ACE also degrades bradykinin (vasodilator) · that's why ACEi cause cough.

↓

💥

Angiotensin II · the most powerful vasoconstrictor your body makes. Constricts efferent > afferent arteriole. Raises BP systemically.

↓

A

Aldosterone released from adrenal zona glomerulosa → Na⁺/H₂O reabsorption in collecting duct. More volume = more pressure.

↓

🧠

ADH (vasopressin) release stimulated + thirst center activated. Even more water retention. The whole system is screaming: RAISE THE PRESSURE.

THE BOSS

Angiotensin II: 6 Ways It Wrecks You

Tap each card to see the mechanism

⭕ Vasoconstriction

Constricts arterioles body-wide. Efferent > afferent in the kidney · this preserves GFR even when perfusion drops.

Think of it like pinching the drain hose harder than the supply hose. Less blood gets out of the glomerulus, so filtration pressure stays up. That's why ACEi can DROP GFR in bilateral RAS · you're removing the only thing keeping filtration going.

🧂 Aldosterone Release

Stimulates zona glomerulosa → aldosterone → Na⁺ reabsorption + K⁺ secretion in the collecting duct.

More sodium reabsorbed = more water follows = more blood volume = higher BP. Also causes hypokalemia. Prolonged activation = metabolic alkalosis from H⁺ loss.

💧 ADH Secretion

Stimulates posterior pituitary to release ADH (vasopressin) → free water reabsorption via aquaporin-2 in collecting duct.

Double whammy: aldosterone grabs sodium, ADH grabs water. The body is pulling every lever to expand volume and raise pressure.

🤔 Thirst

Acts on the subfornical organ and OVLT in the hypothalamus → you feel thirsty → you drink more → volume goes up.

Your brain is literally conspiring with your kidneys to raise your blood pressure. The thirst signal is Ang II reaching the brain through circumventricular organs that lack a blood-brain barrier.

🔄 PCT Na Reabsorption

Stimulates the Na⁺/H⁺ exchanger in the proximal convoluted tubule. Grabs sodium early, before the loop even sees it.

This is the sneaky one. Aldosterone works on the collecting duct, but Ang II works on the PCT · hitting sodium reabsorption at two different sites along the nephron.

❤️ Cardiac Remodeling

Promotes hypertrophy and fibrosis in the heart and blood vessels. Long-term exposure → stiff, thick-walled ventricle.

This is why chronic RAAS activation is so destructive. It's not just the blood pressure · Ang II directly damages organs even independent of BP. ARBs and ACEi reduce this remodeling.

THE BACKUP SENSOR

Tubuloglomerular Feedback

The second way your kidney detects low flow

The JGA has a buddy system. While JG cells sense pressure, the macula densaSpecialized cells in the wall of the thick ascending limb / early DCT, right where it touches the afferent arteriole. They sense NaCl concentration in the tubular fluid. Low NaCl = low GFR = release prostaglandins to trigger renin. senses sodium concentration in the tubular fluid.

1

Renal artery stenosis → less blood filtered → less NaCl delivered to the distal tubule

2

Macula densa senses low NaCl → releases prostaglandins (PGE2, PGI2)

3

Prostaglandins signal JG cells → renin release (same endpoint, different trigger)

4

Prostaglandins also dilate the afferent arteriole → trying to increase GFR locally

This is why NSAIDs worsen renal function · they block prostaglandins, removing the kidney's backup rescue mechanism.

ACEi in bilateral RAS: Both kidneys depend on Ang II constricting the efferent arteriole to maintain GFR. Give an ACEi → efferent dilates → filtration pressure collapses → acute kidney injury. Board question: "New creatinine spike after starting lisinopril" = think bilateral RAS.

"Resistant hypertension" on boards = hypertension despite 3+ drugs including a diuretic at max dose. Top 3 causes: renovascular disease (RAS), primary aldosteronism, pheochromocytoma. If the stem mentions smoking + PVD + resistant HTN, it's RAS until proven otherwise.

Two causes of RAS with different demographics: Atherosclerosis (older male, smoker, PVD) affects the proximal 1/3 of the renal artery. Fibromuscular dysplasia (young woman) affects the middle-to-distal renal artery · has a "string of beads" appearance on angiography. 🔑Atherosclerosis = Aortic end (proximal). FMD = Far out (distal).

DECISION TREE

RAAS Manipulation: When to Use Each Drug Class

Pick your goal, follow the branch.

Goal of intervention?

BOARD PRACTICE

Prove You Know the Cascade

5 patients. Their kidneys are mad. Figure out why.

Board-Style Walkthrough

Original board-style vignettes. Shuffled, never-repeat, full explanations for every choice.