Rapid Review·Cardiovascular
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PHYSIOLOGY
T2High yieldCapillary Fluid Exchange & Starling Forces
P299
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Focus on
Four pressures that decide whether fluid leaves the capillary, and the four ways edema happens.
Key takeaways
The four Starling forces
![Starling forces along a single capillary. Hydrostatic pressure falls steadily from the arteriolar to the venular end while osmotic pressure stays roughly constant, so net flow is outward (filtration) at the arteriolar end and inward (reabsorption) at the venular end, crossing over where the two lines meet. The equation shown is net fluid flow = Kf [(Pc - Pi) - sigma (pi c - pi i)].](https://pub-06b3554574054cb79ff921ea5b0daf8c.r2.dev/library/a4d56c05-4382-4c82-98fa-92ce9277eb71/097ae0f0-3f95-4f14-af6b-27a0e33b510d.webp)
What this shows
Pc, capillary hydrostatic pressure
pushes fluid OUT.Pi, interstitial hydrostatic pressure
pushes fluid IN.πc, plasma oncotic pressure
pulls fluid IN.πi, interstitial oncotic pressure
pulls fluid OUT.The equation
Jv = Kf [(Pc − Pi) − σ(πc − πi)].- Kf is capillary permeability to fluid.
- σ, the reflection coefficient, measures capillary impermeability to protein.
Filtration happens at the arteriolar end and reabsorption at the venular end
because hydrostatic pressure falls along the capillary while oncotic pressure stays roughly constant. The crossover is drawn on the figure where the two lines meet.| Force | Full name | Direction |
|---|---|---|
| Pc | Capillary hydrostatic pressure | Out |
| Pi | Interstitial hydrostatic pressure | In |
| πc | Plasma oncotic pressure | In |
| πi | Interstitial oncotic pressure | Out |
The four ways edema happens
| Mechanism | Change | Classic causes |
|---|---|---|
| Raised capillary hydrostatic pressure | Pc up | Heart failure |
| Raised capillary permeability | Kf up | Toxins, infections, burns |
| Raised interstitial oncotic pressure | πi up | Lymphatic blockage |
| Reduced plasma proteins | πc down | Nephrotic syndrome, liver failure, protein malnutrition |
| Pattern | Think |
|---|---|
| Bilateral pitting | Cardiac (RHF), hepatic (cirrhosis), nephrogenic (proteinuria), drugs (amlodipine, imatinib), dietary, pregnancy |
| Unilateral non-pitting | Lymphatic: malignancy, mastectomy, Wuchereria bancrofti |
| Pretibial myxedema | Graves disease only |
A patient on amlodipine has new ankle edema. Is this heart failure?
No. Dihydropyridine calcium channel blockers cause it directly by dilating the precapillary arteriole.
How it's tested
The four mechanisms of edema map one-to-one onto the four Starling forces, so a stem naming nephrotic syndrome is testing reduced plasma oncotic pressure and one naming burns is testing increased capillary permeability. Mehlman adds the practical layer that catches people clinically: a patient on amlodipine with ankle edema does not have heart failure, because dihydropyridine calcium channel blockers cause it directly by dilating the precapillary arteriole.
Go deeper
First Aid 2026 — CV/Physiology (p.299) · B&B — Capillary fluid exchange · Mehlman — HY Cardio (edema mechanisms, unilateral non-pitting)
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Flashcards for this page
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Jv = (...) × [ ( (...) − (...) ) − (...) ( (...) − (...) ) ]
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