The relationship between interstitial fluid pressure and volume in rat trachea.

Abstract

A change of interstitial fluid volume (IFV) will normally change the interstitial fluid pressure (Pif) so as to counteract further fluid movement across the capillaries and changes in IFV. Contrary to this, several acute inflammatory reactions in the trachea are associated with increased negativity of Pif, which will 'actively' generate oedema. To outline further the role of Pif in interstitial fluid balance in the trachea, interstitial compliance (delta IFV/delta Pif) was measured in pentobarbital anaesthetized rats. IFV was measured as the plasma equivalent extravascular distribution space of [51Cr]EDTA. Pif was measured in the same animal with sharpened glass pipettes (diameter 3-6 microns) connected to a servocontrolled counterpressure system. In dehydration (30 mL saline i.v., n = 10) interstitial compliance was 0.083 mL g dry wt-1 mmHg-1. Since control IFV was 1.046 mL g dry wt-1 (n = 10) the interstitial compliance is 8% of IFV per mmHg. In overhydration (30 mL NaCl, n = 10) and dextran anaphylaxis (1 mL dextran 70, n = 10) compliance remained the same for the first 15% increase in IFV and then increased several-fold since Pif did not increase more than 2 mmHg above control level. The increased negativity of Pif by -10 mmHg associated with acute inflammation will require a reduction of IFV by 80% when interstitial compliance is 8% per mmHg. A more likely explanation is therefore that structural rearrangements are responsible for the events leading to increased negativity of Pif in acute inflammation.