Isofiltration capillary pressure and compartmental vascular resistances following elevation of venous pressure in subcutaneous tissue.

Isofiltration technique was used to measure the functional capillary pressure in 10 canine hindpaw (mainly subcutaneous tissue) preparations. With simultaneous determinations of arterial pressure, blood flow and venous pressure, the changes in total, precapillary and postcapillary resistances were assessed at different levels of venous pressure. When the tibial arterial pressure was 100 mm Hg. at an initial venous pressure of 5 mm Hg, the isofiltration capillary pressure averaged 12.8 mm Hg, the ratio of pre-to post-capillary resistance 9.9. The precapillary resistance accounted for 90.8% of the total vascular resistance. The isofiltration capillary pressures were 24.9 and 38.0 mm Hg, respectively when venous pressure was elevated to 20 and 35 mm Hg. Capillary pressure (Pc) was approximately related to venous pressure (Pv) by: Pc=0.82Pvm8. Upon elevation of venous pressure, the total and precapillary resistances were not significantly altered. The paw resistance vessels possess neither myogenic nor metabolic autoregulation. With increasing venous pressure, presumably due to passive distension of venules, the postcapillary resistance fell and the ratio of pre-to post-capillary resistance increased.