CHANGES IN INTERSTITIAL PRESSURE DURING AND AFTER BLOOD-VOLUME EXPANSION IN RATS

Interstitial fluid pressure was measured via a chronically implanted capsule before, during after acute isotonic, iso-oncotic blood expansion in normal or in 48 h dehydrated rats. Patterns of body fluid distribution, selected renal responses and mean arterial and mean central venous pressure responses were simultaneously studied. Dry tissue weight (DTW) was determined by freeze drying of the shaved carcass. Dehydration decreased plasma vol and interstitial fluid vol significantly. The initial intracapsular pressure in dehydrated animals was not significantly different from that in normal rats, but dehydrated rats initially showed a significantly lower effective interstitial compliance (0.0005 ml/mm Hg/g DTW) than did the normal group (0.0704). Renal response produced an increase in effective interstitial compliance in dehydrated rats but showed no change in normal rats. Neither group completely corrected its elevated blood vol; both returned their central venous pressures to pre-infusion levels; both decreased their interstitial fluid below preinfusion levels and both decreased their intracapsular fluid pressures 1 mm Hg below the level prevailing in non-infused animals at that time. A reduction in interstitial hydrostatic pressure can be a functionally important influence in the apparent control of central venous pressure following acute blood volume expansion.