Step by Step
1
Capillary hydrostatic pressure — pushes fluid OUT
The blood pressure inside the capillary pushes fluid outward into the surrounding interstitium. This pressure is higher at the arterial end of a capillary (~35 mmHg) and lower at the venous end (~15 mmHg).
2
Plasma oncotic pressure — pulls fluid IN
Proteins in the blood (mainly albumin) create an osmotic pull that draws fluid back INTO the capillary. This oncotic pressure stays fairly constant throughout the capillary, at around 25 mmHg.
3
Net result: filtration at the arterial end, reabsorption at the venous end
At the arterial end, hydrostatic pressure (35) exceeds oncotic pressure (25), so fluid filters OUT of the capillary. At the venous end, hydrostatic pressure (15) is now less than oncotic pressure (25), so fluid is reabsorbed back INTO the capillary. Any excess filtered fluid that isn't reabsorbed is drained by the lymphatic system.
4
Edema — when the balance breaks down
Edema (tissue swelling) occurs when filtration exceeds reabsorption plus lymphatic drainage. Causes include increased capillary hydrostatic pressure (as in heart failure), decreased oncotic pressure (from low albumin, due to liver disease or malnutrition), blocked lymphatics, or increased capillary permeability (as in inflammation).
Applied Walkthrough
1
At the arterial end of a capillary, hydrostatic pressure (about 35 mmHg) is much higher than oncotic pressure (about 25 mmHg), so the net force pushes fluid OUT of the capillary into the interstitial space — filtration.
2
As blood moves toward the venous end of the same capillary, hydrostatic pressure has dropped (to about 15 mmHg) while oncotic pressure remains about the same (25 mmHg) — now the net force pulls fluid back INTO the capillary — reabsorption.
3
A patient with liver disease has low albumin levels, reducing plasma oncotic pressure throughout their capillaries. With less inward pull to balance the outward hydrostatic push, more fluid than usual filters out and isn't fully reabsorbed.
4
This fluid imbalance, if it exceeds what the lymphatic system can drain away, results in edema — visible tissue swelling, directly explained by the disrupted Starling forces balance.
Exam Application
Exams test whether you understand how hydrostatic and oncotic pressures shift along the length of a capillary (favoring filtration at the arterial end, reabsorption at the venous end), and whether you can identify the four possible causes of edema.
⚠ Common Trap
The most common trap is assuming hydrostatic and oncotic pressures behave the same way throughout the capillary — hydrostatic pressure actually drops significantly from the arterial to venous end, while oncotic pressure stays roughly constant, which is exactly why filtration dominates early and reabsorption dominates later.
✓ Quick Self-Check
1. What does capillary hydrostatic pressure do to fluid movement?
Pushes fluid OUT of the capillary into the interstitium.
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2. What does plasma oncotic pressure do to fluid movement?
Pulls fluid IN from the interstitium back into the capillary.
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3. Why does filtration dominate at the arterial end of a capillary?
Because hydrostatic pressure is higher there (~35 mmHg) than oncotic pressure (~25 mmHg).
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4. What happens to excess filtered fluid that isn't reabsorbed?
It's drained by the lymphatic system.
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5. Name two causes of edema.
Any two of: increased capillary hydrostatic pressure (heart failure), decreased oncotic pressure (low albumin), blocked lymphatics, increased capillary permeability (inflammation).
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