Compliance of the interstitial space in rats. II. Studies on skin

Interstitial compliance, defined as the ratio between changes in interstitial fluid volume (delta IFV) and interstitial fluid pressure (delta IFP), has been measured for rat skin. IFP has been measured with sharpened glass capillaries connected to a servocontrolled counterpressure system, while IFV has been determined as the extravascular distribution space for 51Cr-EDTA. The measurements were performed at hydration and dehydration of various duration. Compliance was 0.162 ml/g dry tissue per mmHg in dehydration, corresponding to 5.4 ml/100 g wet tissue per mmHg. The volume-pressure curve was similar at 2-4 and 24-28h of dehydration. Up to 20% increase in IFV compliance was similar to that in dehydration, but thereafter increased severalfold upon further increase in IFV. Even at sixfold increase in IFV, IFP did not rise by more than 1 to 2 mmHg. Control IFV in skin amounts to 40 ml/100 g wet tissue and IFV will therefore change by 14% when IFP changes by 1 mmHg from the control level. This change in IFV will cause interstitial fluid colloid osmotic pressure to increase or decrease by somewhat more than 1 mmHg in dehydration or hydration respectively from a normal control level of about 9 mmHg. With further increase in IFV secondary to increased capillary filtration, the interstitial edmaprevention will depend on washout and/or dilution of interstitial proteins since IFP did not increase by more than 1 to 2 mmHg.