Regulation of extracellular volume and interstitial fluid pressure in rat bone marrow

The volume and fluid pressure characteristics of the intact bone marrow is incompletely understood. We used microspheres and lipoproteins for measurem ents of intravascular volume (IVV) and EDTA for interstitial fluid volume ( IFV) within the rat bone marrow. Interstitial fluid pressure (IFP) was dete rmined with micropipettes connected to a servo-controlled counter-pressure system. Both the microspheres and the lipoproteins yielded estimates of IVV of similar to1 ml/100 g. After a brief reactive hyperemia, IVV increased t o 2.5 ml/100 g, whereas IFV decreased with similar to1.5 ml/100 g, so that total extracellular volume did not change. Baseline bone marrow IFP was 9.7 mmHg. The hyperemia led to a transient twofold increase in IFP, whereas a marked blood loss decreased IFP by almost one-half. These novel data sugges t that extracellular volume and IFP within the bone marrow can be measured with tracer methods and the micropuncture technique. The responses of IVV, IFV, and IFP during changes in blood flow to the bone marrow suggest a tigh t regulation and are thus compatible with those for a low-compliant tissue.