Past Papers · SAQ

Cell Transport — Capillary Exchange

Current · V5 (2025) → A.iv Historical · V4 (2023) → E.ii 1 exam appearance

2010A Q15

Exam question

Discuss the important factors in exchange of gases and substrates between capillaries and tissue cells.

CICMWrecks answer

Master answer

Based on 3 main physical principles

Diffusion via Fick’s Law

Rate of movement of solute across semi-permiable membrane J is

J\;={{D\;A\;\Delta C}\over{T}}

where

D=Diffusion\;constant\;\propto\;{{Solubility}\over{\sqrt{Molecular\;Weight}}}

C = concentration (or partial pressure for gasses)
A = cross-sectional area
T = thickness of the membrane or distance over which diffusion takes place.

These factors alter the rate of transfer as per the equation above.
Valid for Most medium – small substances
- Water soluble (H2O, electrolytes, glucose, urea) via intercelular clefts
- Lipid soluble substances (O₂, CO₂) via endothelial cells themselves.

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

Gibbs-Donnan Effect:

“Opposing osmotic and electro-chemical gradients in the presence of a nondiffusable ion resulting in unequal distribution of the diffusable ions”

Diffusible ions move ↓ [ ] gradient
Because of non-diffusable ion, significant opposing electrical potential develops.
This prevents further movement of ions, and an electrochemical equilibrium is reached.
Because of the presence of non-diffusable ions, there is an osmotic disequilibrium.

Osmotic pressure itself can be determined from the Vant Hoff Equation

Osmotic \; pressure \; (\pi) \; = \; {{nRT \times c} \over {M}}

where
n = # of particles into which substance dissociates
c = [ ] (in g/L)
T = Absolute temperature
R = Universal Gas Constant (8.314 J⋅K⁻¹⋅mol⁻¹)
M = Molecular weight of molecules

It depends on:

# of particles
[ ] or molarity of the solution
Temperature
Inversely to the molecular weight

Active Processes

Serve to establish Starlings forces and G-D equlibrium

Facilitated diffusion
◦ Diffusion through the membrane using a specific carrier protein to help
Active transport
◦ Movement of ions or other substances across the membrane in combination with a carrier protein
against an energy gradient.
◦ May be primary (energy derived directly from ATP)
◦ or secondary (occurs via co-transport or counter transport)
Endocytosis/Exocytosis
◦ Vesicular transport by engulfment/extrusion of particle by cellular contents

Gladwin 2016

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