Unit 13 — Urinary System · Question Bank

The renal corpuscle is the spherical filtration head of the nephron, found in the cortical labyrinth. It looks under the microscope like a tight ball of capillaries (the glomerulus) tucked inside a thin epithelial cup (Bowman capsule), with a narrow capsular space between the two layers of the cup. Every drop of urine you will ever pass begins as plasma squeezed across the wall of this little ball — so the architecture of the corpuscle is also the architecture of the filtration barrier itself.

Glomerulus and the two poles

The glomerulus is a tuft of fenestrated capillaries between two arterioles in series. The afferent arteriole enters and the efferent arteriole leaves at the vascular pole. Diametrically opposite is the urinary pole, where Bowman capsule funnels filtrate into the proximal convoluted tubule. The double-arteriole arrangement is unique in the body and is what allows the kidney to hold glomerular capillary pressure stable while systemic blood pressure fluctuates — the basis of autoregulation. Examiners frequently swap the two poles, so anchor: urinary → PCT leaves; vascular → afferent in, efferent out, DCT touches.

Bowman capsule — two epithelial layers

The capsule has an outer parietal layer of simple squamous cells and an inner visceral layer of octopus-shaped podocytes draped over every capillary loop. Podocyte processes branch into primary processes and again into fine secondary processes called pedicels. Adjacent pedicels interdigitate around each capillary, leaving narrow ~25 nm gaps — the filtration slits — that are bridged by a zipper-like membrane built of nephrin, the slit diaphragm. Between the capillary loops sit the intraglomerular mesangial cells, which secrete supporting matrix, phagocytose trapped immune complexes off the GBM, and contract to modulate filtering surface area.

The filtration barrier — three layers

Plasma crosses three serial sieves to become filtrate. The first is the fenestrated endothelium of the glomerular capillary, with large 70–90 nm pores that uniquely lack a diaphragm; it acts as the coarse cellular screen. The second is the glomerular basement membrane (GBM), the only place in the body where two basement membranes (endothelial + podocyte) fuse into one thick sandwich. On electron microscopy the GBM resolves into a central lamina densa (a meshwork of type IV collagen plus laminin that sets the size cutoff at 4–8 nm) flanked by two laminae rarae rich in heparan sulphate proteoglycan whose anionic charge repels negatively charged albumin. The third layer is the podocyte slit diaphragm, the finest sieve, dependent on nephrin.

Mesangium and supporting cells

Mesangial cells are not a layer of the barrier but are essential to its function. They sit between capillary loops, hold the tuft together with mesangial matrix, clear trapped immune complexes by phagocytosis and contract under angiotensin II to vary the filtering surface. Their cousins outside the glomerulus — the extraglomerular mesangial (lacis) cells — sit at the vascular pole between the arterioles and form the connecting cell of the JGA.

Clinical anchor

Each layer corresponds to a recognisable disease. Lose the anionic charge of the GBM (cytokine-driven, in minimal change disease) and albumin pours through; podocyte foot processes are simultaneously effaced on EM though LM looks normal. Thicken the GBM with subepithelial immune deposits (membranous nephropathy, anti-PLA2R antibody) and you get non-selective proteinuria with a "spike-and-dome" silver pattern. Mutate the α5 chain of type IV collagen (Alport syndrome) and the GBM splits and laminates — haematuria, sensorineural deafness, lens defects. Antibodies against the α3 chain attack both the GBM and the alveolar BM, producing Goodpasture syndrome with crescentic glomerulonephritis and pulmonary haemorrhage.

The cortical labyrinth is the region of cortex between the medullary rays — the tangle of convoluted structures grouped around each renal corpuscle. A full-mark drawing identifies the corpuscle at the centre and the four tubular profiles cycling around it, and the line of cells where the DCT brushes its own afferent arteriole.

Renal corpuscle

Draw it as a sphere about 200 µm across. Inside, sketch the loops of the glomerular capillary tuft with red cells indicated; around it draw the thin parietal layer of Bowman capsule (simple squamous) and label the clear capsular (urinary) space. Mark the vascular pole where the afferent arteriole enters and the efferent leaves, and the diametrically opposite urinary pole where the capsule opens into the PCT. Show the podocytes as cells with foot processes draped on the capillary; the intraglomerular mesangial cells fill the angles between loops.

Proximal convoluted tubule (PCT)

Draw multiple irregular ring profiles in cross-section around the corpuscle. Each profile should be larger than a DCT ring and the lumen should look narrow and fuzzy. Use deep eosinophilic shading because the cytoplasm is packed with mitochondria. Label the brush border as a pink fringe inside the lumen. Show only a few visible nuclei per ring because each cuboidal cell is large and the borders between cells are indistinct.

Distal convoluted tubule (DCT)

Draw smaller, neater ring profiles with a wide, clear lumen and pale cytoplasm. There is no brush border, so the apical edge is sharp and smooth. More nuclei should be visible per ring because the cells are smaller. The lumen looks empty — this is the giveaway for the DCT on a slide.

Macula densa

Wherever the DCT touches the afferent arteriole at the vascular pole of its own corpuscle, the DCT cells are modified into a strip of tall, narrow, crowded cells whose nuclei pile up tightly — the macula densa. Draw it as a dense little island of nuclei on the wall of one DCT loop, directly opposite the afferent arteriole. Note it remains a simple (single-layered) epithelium even though it looks "stratified" at low power.

Supporting elements and clinical anchor

Label the surrounding loose connective tissue with peritubular capillaries (these supply the cortex with the post-glomerular blood and contain the fibroblast-like interstitial cells that secrete erythropoietin). Note the absence of medullary structures (no thin limbs, no collecting ducts) — if those appear, you have drawn a medullary ray instead. Clinically, this is the field a pathologist looks at first to grade conditions like diabetic nephropathy (GBM thickening, mesangial expansion, nodular Kimmelstiel–Wilson lesions) or to spot muddy brown granular casts in dilated PCT lumina (acute tubular necrosis).

The uriniferous tubule is the full drainage tree of a single filtration unit, made of two parts joined at the connecting tubule: the nephron (metanephric blastema origin) and the collecting duct (ureteric bud origin). Each segment has an epithelial type matched to its job. Follow filtrate from Bowman space and the morphology tells you exactly what is being reabsorbed or secreted.

Proximal convoluted tubule (PCT)

Filtrate enters the PCT at the urinary pole. The lining is simple cuboidal with tall, eosinophilic cells, abundant mitochondria, basolateral infoldings with lateral interdigitations and a tall apical brush border. This architecture is the histological signature of bulk reabsorption: ~65% of filtered Na and water, all filtered glucose and amino acids, and most of the bicarbonate are pulled back into the peritubular capillaries here. The PCT continues into the proximal straight tubule that enters the medulla as the thick descending limb.

Loop of Henle

The loop is a hairpin in the medulla. The thin descending and thin ascending limbs are flat simple squamous — so thin you can mistake them for capillaries until you look for red cells. The thin descending limb is permeable to water but not solute; the thin ascending limb is the opposite, setting up the countercurrent gradient. The thick ascending limb (TAL) returns to simple cuboidal and runs the famous Na/K/2Cl cotransporter (target of furosemide), pumping salt out of an epithelium impermeable to water — leaving dilute filtrate and a salty medulla.

Distal convoluted tubule (DCT)

The DCT is also simple cuboidal but with smaller cells, pale cytoplasm, no brush border and a wide clear lumen with more nuclei visible per ring. It runs a Na/Cl cotransporter (target of thiazide diuretics) for fine-tuned sodium handling. Where its wall brushes its own afferent arteriole it is modified into the macula densa, the NaCl sensor of the JGA.

Connecting tubule and collecting duct

A short connecting tubule links the DCT to the collecting duct — the boundary where embryologic lineages meet. The collecting duct is lined by simple cuboidal epithelium that becomes columnar in the inner medulla; the cells stain pale with distinct cell borders (a useful slide tell). Two cell types share the wall: principal cells, which reabsorb Na under aldosterone (via ENaC) and water under ADH (via aquaporin-2), and scattered darker intercalated cells (α for H⁺ secretion, β for HCO3⁻ secretion). The duct terminates as a papillary duct (of Bellini) at the apex of the medullary pyramid, dripping urine into a minor calyx.

Functional integration and clinical anchor

The uriniferous tubule reduces 180 L of glomerular filtrate per day to ~1.5 L of urine — a ~99% reabsorption. Each segment has a signature pathology: acute tubular necrosis kills PCT cells first (muddy brown casts), Bartter syndrome mimics chronic furosemide (broken TAL Na/K/2Cl), Gitelman syndrome mimics chronic thiazide (broken DCT Na/Cl), and central or nephrogenic diabetes insipidus arises when ADH or its V2 receptor on principal cells fails, producing copious dilute urine.

The juxtaglomerular apparatus (JGA) is the kidney's anatomical thermostat. It sits at the vascular pole of every renal corpuscle, where the tubule of that same nephron loops back and brushes its own afferent arteriole. By placing a sensor, a wire and an effector together at this point, the kidney links the chemistry of tubular fluid to the calibre of the blood supply — closing a fast local feedback loop that also feeds the systemic renin–angiotensin–aldosterone cascade.

Macula densa — the sensor

The macula densa is a plaque on the wall of the DCT, right where it touches the afferent arteriole. Its epithelial cells become tall, narrow and crowded so that their nuclei pile up tightly — "dense spot" — but the epithelium remains simple, not stratified (a classic exam trap). The cells read the NaCl concentration of the tubular fluid via apical NKCC2 cotransporters; rising NaCl → ATP release across gap junctions; falling NaCl → the opposite. When the macula densa "feels" low salt it interprets this as low renal perfusion and fires the renin response.

Juxtaglomerular (JG, granular) cells — the effector

The JG cells are modified smooth muscle cells in the wall of the afferent arteriole — not the efferent. They are swollen with PAS-positive granules of pro-renin and have lost most of the contractile filaments. They release renin in response to three independent signals: (1) the macula densa via gap junctions; (2) intrinsic baroreceptor function — a drop in afferent wall tension; and (3) sympathetic β1 activation. Renin cleaves angiotensinogen to angiotensin I, which ACE converts to angiotensin II — a potent vasoconstrictor (preferentially efferent arteriole) and stimulus for adrenal aldosterone release.

Extraglomerular mesangial (lacis) cells — the wire

The extraglomerular mesangial cells fill the triangular space between afferent and efferent arterioles, just outside the glomerulus. They are pale, stellate cells linked by extensive gap junctions to both the macula densa and the JG cells, transmitting the chemical signal between them. They are continuous with the intraglomerular mesangium but functionally distinct — they sit outside the basement membrane and never see plasma. The classical synonym, lacis cells, comes from their net-like arrangement.

Integrated function and clinical anchor

When you stand up from a chair, blood pools in your legs, renal perfusion drops, the macula densa senses low NaCl, the JG cells release renin, angiotensin II constricts vessels and aldosterone makes the principal cells retain salt — restoring blood pressure within seconds to minutes. Pathological versions of this loop define several diseases. Renal artery stenosis chronically under-perfuses the kidney → JG cells dump renin → secondary hypertension that responds dramatically to ACE inhibitors (which break the angiotensin II step) but only modestly to standard antihypertensives. Bartter syndrome mimics chronic furosemide — defective TAL Na/K/2Cl means the macula densa always senses low salt, runaway renin produces hypokalaemic metabolic alkalosis with paradoxically normal blood pressure (the loop is broken in opposite directions). JG-cell tumours (reninomas) are a rare but curable cause of severe hypertension with hypokalaemia in young patients.

The ureter and urinary bladder are the muscular conduit and the muscular reservoir of the urinary tract. They share a single lining — transitional epithelium (urothelium) — whose entire architecture is engineered around two contradictory demands: it must stretch with passing or pooling urine, and it must remain impermeable at every degree of stretch. Below the urothelium each organ adapts its muscular wall to its specific job: rhythmic peristalsis in the ureter, mass evacuation in the bladder.

Urothelium (transitional epithelium)

The urothelium is a stratified epithelium 3–6 cell layers thick. The basal layer is small cuboidal, the middle layers are polyhedral, and the surface is covered by huge dome-shaped umbrella cells that may be binucleate or polyploid. The apical membrane of these umbrella cells is studded with rigid plaques of the integral protein uroplakin, joined by accordion-like hinge regions of normal lipid bilayer. When the bladder is empty the hinges fold and the plaques stack; when it fills the hinges unfold and the surface flattens into a smooth sheet. The umbrella cells are joined by tight junctions and the whole epithelium is waterproof at every volume.

Ureteric wall

The ureteric mucosa folds into a stellate lumen when empty, lined by urothelium on a loose vascular lamina propria. There is no muscularis mucosae and no true submucosa. The muscularis is unusual: in the upper two-thirds it has an inner longitudinal + outer circular arrangement — the opposite of the gut. The lower one-third adds a third outer longitudinal layer that helps drive urine into the bladder at the ureterovesical junction. The outer coat is fibrous adventitia blending into perirenal fat. Functionally the muscle generates peristaltic waves at ~1–5/min that propel a urine bolus toward the bladder — the basis of the colicky pain when a stone obstructs the lumen.

Bladder wall

The bladder mucosa is again urothelium on a loose lamina propria — with no muscularis mucosae and no submucosa, so the mucosa sits directly on the muscular wall. The detrusor is a thick, interwoven smooth muscle in three indistinct layers (inner longitudinal, middle circular, outer longitudinal). At the bladder neck the middle circular fibres form an internal urethral sphincter under autonomic control. Externally most of the bladder has a fibrous adventitia, but the dome carries true serosa (mesothelium) where it is covered by peritoneum — an important surgical landmark.

Function and clinical anchor

The combination of stretchy waterproof urothelium plus a powerful detrusor lets the bladder store 300–500 mL between voids and then expel it as a coordinated wave under parasympathetic control. The ureter's peristaltic, opposite-direction muscularis squeezes urine downstream against gravity. Clinically, the urothelium is the substrate of urothelial (transitional cell) carcinoma — the dominant bladder cancer in industrialised countries, with aniline dye and tobacco exposure as classic risks; it can arise anywhere urothelium runs, from renal pelvis to ureter to bladder. Schistosoma haematobium infection drives squamous metaplasia of the urothelium and predisposes instead to squamous cell carcinoma in endemic areas. Cystitis appears histologically as urothelial denudation, oedema and a neutrophil-rich lamina propria; chronic cystitis can produce reactive nests (von Brunn nests) and glandular metaplasia (cystitis glandularis).