MCAT content library / Biology & biochemistry / Capillary exchange and lymph flow
Predict fluid and solute movement across a capillary from the supplied pressure model, then decide what lymphatic return contributes.
About 17 minutes, including the simplified Starling-force example and tracer application.
Useful foundations
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Separate diffusion from bulk fluid movement
Small gases and solutes can cross capillary walls by diffusion according to concentration or partial-pressure gradients. Water and dissolved material can also move with bulk fluid filtration across the endothelial barrier. These are related but distinct processes: a molecule can diffuse efficiently even when net fluid movement is small, and a change in hydrostatic pressure can alter bulk fluid movement without changing a solute’s concentration gradient.
Large plasma proteins cross many capillary beds much less freely than water and small solutes. Their restricted movement helps create a colloid-osmotic, or oncotic, pressure difference between plasma and interstitial fluid.
Concept: hydrostatic and oncotic gradients compete
Capillary hydrostatic pressure tends to push fluid outward from blood into the interstitium. Plasma oncotic pressure generated by retained proteins tends to oppose that outward movement. Interstitial hydrostatic and oncotic pressures contribute as well, and the actual flux also depends on capillary permeability and surface area.2
For an MCAT calculation, use the model stated in the problem. A simplified teaching model may define a net outward driving pressure as:
NFP = (Pc − Pi) − (πc − πi)
where P terms are hydrostatic pressures and π terms are oncotic pressures. A positive value represents net outward filtration under this simplified convention.
Worked application: use the model that is supplied
An original capillary model sets Pc = 30 mmHg, Pi = 0 mmHg, πc = 25 mmHg, and πi = 5 mmHg. Assume the reflection coefficient is 1 and the filtration coefficient is unchanged between comparisons.
The net outward driving pressure is:
(30 − 0) − (25 − 5) = 10 mmHg outward.
| Condition | Pc | Pi | πc | πi | Simplified NFP |
|---|---|---|---|---|---|
| Reference | 30 | 0 | 25 | 5 | +10 |
| Higher capillary pressure | 36 | 0 | 25 | 5 | +16 |
| Lower plasma oncotic pressure | 30 | 0 | 18 | 5 | +17 |
Both higher capillary hydrostatic pressure and lower plasma oncotic pressure increase outward filtration in this stipulated model. The table does not claim that every real capillary has these pressures or that pressure stays constant along a capillary.
Pattern: lymph returns filtered fluid and protein
Net capillary filtration continuously contributes fluid to the interstitial space. Lymphatic vessels collect excess interstitial fluid and proteins and ultimately return them to the circulation. Lymphatics also transport immune cells and dietary lipids absorbed from the intestine.3
In an original tracer experiment, a large labeled protein is injected into the interstitial space. With normal lymph flow, label appears progressively in a draining lymph vessel and later in venous blood. After selective lymphatic obstruction, interstitial label persists and tissue volume increases while venous recovery falls. That pattern identifies failed return rather than increased capillary entry as the manipulated step.
Rule: do not assume universal venous-end reabsorption
A traditional simplified diagram often shows filtration at the arterial end of a capillary and reabsorption at the venous end. Real microvascular exchange is more nuanced. Many tissues exhibit sustained net filtration under steady conditions, with lymphatic drainage returning the excess fluid and protein to the circulation.4
For an exam problem, follow the explicitly supplied model. If the stem gives a classic pressure table and asks for inward versus outward movement, calculate from those values. If it asks about physiological lymph return, do not assume that all filtered fluid must be reabsorbed directly at the venous capillary end.
ACTIVE PRACTICE · FLUID AND LYMPHATIC PHYSIOLOGY
Change a Starling Force Before Predicting Filtration
Original simplified capillary data: capillary hydrostatic pressure Pc = 30 mmHg, interstitial hydrostatic pressure Pi = 0, plasma oncotic pressure πc = 25, and interstitial oncotic pressure πi = 5. Assume reflection coefficient 1 and a fixed filtration coefficient. Positive net driving pressure denotes outward filtration.
Net driving pressure = (Pc − Pi) − (πc − πi) = mmHg.
Positive: outward fluid filtration in the stipulated model.
This is the lesson’s simplified Starling-force comparison, not a claim that all real capillaries reabsorb at the venous end. Actual net fluid flux also depends on filtration coefficient, vascular glycocalyx and lymphatic return.
Check your reasoning
Choose an answer before opening its feedback.
Check 1
In the supplied simplified model, capillary hydrostatic pressure rises while all other terms remain constant. What happens to net outward filtration?
A. It increases.
Correct. A larger outward hydrostatic term raises the net outward driving pressure.
B. It decreases.
Review this choice. The changed term favors outward movement.
C. It must become exactly zero.
Review this choice. No values support that conclusion.
D. It becomes independent of pressure.
Review this choice. Hydrostatic pressure is part of the supplied model.
Check 2
A large interstitial protein tracer normally returns to venous blood through lymph. Lymphatic drainage is blocked without changing capillary pressure. Which result is most expected?
A. Greater interstitial retention of the tracer
Correct. The return pathway has been blocked.
B. Immediate disappearance of all interstitial fluid
Review this choice. Blocking lymph return favors fluid accumulation rather than disappearance.
C. Conversion of the protein into oxygen
Review this choice. The experiment concerns transport, not chemical conversion.
D. Guaranteed fall in capillary permeability
Review this choice. Permeability was not manipulated.
Check 3
Which process most directly moves oxygen from capillary blood into a tissue when tissue PO₂ is lower?
A. Diffusion down a partial-pressure gradient
Correct. Oxygen crosses primarily by diffusion according to its partial-pressure gradient.
B. Platelet aggregation
Review this choice. Platelets mediate hemostasis, not routine oxygen diffusion.
C. DNA replication
Review this choice. DNA replication does not drive capillary gas exchange.
D. Lymphocyte clonal selection
Review this choice. That is an immune process.
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