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Cardiovascular Physiology Concepts

Richard E. Klabunde, PhD


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Mechanisms of Capillary Exchange

Fluid, electrolytes, gases, small and large molecular weight substances can transverse the capillary endothelium by several different mechanisms: diffusion, bulk flow, vesicular transport, and active transport. These mechanisms are depicted in the following figure and discussed in the subsequent text:

mechanisms of molecules moving across capillaries


Where dn/dt is flux in moles/sec, D is diffusion constant, A is surface area, ΔC is concentration difference, and ΔX is thickness of barrier to diffusion.

The movement (or flux) of a molecule is directly related to its diffusion constant across the barrier, the surface area available for diffusion, and the concentration gradient across the barrier. In the case of O2 diffusing from the exchange vessels into the tissue, increasing the partial pressure of oxygen (PO2) in the plasma, or increasing the surface area for exchange (i.e., increasing the number of open capillaries), increases the total amount of O2 per unit time moving out of the blood and into the tissue.

Bulk Flow (Convection)

Vesicular Transport

Active Transport

Revised 4/30/2014

DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice.