20.1 The Swollen Ankles That Point Back to the Kidney

Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.

I · Two errands

By 2025 her socks left deepAway from the surface of the body. rings pressed into her ankles by evening, and her face was faintly puffy when she woke. Press a fingertip into the ankle and it pitted — the dent held for a moment and slowly filled. The labs carried flags: albumin low, calcium low, bicarbonate a little low. Her urineThe liquid waste excreted by the kidneys., which had shown “a little protein” the year before, now carried nearly four grams a day. The visit ended the way so many of her visits had ended, with a tidy two-part instruction that treated each number as its own small problem. Drink less salt for the swelling. Take some calcium for the calcium. Nobody drew a line from either errand back to the filter the last module had shown was leaking.

See it move: Scene 1 · Socks That Leave Rings · Scene 5 · Read the Visit Note

II · Three balances, one organ

Most of you is water — about sixty percent of body weight in an adult, less with age and with fat — and where it sits matters. The Introduction lecture parcels it into compartments: intracellular fluidThe fluid inside a cell, primarily composed of cytosol., everything poured out of every cell, about forty percent of body weight; and extracellular fluid(ECF) Fluid outside cells, including plasma and interstitial fluid., the twenty percent outside cells, split into the plasmaThe liquid component of blood. inside the vessels (four percent) and the interstitial fluidThe fluid surrounding cells within tissues. that bathes the tissues (sixteen percent). Only the plasma trades with the outside world — through the gut, the lungs and the kidney — and the plasma trades with the interstitial fluid, which trades with the cells. Three balances ride on that plumbing. Fluid balance: how much water, and in which tank. Electrolyte balance: the dissolved ionsCharged atoms or molecules. — sodium, potassium, chloride, bicarbonate, calcium, phosphate — that run everything electrical and osmotic. Acid–baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). balance: a blood pHA measure of hydrogen ion concentration in a solution. held between 7.35 and 7.45 while the body makes acid all day. The lungs can move one of those dials. The gut can add to two. The kidney holds the reins on all three: it sets sodium and therefore water, it guards potassium and calcium, it reclaims and manufactures bicarbonate and excretes the day’s fixed acid. One organ, three balances — which is why one failing organ can write three flags on one page.

See it move: Scene 2 · Mostly Water · Scene 3 · Three Balances, One Organ

III · The filter, on a slide

Under the microscope the glomerulus is a ball of capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. whose wall is a three-layer sieve — fenestrated endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces fr, a basement membrane, the foot processes of podocytesSpecialized cells in the glomerulus that form filtration slits, preventing large molecules from ente — built to let water and small solutes through and keep albumin and cells back. The Urinary module put amyloid in that wall. A sieve with amyloid in it leaks protein: nearly four grams a day in her case, most of it albumin, the protein that holds water in the vessels and carries close to half the calcium in the blood. Read with that slide in hand, the four flags stop being four problems. The swelling is fluid that left the plasma because its anchor is gone. The “low” calcium is largely calcium that rides on albumin going missing from the assay. The low bicarbonate is the same organ, stiffening, losing its grip on acid. The chapters that follow earn each of those sentences, one balance at a time.

See it move: Scene 4 · The Filter That Leaks, on a Slide · Scene 5 · Read the Visit Note

5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

Scene 4 of 5

The Filter That Leaks, on a Slide

Glomerulus, slide 209 — where four grams of albumin a day are going.

Slide · a still to study

The Renal Corpuscle and Its Sieve

The filter

• The glomerulus is a ball of capillaries inside Bowman’s capsule.
• Its wall has three layers: fenestrated endothelium, the basement membrane, and podocyte foot processes with slit diaphragms between them.
• Water, ions, glucoseA simple sugar that is the main source of energy for cells. and urea pass. Albumin and cells are held back.

Her filter

• The Urinary module’s biopsy found AA amyloid in the mesangium and the capillary walls: Congo red, apple-green under polarizedA resting neuron’s membrane with a more negative charge inside than outside. light.
• A sieve with amyloid in it leaks protein: 3.8 g a day, most of it albumin.

The filter is the upstream organ; everything in this module is downstream of what it lets through.

Prefer the full lecture? Renal Clearance and Transport Max (original video on the site) · histology page: Renal Corpuscles

Game

Through the sieve

Microscope

Sort a tray of particles at the filter, push them through, then lay amyloid into the wall. Last, find the corpuscle on a real slide.

Text description of this game

The glomerular filtration membrane drawn in cross-section at about 20,000 times: capillary blood above; the endothelium with its pores; the basement membrane; the podocyte foot processes with slits between them; and the filtrate in Bowman’s space below. Particles are not drawn to one scale; the text gives their sizes. Step 1: seven particles, water (0.3 nm), sodium (well under 1 nm), urea (about 0.5 nm), glucose (about 0.9 nm), albumin (about 7 nm, negatively charged), IgG (about 11 nm) and a red cell (about 7,500 nm). For each choose passes or held back, and the layer that stops it (none, it passes; the endothelial pores; or the basement membrane and podocyte slits). After you commit, the small particles move into the filtrate. Step 2: set the amyloid in the capillary wall to none, some or heavy; predict what crosses that did not before (nothing new; albumin first; red cells first; or IgG first, then albumin); then run it. A readout lists albumin, IgG, red cells and urine protein. Step 3: choose why albumin is the leak that matters for every chapter after this one. Step 4: slide 209, a light micrograph of kidney cortex stained with H&E, shows one glomerulus inside a clear crescent of Bowman’s space, ringed by tubules. Three numbered pins: label each as glomerulus, Bowman’s space, proximal tubule, collecting duct or renal pelvis. Step 5: choose the part of the filter you cannot see at this power.

Think · not graded

Albumin is small enough to pass a damaged membrane and large enough to matter twice — for water at every capillary and for calcium in a test tube. Which of her four flags would you expect to be downstream of albumin, and which is the one that is not?

Spot-check your thinking: true or false?

Check · Drag and Drop

Drag each label onto slide 209: the glomerulus, Bowman’s space and a proximalCloser to the point of attachment or origin. tubule.

Evidence card 4

Add it to your evidence board

Amyloid in the sieve; albumin in the urine — the leak that will explain the next three chapters.

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: One leaking filter, read four ways, Neither on its own, or Separate problems, separate fixes. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.