20.3 Hydrostatic vs Oncotic Pressure

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 · A pull that is half gone

Her albumin is 2.4 grams per decilitre, and more than three grams of it leave in her urine every day. Her estimated colloid osmotic pressure is about twelve millimetres of mercury, half of normal. Her heart is fine, her neck veins are flat, her lungs are clear, her liver is not enlarged. Her reninAn enzyme secreted by the juxtaglomerular cells that helps regulate blood pressure by triggering the and aldosterone are high and her urine sodium is low: the kidney is retaining salt. And the swelling is worst by evening in the ankles and worst by morning around the eyes, which is what gravity does to fluid that is sitting loose in the tissues. The visit said elevate the legs and cut the salt. Both move a symptom. Neither touches the pull that is gone.

See it move: Scene 1 · Two Pressures at Every Capillary · Scene 4 · Four Doors to Edema

II · Starling forces and the anchor

At every capillary two pressures fight over the fluid, and the Blood Vessels module met them as capillary exchange. Hydrostatic pressure is the push of the blood against the wall, about thirty-five millimetres of mercury at the arteriolar end and fifteen at the venular end, driving waterThe universal solvent essential for life. and small solutes out through the clefts. Oncotic — colloid osmotic — pressure is the pull of the plasma proteins too large to leave, about twenty-five to twenty-eight millimetres, holding water in by osmosis; the interstitial fluidThe fluid surrounding cells within tissues. has little protein, so the pull is nearly all on the vessel’s side. At the arteriolar end push wins and fluid filters out; near the venular end pull wins and most of it is drawn back; the three litres a day that are not come home through the lymphatics, blind-ended vessels that collect the surplus and return it at the subclavian veins. Albumin is the anchor: fifty-eight percent of the plasma protein, made by the liver, the molecule that creates most of the oncotic pull — and the carrier of hormones, drugs and close to half the blood’s calcium. The site’s IV mini-lecture makes the clinical point: normal saline has no protein in it and cannot restore colloid osmotic pressure, which is why patients with severe protein loss are given albumin, not more salt water. Edema has four doors. Raise the push — a failing heart, a clot in a vein, a pregnancy on the veins of the legs. Open the wall — a bruise, a burn, inflammation, the histamine of an allergic reaction. Block the drain — a lymphatic cut by surgery or clogged by a parasite. Or lose the pull — hypoproteinemia: a liver that cannot make albumin, a gut that cannot absorb protein, or a kidney that lets it out. The fluid of the first three is different; the fluid of the fourth is thin, protein-poor, pits deeply and moves with gravity. That is hers.

See it move: Scene 1 · Two Pressures at Every Capillary · Scene 2 · The Anchor: Albumin · Scene 4 · Four Doors to Edema

III · The anchor leaving, on a slide

On the site’s slides the glomerular wall is three layers in a micrometre: the fenestrated endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces fr with its pores, the glomerular basement membrane, and the podocytesSpecialized cells in the glomerulus that form filtration slits, preventing large molecules from ente whose foot processes interlock around the capillary with slit diaphragms between them — the layer that actually holds albumin back, by size and by charge. The Urinary module’s biopsy put AA amyloid in the mesangium and in the capillary walls, Congo red and apple-green under polarizedA resting neuron’s membrane with a more negative charge inside than outside. light. Amyloid in that wall widens the sieve: albumin passes, the liver cannot make it fast enough, the plasma’s pull falls, and at every capillary in the body the push wins a little more. Then the kidney, reading a plasma tank that has shrunk because its contents are sitting in the tissues, does what a kidney does for low effective volume: renin, aldosterone, retain sodium and water — which cannot refill a plasma whose anchor is gone and so lands in the ankles too. That is why “cut the salt” is half right: she is retaining salt. And why it is the wrong lever: the salt is being retained because the pull is gone, and the pull is gone because the sieve is leaking. Restore the filter, or the protein, and the salt takes care of itself.

See it move: Scene 3 · Remove the Anchor · Scene 5 · Where the Albumin Went

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 5 of 5

Where the Albumin Went

The filtration membrane and the podocyte, on a slide.

Slide · a still to study

The Sieve, Three Layers in a Micrometre

Three layers and a scaffold

• Fenestrated endothelium: pores about 70 nm wide. They stop cellsThe basic structural and functional units of life., not albumin.
• Glomerular basement membrane: its negative charge helps repel albumin.
• Podocyte foot processes and slit diaphragms: the final size barrier.
• Mesangial cells sit between the capillary loops.

What amyloid does

• In the Urinary module’s biopsy, AA amyloid sat in the mesangium and along the capillary walls, and Congo red glowed apple-green.
• The podocytes were effaced, so the sieve leaked albumin.
• On a routine stain at this power the damage barely shows; the special stain reveals it.

The whole module is downstream of one micrometre of damaged wall.

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

Explore

Widen the sieve

A sieve with three dials

Three dials on a three-layer sieve. Predict the albumin that passes, overlay the amyloid, time the labs, then compare two slides.

Text description of this explore

A cross-section of the glomerular filtration membrane, drawn: blood carrying gold albumin dots on top, then the endothelium with its pores, the basement membrane marked with negative charges, the podocyte feet with slit diaphragms between them, and the urine side (Bowman’s space) below. Step 1: three dials set the endothelial pores (normal, about 70 nm, or twice as wide), the basement-membrane charge (negative or lost) and the slit diaphragms (normal, widened, or torn open in places). Predict how much albumin passes each day (under 0.03 g, 0.03 to 0.3 g, 0.3 to 3 g, or over 3 g), then run the counter: gold dots appear on the urine side and a readout gives grams per day, the normal value, Stina’s 3.1 g and the layer holding the most back. Step 2: with amyloid overlaid (pink lumps packed in the basement membrane, the podocyte feet flattened over them), decide which layer each of four descriptions names: endothelium, basement membrane, podocyte slits, or the mesangium between the loops. Step 3: decide when each lab line moves (first, within days; next, over two to three weeks; or last, when the lymph falls behind): urine albumin, plasma albumin, oncotic pressure and ankle fluid. Step 4: compare two panels, the site’s slide 209 on H&E (one normal glomerulus with the crescent of Bowman’s space, among proximal tubules) and a drawing after the Urinary module’s Congo-red panel under polarized light (deposits glowing apple-green on a dark field), and choose the one feature only the stain shows.

Think · not graded

The H&E slide of her glomerulus looks almost normal at 40×; the Congo red shows the amyloid. Which layer of the sieve does amyloid in the capillary wall disable, and why does a damaged sieve leak albumin before it leaks red cells?

Spot-check your thinking: true or false?

Check · Drag the Words

Label the three layers of the filtration membrane.

Evidence card 5

Add it to your evidence board

Amyloid in the capillary wall widens the sieve; albumin leaves; the anchor goes with it.

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: The pull is gone — oncotic edema from a leaking filter, Neither on its own, or The push is too high — a heart or vein problem. 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.