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:
- Particularly important for gases (O2 and CO2) and lipid-soluble substances (e.g., anesthetics); fluid and electrolytes are also exchanged, in part, by diffusion forces (a in preceding figure).
- Fick's First Law of diffusion:
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)
- Bulk flow of fluid and electrolytes occurs through "pores" and intercellular clefts (d,e,f in above figure)
- This mechanism of exchange is particularly important in renal glomerular capillaries; however, it occurs to variable extent in nearly all tissues.
- Bulk flow follows Poiseuilles equation for hydrodynamic flow. Therefore, changes in pressure driving forces (either hydrostatic or osmotic) and in the size of "pores" or intercellular clefts will alter exchange. Contraction of capillary endothelial cells by substances such as histamine and bradykinin increases intercellular pore size and greatly augments fluid and electrolyte movement by increasing the capillary filtration constant.
- There is some evidence that vesicles can fuse together creating pores across endothelial cells (c in above figure).
- Vesicular transport is involved in the translocation of macromolecules across capillary endothelium (b in above figure).
- Some molecules (e.g., ions, glucose, amino acids) can also be taken up by vascular endothelial cells by transport mechanisms; however, this is not normally thought of as a mechanism for exchange between plasma and interstitium, but rather between an individual cell and its surrounding extracellular environment.