Urinary System

Slice a fresh kidney in half along its long axis and the first thing you see is two zones. The outer rim is a pale, granular cortex studded with tiny red dots — the glomeruli. The inner zone is the medulla, arranged into 8–18 conical renal pyramids whose striated, parallel-lined appearance comes from straight tubules and vasa recta all running in the same direction. The apex of each pyramid — the papilla — pokes into a minor calyx, the funnel that catches finished urine.

Look closer at the cortex and you will see vertical stripes running from medulla up into the cortex like rays of sunlight. These are the medullary rays — bundles of straight tubules (proximal straight, thick ascending, collecting duct) that belong to the medulla but invade cortical territory. Between rays sits the cortical labyrinth — a tangle of renal corpuscles and convoluted tubules. This cortex-vs-medulla, labyrinth-vs-ray geography is the single most useful map you can carry into a histology exam.

The functional unit doing all the work is the nephron. Follow a drop of filtrate from start to finish: it begins in the renal corpuscle (glomerulus + Bowman capsule), runs into the proximal convoluted tubule (PCT), dives down as the thin descending limb, hairpins through the thin ascending limb, climbs back up the thick ascending limb, brushes past its own glomerulus at the distal convoluted tubule (DCT), slips through a short connecting tubule and finally joins a collecting duct. The collecting duct is technically not part of the nephron — it is shared real estate where several nephrons drain together — but you must always think of nephron + duct as one drainage tree.

A condensed map of the components:

Collecting ducts receive the DCT but are not part of the nephron — they are a separate embryologic lineage (ureteric bud, not metanephric blastema). Renal corpuscles + convoluted tubules lie in the cortical labyrinth; straight tubules run in the medullary rays and medulla.

The renal corpuscle is the kidney's filtration head. Picture a fist (the glomerulus) pushed into a balloon (Bowman capsule) — the fist invaginates the balloon so deeply that two layers of balloon wall cling to the fist. The outer wall you can still see is the parietal layer (simple squamous); the inner wall now plastered onto the capillary loops is the visceral layer, made of bizarre octopus-shaped cells called podocytes. Between the two layers is the capsular (urinary) space where filtrate collects.

Every corpuscle has two poles. At the vascular pole, the afferent arteriole enters and the efferent arteriole leaves — two arterioles in series, a kidney peculiarity that lets the glomerulus run at high pressure. At the urinary pole, opposite the vascular pole, the capsule funnels filtrate into the proximal convoluted tubule. Examiners love to swap these — remember: distal tubule touches the vascular pole (macula densa); proximal tubule leaves the urinary pole.

The cellular cast:

• Glomerulus: a tuft of fenestrated capillaries (large 70–90 nm pores, no diaphragm) between an afferent (enters) and efferent (leaves) arteriole; supported internally by intraglomerular mesangial cells.
• Bowman's capsule: parietal layer = simple squamous; visceral layer = podocytes wrapping the capillaries with foot processes (pedicels).
• Vascular pole = where arterioles enter/leave; urinary pole = where the capsule becomes the proximal convoluted tubule.

Sitting in the angles between the capillary loops, never on the urinary side of the basement membrane, are the mesangial cells. These are unusual cells with three day-jobs: (1) contractile — they squeeze and relax to change the filtering surface area; (2) phagocytic — they clean trapped immune complexes off the basement membrane (which is why mesangial expansion is a hallmark of immune-complex nephritis); and (3) structural — they secrete the matrix that holds the tuft together. Their cousins outside the glomerulus, the extraglomerular mesangial cells (lacis cells), sit at the vascular pole and form part of the juxtaglomerular apparatus (see 13.4).

13.2.1 — Glomerular Filtration Barrier

Plasma cannot just leak into Bowman space — if it did, you would lose every protein in your blood every minute. Between blood and filtrate sit three molecular sieves stacked on top of each other, collectively the filtration barrier. Each layer rejects something slightly different, and each layer has a signature disease when it breaks.

◆ Define — Filtration Barrier

Filtration barrier: the three layers plasma crosses to form the filtrate — (1) fenestrated endothelium of the glomerular capillary; (2) the fused glomerular basement membrane (a selective macromolecular filter, 4–8 nm); (3) the filtration slits between podocyte foot processes, bridged by a slit diaphragm. Function: filters blood plasma into Bowman's space.

Layer one is the fenestrated endothelium. The pores are wide and open (no diaphragm), so this layer mostly stops blood cells — it is a coarse screen, not a fine one. Layer two is the glomerular basement membrane (GBM), the only place in the body where two basement membranes (endothelial + podocyte) fuse into one thick sandwich. On EM you see three sublayers: lamina rara interna, lamina densa in the middle, and lamina rara externa. The middle lamina densa is a meshwork of type IV collagen (sets the size cutoff) plus laminin, while the rare layers are studded with heparan sulphate proteoglycan — intensely negatively charged. Albumin is also negatively charged at physiological pH, so it is repelled before it even reaches the slits. Layer three is the podocyte slit diaphragm: between pedicels of adjacent podocytes a zipper-like membrane made of the protein nephrin stretches across a ~25-nm slit. Nephrin is the final, finest sieve.

◆ Intuition — three sieves, three diseases

Lose the charge of the GBM (T-cell cytokines neutralise heparan sulphate) → albumin pours through → minimal change disease — on EM the podocyte foot processes are also flattened ("effacement") but the membrane looks normal on LM. Thicken the GBM with immune deposits and you get membranous nephropathy (anti-PLA2R antibody, "spike-and-dome" silver stain). Mutate nephrin (congenital Finnish nephrotic syndrome) and the zipper itself is broken. The filtration barrier is one of the rare structures where you can read a histology lecture and a pathology lecture as a single story.

◆ Clinical anchor

Alport syndrome — X-linked mutation in the α5 chain of type IV collagen; the GBM splits and laminates ("basket-weave" on EM) → haematuria, deafness, lens defects. Goodpasture syndrome — IgG antibodies against the α3 chain of type IV collagen attack GBM and alveolar basement membrane → crescentic glomerulonephritis + lung haemorrhage. Diabetic nephropathy — non-enzymatic glycation thickens the GBM and expands the mesangium (Kimmelstiel–Wilson nodules) — histological correlate of microalbuminuria.

◆ Exam Q&A

Q: Which is the WRONG statement about the renal corpuscle? (...) (C) at the urinary pole it is continuous with the distal tubule (...)

A: (C) — at the urinary pole the corpuscle is continuous with the proximal convoluted tubule, not the distal. (2021 final MCQ 16.)

Q: The podocytes form the visceral layer of Bowman's capsule. (T/F)

A: True. (Other-final T/F 3.)

◆ Recall this subtopic Three layers: fenestrated endothelium + fused GBM (type IV collagen + laminin + heparan sulphate, anionic) + podocyte slit diaphragm (nephrin). Mesangial cells = phagocytic + contractile (not part of barrier). Minimal change = charge loss + foot-process effacement. Membranous = thick GBM, anti-PLA2R. Alport = α5 type IV. Goodpasture = anti-α3 type IV.

After filtrate leaves Bowman space at the urinary pole, the very first thing that happens is bulk reabsorption — not because the kidney can't filter less, but because evolution chose to filter generously (~180 L/day) and reclaim almost everything downstream. That reclamation begins in the proximal convoluted tubule (PCT), where ~65% of filtered sodium and water, all filtered glucose, all filtered amino acids, and the bulk of bicarbonate go back into the blood.

To do this work the PCT cell is built like a tiny absorption factory. Look at the apical surface and you will see a tall brush border of microvilli — massive surface-area amplification for transporters. Look at the cytoplasm and it is packed with mitochondria (because reabsorption against an electrochemical gradient costs ATP), making the cytoplasm stain a deep eosinophilic pink on H&E. Look at the basolateral surface and you will see deep infoldings with lateral interdigitation — more surface area, more Na/K-ATPase. The lumen looks narrow and fuzzy because microvilli fill it. There are fewer visible nuclei per ring of tubule because each cell is so large.

The loop of Henle is a hairpin diving into the medulla. The thin descending and thin ascending limbs are lined by a flat simple squamous epithelium — so thin and so pale they are easy to mistake for capillaries until you look for red cells inside. Their job is passive: water leaves the descending limb (it is permeable to water but not solute) while solutes leave the ascending limb (the reverse). The thick ascending limb (TAL) is back to simple cuboidal and runs the famous Na/K/2Cl cotransporter — the target of furosemide. The TAL is impermeable to water but pumps salt out, leaving dilute filtrate and a salty medulla. This is the engine of the countercurrent multiplier.

The distal convoluted tubule (DCT) is structurally the reverse of the PCT — simple cuboidal but smaller cells, NO brush border, fewer mitochondria, less eosinophilic, wider clearer lumen, more nuclei visible per ring. Functionally the DCT runs the Na/Cl cotransporter (target of thiazide diuretics) and is the site where the wall is modified into the macula densa where it grazes its own afferent arteriole.

After the TAL climbs back up to the cortex, it does something extraordinary: the rising tubule swings around and touches the same glomerulus that spawned it, brushing against its own afferent arteriole at the vascular pole. Why? So that one nephron can feel what its own filtration is doing and feed back on it. This anatomical handshake is the juxtaglomerular apparatus (JGA), and it has three cellular characters.

The first is the macula densa — literally "dense spot" — a patch of DCT epithelium where the cells become tall, narrow and crowded so that their nuclei pile up tightly. Their job is to sense the NaCl concentration in the tubular fluid. If filtration is too brisk, NaCl arrives in excess; if too sluggish, NaCl is low. The macula densa reads this number and signals next door. The second character is the juxtaglomerular (JG) cell — modified smooth muscle of the afferent arteriole wall, swollen with PAS-positive granules of renin. The third is the extraglomerular mesangial cell (lacis cell), a connector cell in the angle between afferent and efferent arterioles, linked by gap junctions to both the macula densa and the JG cells.

At the vascular pole, three components regulate blood pressure & filtration:

The collecting duct is where the kidney does its final water- and acid-base accounting. Two cell types share its wall. Principal cells are pale-staining with distinct cell borders (those crisp polygonal outlines are a giveaway); they handle Na reabsorption (under aldosterone) and water reabsorption (under ADH, via aquaporin-2 channels inserted into the apical membrane). Intercalated cells are darker and scattered between principals; the α-intercalated cell secretes H⁺ (acid handling), the β-intercalated cell secretes HCO3⁻ (alkali handling). ADH is the master switch — without it the duct is impermeable to water and you produce dilute urine; with it, water is pulled back into the salty medulla and urine concentrates.

From the papilla, urine drips into the minor calyx, pools in the major calyx and renal pelvis, then runs down the ureter. The whole tract from minor calyx to bladder is lined by transitional epithelium (urothelium) — a stretchy 3-to-6-layered epithelium whose surface (umbrella) cells carry rigid plaques of the protein uroplakin. When the bladder is empty the umbrella cells fold like an accordion; when full they stretch flat. This trick lets the epithelium go waterproof at every volume.

The bladder has the same urothelium plus a thick muscular wall — the detrusor — arranged in three interwoven smooth-muscle layers. Crucially, the bladder has no muscularis mucosae and no submucosa, so the urothelium sits directly on a loose lamina propria over the detrusor. The urethra changes lining as it descends: in the male the prostatic portion stays transitional, the membranous and proximal spongy portions are stratified or pseudostratified columnar, and near the glans it becomes stratified squamous (matching the skin). The much shorter female urethra is mostly stratified squamous along its length.

• Collecting ducts: simple cuboidal → columnar epithelium, pale cytoplasm with distinct cell borders; ADH-responsive (water reabsorption); run in medullary rays/pyramids to the papilla.
• Ureter & bladder: lined by transitional epithelium (urothelium) with umbrella cells, on a muscular wall (recall Unit 2).