Effect of Ischemia on Capillary Pressure and Equivalent Pore Radius in Capillaries of the Isolated Dog Hind Limb

¹ Department of Physiology and Biophysics, University of Iowa College of Medicine, Iowa City, Iowa 52242

Measurements were made of filtration coefficients (L_p), area per unit path length (A/x), and equivalent pore radius (r_p) in the control state and following severe ischemia (arrested blood flow) for periods of 30 minutes, 1 hour, and 3 hours. The value of A_s/x for diffusion of all lipid-insoluble substances was not changed after 30 minutes of ischemia, but it was increased after 1 and 3 hours of ischemia. The value of r_p calculated from the theory of restricted diffusion yielded values of 34-35 Å for both the control period and after all three periods of ischemia. Combination of hydrodynamic data (L_p) and diffusion data (A/x) yielded values for r_p of 23 Å for control and ischemic periods. Measurements of plasma-protein osmotic pressure, tissue-protein osmotic pressure, tissue hydrostatic pressure, and capillary hydrostatic pressure supported the conclusion that extended periods of arrested blood flow did not affect muscle capillary membrane porosity. In 5 of 16 hind limbs, there appeared to be a porosity change following 3 hours of arrested blood flow. This change was demonstrated by a net decrease in plasma-protein osmotic pressure and an increase in r_p from 34 Å to 54 Å. L_p was not changed after 30 minutes but was increased after 1 hour of ischemia; the increase was associated solely with an increase in capillary surface area. After 3 hours of ischemia, the primary change in 11 of 16 hind limbs was an increase in capillary surface area, although an increase in the size of the pores per unit membrane area could not be rigorously excluded. In 5 hind limbs after 3 hours of ischemia, an increase in r_p was the primary change and an increase in capillary surface area was of secondary importance. The data indicate that the edema which occurs subsequent to reperfusion of the vasculature after moderately long periods of severe ischemia results from an increase in capillary hydrostatic pressure augmented by an increase in capillary surface area not associated with an increase in membrane porosity. The rise in capillary pressure for any given arterial or venous pressure involves a decrease in precapillary resistance, but postcapillary resistance does not change for any given flow.

Submitted on August 30, 1973
Accepted on March 27, 1974