Cell Transport — Starling Forces

Current · V5 (2025) → D4.v Historical · V4 (2023) → E.ii 2 exam appearances

2025A Q12

Exam question

(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).

CICMWrecks answer

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
J_v is the trans endothelial solvent filtration volume per second
( [ P_c – P_i ] – σ [ π_p – π_i ] ) 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 = L_pS = 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
P_c ~35 → 15mmHg (Arterial → venous) Capillary hydrostatic pressure Pressure moving fluid out of capillary	π_p ~ 20mmHg Plasma oncotic pressure Pressure keeping fluid within capillary
P_if = 5mmHg Interstitial hydrostatic pressure Pressure moving fluid into capillary	π_if ~ 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 ↓π_c, ↑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

Reusable content

Formulae used in this answer

Starling Forces

CWF-0125

J_v={\kappa \; ([P_{capil} - P_{interstit}] - \sigma \; [\pi_{plasma} - \pi_{interstit}])}

Starling Forces

CWF-0126

P_{cap} \; \propto \; {{Post-capil\;resist} \over {Pre-capil \; resist}}

Answer by JC

Past papers

Exam appearances

2 appearances

Exam	Exact exam wording	Candidate 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).	34%
2018B Q16	Describe the forces that result in fluid exchange across capillary membranes.	57%