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Urinary System

A guided journey in five stops

Anatomy & Physiology • Prof. Amy Fenwick

How to use this page

This journey covers the urinary system from gross kidney down to the bladder wall, in five stops spread across one week. Each stop takes about 20–30 minutes and follows the same rhythm:

One topic per day, organized so the toughest skill (telling tubules apart) gets two full stops:

Click any stop to expand. Progress is saved automatically in this browser.

1 Map the territory Mon • ~25 minutes • Cortex / medulla / calyx / renal basin / ureter ✓
Why this stop matters: Before any micrograph makes sense, you need to know where you are. This stop builds the gross-anatomy framework that every later slide will plug into.
TEXT

The kidney from outside in

The kidney has two jobs and the tissue is split between them. The cortex is the outer band — that's where the filtering happens, in millions of tiny units called nephrons. The medulla is the inner triangular region, organized into renal pyramids. The medulla concentrates the filtrate as it drains downward.

Each pyramid points inward, with its tip — the renal papilla — opening into a small cup called a minor calyx. Several minor calyces merge into a major calyx, and all the major calyces drain into the central reservoir of the kidney: the renal pelvis, also called the "renal basin". From there, urine leaves through the ureter.

Two terms, one structure: renal pelvis = renal basin. Different textbooks use different names. Recognize both.

The renal basin central drainage chamber
The renal pelvis (renal basin) is the wide funnel that collects urine from every major calyx before it enters the ureter. Clinically, this is where kidney stones love to lodge — when the basin is obstructed, urine backs up into every pyramid at once.
Coronal section of the kidney with labeled cortex, medulla, papilla, calyces, renal pelvis, and ureter.
INTERACTION

Click to reveal — the kidney parts

Try to name each numbered structure on the diagram below before clicking the blank to reveal the answer.

Unlabeled coronal kidney with numbered markers.
  • 1click to revealCortex
  • 2click to revealMedulla (renal pyramid)
  • 3click to revealRenal column
  • 4click to revealRenal papilla
  • 5click to revealMinor calyx
  • 6click to revealMajor calyx
  • 7click to revealRenal pelvis ("renal basin")
  • 8click to revealUreter
H5P ACTIVITY — Image Hotspots: Transverse kidney section
Loads live from bettybroadbent.com — needs an internet connection.
CONFIRMATION

Quick self-check

A drop of urine has just exited a renal papilla. Which structure does it enter NEXT?
Order: papilla → minor calyx → major calyx → renal pelvis (renal basin) → ureter. The minor calyx is the first cup that catches urine from a single papilla.
Which of these is ANOTHER name for the renal pelvis?
Renal pelvis = renal basin. Same chamber, two names. Both are fair game on the practical.
On a low-power kidney slide, you see a wide outer band of round dots mixed with twisted tubules and a narrower inner band of parallel straight tubes with no dots. The first band is the:
Round dots = renal corpuscles, which only exist in the cortex. The parallel-tube band with no corpuscles is the medulla.
2 The glomerulus & renal corpuscle Tue • ~20 minutes • The filter at the start of every nephron ✓
Why this stop matters: The glomerulus is the start of urine — every drop you've ever made began as plasma forced across this capillary wall. It's also the most recognizable structure on a kidney slide, so it doubles as your orientation landmark.
TEXT

What is a renal corpuscle?

A renal corpuscle is the filtering apparatus at the start of each nephron — about a million of them per kidney. It has two parts that fit together like an egg in a cup:

  • The glomerulus — a tangled ball of capillaries.
  • The glomerular capsule (renal capsule) — a hollow cup wrapped around the glomerulus.

Blood enters the glomerular capillaries under pressure. Plasma (and small dissolved things — water, salts, glucose, amino acids, urea) is forced ACROSS the capillary wall into the hollow space inside the capsule. Cells and large proteins are too big to cross, so they stay in the blood.

The three parts on the curriculum list

Glomerular capillaries the tangled red ball
A knot of capillaries inside the corpuscle. They're fenestrated — peppered with little windows — so plasma leaks across very easily. On a slide, this looks like a deep pink or red ball in the middle of the corpuscle.
Capsular space also called capsular space
The clear ring around the capillary tuft. This is where the filtrate first appears — it's the very beginning of the urine stream. From here, filtrate exits the capsule through the urinary pole and enters the PCT.
Parietal layer the outer wall of the capsule
The thin outer wall of the glomerular capsule. It's made of simple squamous epithelium — basically a one-cell-thick barrier that holds the capsular space in place. The visceral layer (podocytes) wraps the capillaries directly, but parietal is what's on your list.
Renal corpuscle close-up showing glomerular capillaries, capsular space, parietal layer, vascular pole, urinary pole.
ID hint on a real micrograph: the renal corpuscle is the easiest thing to find on a kidney slide. Just scan the cortex for a round structure that looks like a deep-pink ball with a clear ring around it — that ball-and-ring is the corpuscle.

Now look at a real glomerulus

Move from the schematic to a real micrograph. Find the glomerular capillary tuft, the capsular space around it, and the thin parietal layer.

H5P ACTIVITY — Microscopic glomerulus
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INTERACTION

Click to reveal — the renal corpuscle

Name each part of the corpuscle before revealing the answer.

Unlabeled renal corpuscle with numbered markers.
  • 1click to revealGlomerular capillaries
  • 2click to revealCapsular space
  • 3click to revealParietal layer (simple squamous)
  • 4click to revealUrinary pole (filtrate exits to PCT)
  • 5click to revealVascular pole (afferent in / efferent out)
H5P ACTIVITY — Image Hotspots: Renal corpuscle
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CONFIRMATION

Quick self-check

In a renal corpuscle, what is the parietal layer made of?
The parietal layer is the OUTER wall of the glomerular capsule — simple squamous, basically a thin barrier. The visceral layer (podocytes) wraps the capillaries directly, but isn't on this curriculum's list.
You're looking at the cortex at high power. You see a round structure that looks like a tangled ball of capillaries with a clear ring around it. What is the clear ring?
The clear ring around the glomerulus is the capsular space. This is where filtrate first collects — the very beginning of urine formation.
Which is found ONLY in the renal cortex?
Renal corpuscles live exclusively in the cortex. Their presence is THE giveaway that you're looking at cortex tissue — even one round corpuscle in the field tells you you're in the cortex.
3 PCT & nephron loop Wed • ~30 minutes • The first half of the nephron tubule path ✓
Why this stop matters: The PCT is the workhorse of reabsorption — the most numerous tubule on any cortex slide and the easiest to mis-identify. The thin nephron loop is the only squamous tubule in the kidney, which makes it the easy outlier once you know to look for it.
TEXT

The first half of the journey: PCT → Loop

Filtrate exits the corpuscle and immediately enters the Proximal Convoluted Tubule (PCT), which winds around for a while in the cortex doing the bulk of reabsorption. Then it dives down into the medulla as the descending limb of the nephron loop, makes a hairpin turn, and rises back up to the cortex.

PCT — the workhorse of reabsorption

The PCT reabsorbs about two-thirds of everything in the filtrate: water, glucose, amino acids, sodium, bicarbonate, most small ions. To do that much work, the apical surface of each PCT cell is covered in microvilli — collectively called the brush border. The brush border is what makes the PCT lumen edge look fuzzy or out-of-focus under the microscope.

PCT — Proximal Convoluted Tubule simple cuboidal + brush border
Where: renal cortex, near the corpuscle. Usually the most numerous tubule you see.
What you see: simple cuboidal cells with a fuzzy/blurry lumen edge from the brush border (microvilli). Few sparse round nuclei.
Watch out for: easy to confuse with DCT — both are simple cuboidal in the cortex. The brush border is the one feature that reliably tells them apart.

Nephron loop — the medullary U-turn

From the PCT, filtrate enters the descending limb of the loop and travels straight DOWN into the medulla. The descending limb is permeable to water — water leaves the tubule, and the filtrate inside becomes more concentrated.

At the bottom of the loop, the tubule makes a hairpin turn and becomes the ascending limb, traveling straight UP back into the cortex. The thin part of the ascending limb is permeable to salt — salt leaves the tubule, and the filtrate becomes more dilute.

Together, the descending and ascending limbs build the salt gradient in the medulla that makes urine concentration possible later, in the collecting duct.

Nephron loop (thin segment) simple squamous — the only flat tubule
Where: renal medulla.
What you see: very thin wall, almost flat — simple squamous epithelium. Looks like a capillary, but with no red blood cells inside.
Watch out for: students often mistake thin loops for capillaries. Trick: capillaries have RBCs; thin loops don't.
Schematic of one nephron showing corpuscle, PCT, nephron loop, DCT, and collecting duct.

Side-by-side comparison

TubuleRegionEpitheliumID give-away
PCTCortexSimple cuboidalFuzzy lumen edge from brush border; few sparse round nuclei
Nephron loop (thin)MedullaSimple squamousAlmost flat; looks like a capillary but no RBCs inside

The first two steps of the decision card

For PCT & Loop, ask in this order:

Q1. Are the cells flat (squamous), with almost no wall?
→ YES: it's a thin nephron loop (in the medulla).
Q2. Is the lumen edge fuzzy / blurry?
→ YES: PCT. The fuzz is the brush border (microvilli).
Otherwise — NOT one of these. Move on to the DCT / collecting duct rules in tomorrow's stop.

Now look at real tissue

The schematic teaches you what to expect; real micrographs are what you'll be tested on. Compare these to the schematic above.

Kidney micrograph, PAS stain, with PCT brush borders highlighted.

Kidney slide #210, PAS stain, labeled tubules. The PAS stain highlights the brush border on the apical surface of PCT cells — that's what makes those lumen edges look fuzzy on H&E too.

Kidney medulla micrograph showing labeled nephron loop.

Kidney slide #206, medulla, labeled nephron loop. Notice how thin the wall is — almost flat. The nuclei sit ON the wall like beads on a string.

INTERACTION

Click to reveal — the nephron path

Find the PCT and the descending and ascending limbs on the labeled nephron.

Unlabeled nephron with numbered markers.
  • 1click to revealGlomerular capillaries
  • 2click to revealCapsular space
  • 4click to revealPCT
  • 5click to revealNephron loop (descending)
  • 6click to revealNephron loop (ascending)
  • 7click to revealDCT (covered tomorrow)
  • 8click to revealCollecting duct (covered tomorrow)
H5P ACTIVITY — Blood Flow and Nephrons
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CONFIRMATION

Quick self-check

A tubule cross-section has a fuzzy lumen edge and only a few sparse, round nuclei. What is it?
Fuzzy lumen edge = brush border = PCT. The brush border is microvilli that increase the surface area for reabsorption. The DCT lumen is sharp; the collecting duct has distinct cell borders; the thin nephron loop is squamous and almost flat.
Where does the nephron loop make its hairpin turn?
The descending limb dives DOWN into the medulla. The U-turn is deep in the medulla. The ascending limb comes back UP into the cortex. That's why the medulla is full of parallel tubes — it's loops and collecting ducts running side by side.
A medulla micrograph shows a thin-walled circle that looks almost flat — like a capillary — but has no red blood cells inside. What is it MOST likely?
Thin nephron loops are simple squamous — they look almost like capillaries except there are no RBCs inside. PCTs are cuboidal with a brush border; collecting ducts have distinct cell borders.
What is the BRUSH BORDER actually made of?
The PCT brush border is microvilli — tiny finger-like extensions of the apical membrane that massively increase the surface area for reabsorption. They're not cilia (no central core, no movement).
4 DCT & Collecting duct Thu • ~30 minutes • The trickiest tubules to tell apart ✓
Why this stop matters: Telling DCT from collecting duct is the practical-exam trip-up. They're both cuboidal, both have sharp lumens, both are in the cortex. Today's job is internalizing the one feature that separates them.
TEXT

The second half of the nephron: DCT → CD

After the nephron loop, filtrate enters the Distal Convoluted Tubule (DCT), which winds around briefly in the cortex doing fine-tuning under hormonal control. From there, several DCTs from many nephrons all dump into a shared collecting duct. The collecting duct travels straight DOWN through the medulla and opens at the renal papilla.

DCT — the fine-tuner

The DCT is shorter than the PCT and its cells are smaller, so a cross-section packs in more nuclei per unit area. There's no brush border, so the lumen edge is sharp and clean.

The DCT does the fine-tuning of urine composition under hormonal control: aldosterone tells it to reabsorb more sodium (and dump potassium); parathyroid hormone tells it to reabsorb more calcium.

DCT — Distal Convoluted Tubule simple cuboidal — no brush border
Where: renal cortex, near the corpuscle. Less numerous than PCT.
What you see: simple cuboidal cells with a sharp, clean lumen edge. Many crowded nuclei toward the lumen.
Watch out for: vs. PCT — DCT is missing the brush border, that's the call. vs. collecting duct — DCT cell borders are less distinct.

Collecting duct — the final shaper

Collecting ducts are the last stop before urine leaves the kidney. They're under control of antidiuretic hormone (ADH): when ADH is high (you're dehydrated), the collecting ducts become permeable to water — water leaves and you make small amounts of concentrated urine. When ADH is low (well hydrated), water stays in the duct and you make lots of dilute urine.

Histologically, the giveaway is the clarity of cell borders. You can usually see the membrane between adjacent cells — something that's much harder to spot in the PCT or DCT. The cells are also taller (cuboidal that grades into columnar as the duct descends through the medulla).

Collecting duct simple cuboidal → columnar; distinct cell borders
Where: STARTS in the cortex, runs DOWN through the medulla. Several nephrons feed each duct.
What you see: sharp lumen edge + you can clearly see EACH cell's outline. Cleaner-looking than PCT or DCT. Bigger, fewer cells.
Watch out for: in the medulla, you'll see lots of these in parallel. Don't confuse with thin loops (those are squamous, much smaller).
Four tubule cross-sections side by side: PCT, DCT, collecting duct, and thin nephron loop.

All four tubules — the full decision card

Now that you've met all four tubules, here's the complete decision rule. Use it on every cortex and medulla micrograph for the rest of the unit.

Tubule ID — ask in this order. The first YES wins.

Q1. Does it contain a glomerulus (round red ball)?
→ YES: it's a renal corpuscle. Name the parts.
Q2. Are the cells flat (squamous)?
→ YES: it's a thin nephron loop (in the medulla).
Q3. Is the lumen edge fuzzy / blurry?
→ YES: PCT. The fuzz is the brush border (microvilli).
Q4. Sharp lumen edge with MANY crowded nuclei?
→ YES: DCT.
Q5. Sharp lumen edge and you can clearly see EACH cell's borders?
→ YES: collecting duct.

All four tubules side-by-side

TubuleEpitheliumID give-away
PCTSimple cuboidalFuzzy lumen edge from brush border; few sparse round nuclei
DCTSimple cuboidalSharp lumen edge; many crowded nuclei toward the lumen
Collecting ductSimple cuboidal → columnarSharp lumen + distinct cell borders (you can see each cell)
Nephron loop (thin)Simple squamousAlmost flat; looks like a capillary but no RBCs inside

Now look at real collecting ducts

Two micrographs from different slides. Both show the parallel-tube view of the medulla. Look for the distinct cell borders that mark the collecting ducts — and notice the smaller, thinner-walled tubes near them are the nephron loops.

Renal medulla, slide #210, labeled collecting ducts.

Slide #210 — collecting ducts labeled in the renal medulla.

Renal medulla, slide #206, labeled collecting ducts.

Slide #206 — a different field of medulla, again with collecting ducts called out. The cell borders are the giveaway.

INTERACTION

Quick mental practice — name the tubule

Cover the labels on the four-tubule diagram above and try to ID each one before peeking. When you can do all four without hesitation, you're ready for the H5P question set.

H5P ACTIVITY — Question Set (tubule ID)
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CONFIRMATION

Quick self-check

Which is the BEST single feature for telling PCT from DCT?
Both PCT and DCT are simple cuboidal. The brush border on the PCT is the only feature that reliably tells them apart at standard magnification: PCT has it (fuzzy lumen), DCT does not (sharp lumen).
A tubule cross-section has a sharp lumen edge and you can clearly see the membrane between each cell. What is it?
Distinct cell borders is the collecting duct's signature. PCT and DCT cell borders are much harder to spot. Nephron loop is squamous (almost flat).
Where does the collecting duct EMPTY?
Collecting ducts open at the tip of the renal papilla. From there: minor calyx → major calyx → renal pelvis (renal basin) → ureter.
In the medulla you see a wider tubule with very distinct cell borders running parallel to a much smaller, almost-flat tube. They are:
Medulla contains only collecting ducts and nephron loops (no corpuscles, no PCT, no DCT). The wider tube with distinct cell borders is a collecting duct; the small almost-flat one is a thin nephron loop.
5 Drainage & storage — ureter and bladder Fri • ~25 minutes • Renal basin details, ureter, transitional epithelium, detrusor ✓
Why this stop matters: The drainage system uses one signature epithelium — TRANSITIONAL — that you'll never see anywhere else in the body. Recognize it once, and you know you're in a urinary tract organ.
TEXT

From the renal basin to the bladder

Once urine leaves the collecting duct at the renal papilla, it's done being made — now it just needs to get out. The plumbing path:

renal papilla → minor calyx → major calyx → renal pelvis ("renal basin") → ureter → bladder → urethra

The ureter and bladder share a two-feature signature: transitional epithelium on the inside, smooth muscle outside. The smooth muscle moves urine along the ureter (peristalsis) and squeezes the bladder during urination (the detrusor muscle).

Transitional epithelium — the shape-shifter

Transitional epithelium is unique. It changes shape with stretch:

Empty (relaxed) bladder 5–6 layers, dome-shaped umbrella cells on top
The surface cells bulge upward into rounded "dome" shapes called umbrella cells. Below them, the cells are pear-shaped and stacked 5–6 layers deep. The whole epithelium looks thick and rounded.
Full (stretched) bladder 2–3 layers visible, surface cells flattened
Same cells, just spread out. The umbrella cells flatten into shapes that look almost squamous. Only 2–3 layers are visible because the cells are now wider and shorter. The barrier function — keeping urine from leaking back into the body — stays intact the whole time.
Bladder wall showing lumen, transitional epithelium with dome-shaped umbrella cells, lamina propria, smooth muscle (detrusor), and adventitia.
INTERACTION

Click to reveal — the bladder wall

Unlabeled bladder wall with numbered markers.
  • 1click to revealLumen
  • 2click to revealUmbrella cells
  • 3click to revealTransitional epithelium
  • 4click to revealSmooth muscle (detrusor)
  • 5click to revealAdventitia
H5P ACTIVITY — Drag-the-Words: Path of urine
Loads live from bettybroadbent.com — needs an internet connection.
CONFIRMATION

Quick self-check

In a relaxed (empty) bladder, the surface cells of the transitional epithelium look:
Dome-shaped umbrella cells are the signature of relaxed transitional epithelium. When the bladder fills, those same cells flatten out and the epithelium looks thinner.
What is the smooth muscle of the bladder wall called?
The detrusor muscle is the bladder's smooth muscle. Three indistinct layers (inner longitudinal, middle circular, outer longitudinal) — but you usually can't tell them apart on histology.
A patient passes a kidney stone. Trace its path from the renal pelvis to the bladder. Which structure does it pass through?
URETER carries urine (and stones) from the kidney to the bladder. URETHRA carries it from the bladder to the outside. Easy to confuse — the spelling and pronunciation are similar.
Why does transitional epithelium make sense for the bladder, but stratified squamous (like the esophagus) wouldn't?
Transitional reshapes when stretched — same cells, different geometry. Stratified squamous handles abrasion (great for the esophagus) but doesn't stretch well — it would tear and let urine leak through.