The capillary bed is arranged to satisfy the metabolic needs of parenchymal cells. Transfer to and from the blood stream is accomplished by diffusion and convection processes that are regulated to integrate systemic and local mechanisms.
Hydraulic pressure together with the geometrical distribution and intrinsic activity of arteriolar smooth muscle in the peripheral extreme of the vascular tree can satisfactorily explain the observed characteristics of intravascular flow within microcirculatory units.
However, the processes that adjust the pressure and concentration gradients between the capillary lumen and cells in extravascular spaces and the corresponding interstitial transport mechanisms remain controversial. Advances toward the long-standing but elusive goal of modeling a completely functional capillary-tissue unit will require fresh approaches to both experimental measurements and theoretical analyses.