Renal & Genitourinary Anatomy

Overview

The kidneys are paired retroperitoneal organs that filter blood, regulate fluid and electrolytes, and excrete nitrogenous waste. Their microanatomy - two capillary beds in series and a countercurrent system - underlies glomerular filtration, tubular reabsorption, and urine concentration.[1]

The kidney

• The kidneys lie retroperitoneally at T12-L3; the right sits lower than the left (under the liver). Each has an outer cortex and an inner medulla of 8-18 renal pyramids, whose tips (papillae) drain into minor then major calyces and the renal pelvis.[1]
• A nephron is the functional unit (about one million per kidney).[1]

The nephron segments

• Glomerulus - a capillary tuft invaginated into Bowman's capsule; the filtration barrier (fenestrated endothelium, basement membrane, podocyte foot processes) holds back cells and large proteins.[1]
• Proximal convoluted tubule - reabsorbs the bulk of the filtrate: all glucose and amino acids, about 65 per cent of sodium and water, and secretes organic acids and drugs.[1]
• Loop of Henle - the descending limb is water-permeable (passive water reabsorption into the hypertonic medulla) and the thick ascending limb actively pumps sodium, potassium and chloride (the Na-K-2Cl cotransporter, the loop diuretic target) and is impermeable to water - the countercurrent multiplier that builds medullary hypertonicity.[1]
• Distal convoluted tubule - fine sodium and calcium tuning (the Na-Cl cotransporter, the thiazide target; calcium reabsorption under parathyroid hormone).[1]
• Collecting duct - aldosterone-driven sodium retention (the ENaC, the potassium-sparing diuretic target) and antidiuretic-hormone-driven water reabsorption (aquaporin-2 channels), setting the final urine volume and osmolality.[1]

Renal blood supply - a portal system

Blood passes through two capillary beds in series, unique to the kidney:[1]

1. The afferent arteriole feeds the glomerular capillaries (where filtration occurs); the efferent arteriole drains them.
2. The efferent arteriole then forms the peritubular capillaries (cortex) and the vasa recta (medulla), which reabsorb what the tubule has reclaimed.

This arrangement lets the kidney regulate glomerular pressure by adjusting afferent and efferent arteriolar tone (the basis of renal perfusion in shock and of angiotensin-II-mediated efferent constriction in renal hypoperfusion).[1]

The juxtaglomerular apparatus

• Where the distal tubule meets its own glomerulus, the macula densa (specialised distal-tubule cells) senses distal sodium/chloride delivery and signals the juxtaglomerular cells on the afferent arteriole to release renin.[1]
• Renin activates angiotensinogen to angiotensin I, then II, which constricts the efferent arteriole to maintain glomerular pressure when renal perfusion falls - and drives aldosterone and sodium retention.[1]

Ureters and lower tract

• Each ureter runs 25-30 cm retroperitoneally; its three natural constrictions - the pelviureteric junction, the pelvic brim where it crosses the iliac vessels, and the vesicoureteric junction - are the sites where stones lodge.[1]
• The bladder holds 400-600 mL; the male urethra is about 20 cm (prostatic, membranous, spongy) and the female about 4 cm - the short female urethra explains the higher urinary-tract infection risk.[1]

[1]

Red flags

Macroscopic (gross) anatomy of the kidney

• Position and level. The kidneys lie retroperitoneally on the posterior abdominal wall, spanning T12 to L3; the right kidney sits lower (a half-vertebra) because the liver occupies the right upper quadrant. The left kidney is more medial and lies between T12 and L3, with its hilum at about L1.[1][1]
• Surface relations. Anterior relations differ by side: the right kidney is related to the liver (hepatic impression), the second part of the duodenum (medially, over the hilum), the hepatic flexure of the colon, and the suprarenal gland superomedially; the left kidney is related to the stomach (upper pole, separated by the lesser sac), spleen, tail of the pancreas (across the hilum), splenic flexure and descending colon, and the left suprarenal gland. The pancreas tail and duodenum are the operative danger - mobilising these organs risks injury during retroperitoneal access.[1][1]
• Posterior relations. Both kidneys rest on the diaphragm (lower), quadratus lumborum and psoas major; the subcostal nerve (T12), iliohypogastric and ilioinguinal nerves (L1) run obliquely across the lower pole beneath the fascia - a pleural reflection (costodiaphragmatic recess) crosses the upper pole and is the reason a high flank incision or a mal-placed nephrostomy causes a hydrothorax.[1]
• Three layers of investing fascia and fat. From within out: (1) the fibrous capsule (capsula fibrosa) - thin, invests the parenchyma, strips easily and is the surgical plane for partial nephrectomy and decapsulation (used to relieve intrarenal tamponade); (2) the perirenal fat (adipose within the perirenal space); (3) the renal fascia of Gerota (perirenal fascia), which envelops kidney, adrenal and perirenal fat, closes inferiorly (the inferior cone is open - the route of pancreatic fluid tracking to the groin); (4) the pararenal fat external to Gerota's fascia. These planes matter for nephrostomy, abscess, and the spread of retroperitoneal haemorrhage in anticoagulated patients.[1][1]
• Hilum and sinus. The renal hilum is the medial concavity; from anterior to posterior lie the renal vein, renal artery, and renal pelvis (mnemonic V-A-P); the ureter is most posterior and inferior. From superior to inferior at the hilum: artery-vein-artery-ureter is variable, but vein anterior, artery middle, pelvis posterior is constant and is the order encountered at nephrectomy. The renal sinus houses the pelvis, calyces, vessels and fat.[1][1]
• Internal architecture. Section shows an outer cortex (granular, contains glomeruli and convoluted tubules) and an inner medulla of 8 to 18 renal pyramids (one per lobe); the cortex dips between pyramids as the columns of Bertin. Each pyramid tapers to a renal papilla that projects into a minor calyx; 7 to 13 minor calyces fuse into 2 to 3 major calyces, which drain into the renal pelvis. The pelvis narrows at the pelviureteric junction to become the ureter.[1][1]

The renal arterial tree - segmental, end-arterial, and the reason for lobar infarction

The kidney's blood supply is the most surgically and radiologically important branch of the aorta, and its segmental, end-arterial organisation explains why a focal renal infarct is wedge-shaped and why a partial nephrectomy follows avascular planes.[1][1]

[1]

• Venous drainage largely mirrors the arteries but, unlike the arteries, the veins freely anastomose - which is why the kidney tolerates segmental venous ligation but not segmental arterial interruption. The left renal vein is longer and crosses between the superior mesenteric artery and the aorta - the nutcracker (SMA) space whose compression produces left renal vein entrapment (nutcracker syndrome: haematuria, left flank pain, varicocele). It receives the left adrenal vein and the left gonadal (testicular/ovarian) vein, then drains to the IVC; the right renal vein is short and drains directly into the IVC. The left gonadal vein drains into the left renal vein whereas the right drains directly into the IVC - the anatomical reason a left varicocele may signal left renal vein obstruction or tumour, while a right varicocele (draining directly to IVC) more often indicates a retroperitoneal mass.[1][1]
• Lymphatics drain via nodes at the hilum to para-aortic (lumbar) nodes; renal-cell carcinoma therefore spreads to para-aortic nodes first (the rationale for nodal dissection at nephrectomy).[1]

The filtration barrier - three layers, two selectivities

Glomerular filtration is the kidney's first and most selective step. The barrier holds back cells and nearly all protein while passing water and small solutes at ~180 L/day. It has three structural layers and a shared charge.[1][1]

• Mesangial cells sit between capillary loops: contractile, phagocytic, supportive, and they secrete and turnover the matrix. They respond to angiotensin II (contraction reduces filtration surface area) - one reason mesangial IgA deposition and mesangial proliferative diseases disturb filtration.[1]

The nephron - cortical versus juxtamedullary, and the drug targets along its length

• There are about one million nephrons per kidney, formed entirely before birth; loss is irreversible and accelerates with age and disease. Nephrons fall into two types distinguished by glomerular depth and loop length, with direct consequences for concentrating ability.[1][1]
• Cortical nephrons (~85 per cent) sit in the outer cortex; their loops of Henle dip only to the outer medulla and their efferent arterioles feed cortical peritubular capillaries - they do most of the bulk reabsorption.
• Juxtamedullary nephrons (~15 per cent) have glomeruli at the corticomedullary junction and long loops of Henle that reach the tip of the papilla; their efferent arterioles give rise to the vasa recta. Although few, they generate the entire medullary osmotic gradient - the concentrating engine of the kidney.

The juxtaglomerular apparatus and the renin-angiotensin-aldosterone system

• At the vascular pole, where the distal tubule returns to its own glomerulus, three cell types form the juxtaglomerular apparatus (JGA): the macula densa (a plaque of specialised, densely-packed distal-tubule cells sensing tubular Na+/Cl- and osmolality via the NKCC2 transporter); the juxtaglomerular (granular) cells - modified smooth-muscle cells in the afferent arteriole wall that store and release renin; and the extraglomerular mesangial (Lacis) cells, which couple the two.[1][1]
• Three stimuli for renin release: (1) reduced stretch of the afferent arteriole (a baroreceptor, falls in perfusion pressure); (2) reduced NaCl delivery sensed at the macula densa (tubuloglomerular feedback); (3) increased sympathetic tone (beta-1 effect on JG cells). Renin cleaves circulating angiotensinogen (liver-derived) to angiotensin I, which ACE (mostly pulmonary, on endothelium) converts to angiotensin II.[1][1]
• Angiotensin II has four renal effects that all protect GFR when perfusion falls: it constricts the efferent arteriole (more than afferent) to maintain intraglomerular pressure; it stimulates aldosterone (zona glomerulosa) for distal Na+ retention; it directly stimulates proximal Na+ reabsorption; and it stimulates thirst and ADH. It also constricts mesangial cells (reducing filtration surface).[1]

[1] [1]

Autonomic and sensory innervation of the kidney, ureter and bladder

• Sympathetic preganglionic fibres arise from T10 to L1 (greater, lesser and least splanchnic nerves) and synapse in the aorticorenal and renal ganglia; postganglionic fibres travel with the renal artery in the renal plexus to the vessels, tubules and juxtaglomerular cells. Effects: afferent and efferent arteriolar vasoconstriction (alpha-1), renin release (beta-1), and proximal Na+ reabsorption. Renal denervation (catheter-based renal artery ablation) exploits the fact that these nerves run in the renal artery adventitia.[1][1]
• Parasympathetic supply is vagal and weak; its functional role in the kidney is minor. The clinically important parasympathetics are the pelvic splanchnic nerves S2-S4 (nervi erigentes), which supply the bladder detrusor (via the pelvic nerves) and the distal bowel and genitalia.
• Pain fibres from the kidney, renal pelvis and upper ureter travel with the sympathetic nerves to T10-L1, so renal pain is referred to the flank, loin and lower abdomen (the classic loin-to-groin radiation of ureteric colic follows the genitofemoral and ilioinguinal nerves, L1-L2). Lower ureteric and bladder pain is referred via S2-S4 to the suprapubic region, perineum and groin.[1][1]
• The renal capsule is richly innervated and stretched by swelling (acute pyelonephritis, obstruction, haemorrhage) producing a dull, aching flank pain distinct from the colic of ureteric spasm. Cystoscopy, ureteric stents and bladder distension below a cord lesion can trigger autonomic dysreflexia (lesions at/above T6) through unopposed sympathetic discharge.[1]

[1]

The ureter - course, constrictions and blood supply

• Each ureter is 25-30 cm long and runs retroperitoneally from the renal pelvis to the bladder, crossing the pelvic brim at the bifurcation of the common iliac artery. It is crossed anteriorly by the gonadal vessels and, on the right, by the second part of the duodenum; on the left it is crossed by the vas deferens (male) or uterine artery (female) - the mnemonic 'water under the bridge' (ureter under the uterine artery/vas) is the surgical warning during hysterectomy or colectomy.[1][1]
• Three natural constrictions where calculi lodge (in descending order of frequency): the pelviureteric junction; the pelvic brim where the ureter crosses the iliac vessels; and the vesicoureteric junction (the narrowest, ~3 mm). The vesicoureteric junction's oblique, tunnelled entry through the bladder wall is the anti-reflux mechanism; its shortening or malformation causes vesicoureteric reflux (and recurrent pyelonephritis).[1][1]
• Segmental blood supply (surgical relevance): the upper third from the renal artery; the middle third from the gonadal and common iliac arteries; the lower third from the superior and inferior vesical arteries. The vessels enter on the medial side and there is a relative watershed at the pelviureteric junction - the reason ureteric ischaemia and stricture form after retroperitoneal surgery, aortic grafting or prolonged stenting, and why ureteric mobilisation must preserve the adventitia.[1]

The bladder and urethra

• The bladder sits behind the pubic symphysis, extraperitoneal, with peritoneum covering only its dome (hence a distended bladder pushes peritoneum up - the basis of safe suprapubic aspiration/cystostomy above the pubic symphysis to avoid bowel). Capacity is 400-600 mL; first urge is felt at ~150 mL and fullness at ~400 mL.[1][1]
• The trigone is the smooth triangular area between the two ureteric orifices and the internal urethral meatus; its interureteric ridge (Mercier's bar) is embryologically mesodermal (unlike the rest of the detrusor, endodermal) and is a fixed landmark at cystoscopy. The detrusor is interlacing smooth muscle; contraction (parasympathetic M3) raises intravesical pressure and opens the bladder neck.[1]
• Two sphincters: the internal urethral sphincter (smooth muscle, involuntary, sympathetic alpha-1, T11-L2) at the bladder neck - the site of physiological continence; and the external urethral sphincter (skeletal, voluntary, somatic pudendal S2-S4 - Onuf's nucleus) in the urogenital diaphragm - the site of socially appropriate continence. The male urethra is ~20 cm (prostatic 3 cm, membranous 1-2 cm - the narrowest and least distensible - and spongy ~15 cm); the female is ~4 cm, opening into the vestibule, which is why women have more UTIs and why catheterisation is anatomically straightforward.[1][1]

Renal replacement therapy access - the anatomy of dialysis

Dialysis access is a recurring CICM/FFICM viva topic because the choice and the complications flow directly from vascular anatomy.[1][1]

[1]

Surface anatomy and clinical examination landmarks

• Ballottement (renal punch / Murphy's punch sign). With the patient supine, one hand placed in the costovertebral angle (the angle between the 12th rib and the erector spinae) is struck sharply by the other hand; transmitted tenderness suggests renal capsule inflammation (pyelonephritis). The kidney is ballotable only when enlarged to at least twice normal size (polycystic, hydronephrotic, tumour) or displaced.[1][1]
• The lumbar triangle of Petit - the weak area bounded by the external oblique anteriorly, latissimus dorsi posteriorly and the iliac crest inferiorly, with internal oblique as its floor - is the portal for the muscle-splitting (lumbodorsal) approach to the kidney (open nephrectomy, percutaneous nephrostomy, retroperitoneal abscess drainage) and the route through which a lumbar hernia (Petit's hernia) may protrude.[1]
• Renal percussion and the relationship to the 12th rib defines the safe upper limit for a flank incision and the risk of pleural breach - the costodiaphragmatic recess crosses the 12th rib, so a supra-12th-rib incision risks a hydrothorax.[1]

Embryology - why ectopic, horseshoe and pelvic kidneys occur

• The definitive kidney (metanephros) develops from two primordia: the ureteric bud (a diverticulum of the mesonephric/Wolffian duct) which branches to form the ureter, renal pelvis, calyces and collecting ducts; and the metanephric blastema (sacral intermediate mesoderm) which forms the nephrons (glomerulus to DCT). The two must meet and induce each other - failure produces renal agenesis, multicystic dysplastic kidney or ectopia.[1]
• The kidneys ascend from the pelvis to T12-L3 during weeks 6-9, acquiring successively higher segmental arteries; failure to ascend gives a pelvic kidney, and fusion before ascent produces a horseshoe kidney (typically fused at the lower poles, caught under the inferior mesenteric artery and supplied by multiple anomalous vessels). The horseshoe kidney is the most common fusion anomaly (~1 in 400) and predisposes to ureteropelvic junction obstruction, stones and infection.[1][1]

Exam practice — SAQs

[1] [1]

Clinical pearls for the CICM / FFICM / EDIC exam

[1]

Glossary of anatomical terms an examiner may press

• Columns of Bertin - extensions of renal cortex between the medullary pyramids; sometimes hypertrophied (septum of Bertin) and mistaken for a tumour on imaging.[1]
• Brodel's line / white line - the relatively avascular plane on the convex renal border between anterior and posterior segmental arterial territories - the incision line for anatrophic nephrolithotomy and partial nephrectomy.[1]
• Renal fascia of Gerota - the perirenal fascia enclosing kidney, adrenal and perirenal fat; defines the perirenal space and the route of spread of retroperitoneal collections.[1][1]
• Vasa recta - straight, hairpin capillaries from the juxtamedullary efferent arterioles that run with the loops of Henle - the countercurrent exchanger that preserves medullary hypertonicity.[1]
• Macula densa / Lacis cells - the sensor and the coupler of the JGA; Lacis (extraglomerular mesangial) cells transmit the macula-densa signal to the JG cells.[1][1]
• Trigone of the bladder - the smooth triangular region between the ureteric orifices and the internal meatus; embryologically mesodermal (unlike the endodermal detrusor) and the site where ureteric ectopia and reflux are anatomically determined.[1]

References

1. [1]Maschio G, Alberti D, Janin G, et al Effect of the angiotensin-converting-enzyme inhibitor benazepril on the progression of chronic renal insufficiency. The Angiotensin-Converting-Enzyme Inhibition in Progressive Renal Insufficiency Study Group N Engl J Med, 1996.PMID 8596594
2. [2]Ruggenenti P, Perna A, Gherardi G, et al Renoprotective properties of ACE-inhibition in non-diabetic nephropathies with non-nephrotic proteinuria Lancet, 1999.PMID 10437863
3. [3]Dember LM, Beck GJ, Allon M, et al Effect of clopidogrel on early failure of arteriovenous fistulas for hemodialysis: a randomized controlled trial JAMA, 2008.PMID 18477783
4. [4]Parienti JJ, Mongardon N, Megarbane B, et al Intravascular Complications of Central Venous Catheterization by Insertion Site N Engl J Med, 2015.PMID 26398070