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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:
Diffusion
- 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, DC is
concentration difference, and DX 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 preceding 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 Poiseuille’s 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 preceding figure).
Vesicular
Transport
- Vesicular transport is involved in the translocation of macromolecules across capillary
endothelium (b in preceding figure).
Active
Transport
- 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.
RK Revised
04/16/2007
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Cardiovascular Physiology Concepts
