Diffusion is the movement of substances down a concentration gradient. Across the capillary endothelium the diffusion of lipid insoluble molecules and ions occurs at defects such as intercellular junctions, pores, fenestrations and larger gaps. Gases and lipid soluble molecules diffuse directly across cell membranes as well as at these endothelial defects. The rate of diffusion of a substance is dependent on concentration gradient (CO -Ci), capillary permeability (P), and capillary surface area (S) and is given by an adaptation of Fick's law:
Rate of diffusion of substance = PS (CO - CI), where
C0 = concentration in the capillary
C1 = concentration in the interstitial fluid.
The capillary permeability is a constant combining the various factors that affect the diffusion of different substances such as:
• Molecular size
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