Canonical Question
Cell Transport
Variant family: at least one exam appearance has different scope. The appearance record controls whether this master answer is safe to display on that Past Paper.
Master answer
Introduction
Capillaries contain semipermeable membranes to allow the movement of fluid and solutes.
- it is normally impermeable to large protein
- Plasma ultrafiltrate is filtered by bulk flow through the capillary wall by the action of opposing hydrostatic and oncotic pressures
Starling Forces
- The NET flux across the membrane is the balance of hydrostatic pressure and oncotic pressure, as defined by the Classic Starling Equation:
\[J_v={\kappa \; ([P_{capil} – P_{interstit}] – \sigma \; [\pi_{plasma} – \pi_{interstit}])}\]
where
Jv is the trans endothelial solvent filtration volume per second
( [ Pc – Pi ] – σ [ πp – πi ] ) is the net driving force
P = hydrostatic pressure
π = oncotic pressure
σ = Staverman’s reflection coefficient ie. Permeability of membrane to protein
κ = filtration constant = LpS = Hydraulic conductivity x Surface Area
\[P_{cap} \; \propto \; {{Post-capil\;resist} \over {Pre-capil \; resist}}\]
- Typically quoted values for the variables in the classic Starling equation:
| Hydrostatic pressure | Oncotic pressure |
|---|---|
| Pressure moving fluid | pressure exerted by proteins which draw water into and keep it within a compartment |
| Pc ~35 → 15mmHg (Arterial → venous) Capillary hydrostatic pressure Pressure moving fluid out of capillary | πp ~ 20mmHg Plasma oncotic pressure Pressure keeping fluid within capillary |
| Pif = 5mmHg Interstitial hydrostatic pressure Pressure moving fluid into capillary | πif ~ 0mmHg Interstitial fluid oncotic pressure Pressure keeping fluid out of capillary |
- In general, at the arterial end of capillary NFP is positive (filtration) +10mmHg
- At the venous end NFP is negative (absorption) -10mmHg
- Approx. 24L fluid filtered / day
- 85% reabsorbed into capillaries
- Rest reabsorbed via lymphatics (~3.5L/day) = Net fluid loss from filtration
Role of plasma oncotic pressure
- largely fixed as capillary is impermeable to oncoproteins (1° albumin)
- If ↓πc, ↑net movement of fluid into interstitium
- Once this exceeds the drainage capacity of lymphatics → oedema occurs
- Especially dangerous in areas where minimal interstitial distance is required b/n capillary and cell to continue normal functioning (e.g. lung)
JC 2019
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2025A Q12 | (a) Outline the structural features of the capillary membrane that facilitate the movement of water (15% marks). (b) Outline the mechanisms by which water moves across the capillary membrane (10% marks). (c) Describe the forces that influence fluid movement across the capillary membrane (75% marks). | variant | 34.00% |
| 2018B Q16 | Describe the forces that result in fluid exchange across capillary membranes. | historical_member | — |