Pressure‐Volume Characteristics of the Interstitial Fluid Space in the Skeletal Muscle of the Cat

Pressure‐volume characteristics of the interstitial fluid space in cat skeletal muscle were determined by plethysmographic recordings of the tissue volume changes following net capillary filtration or absorption. The concomitant changes in interstitial fluid pressure (P if ) were assessed by estimating the isovolumetric capillray pressure (P ci ). In resting muscle P ci averaged 13.7 mni Hg during perfusion with blood, having an average colloid osmotic pressure of 18.3 mm Hg. As there is no net fluid exchange, these findings indicate a balancing filtration iorce of 4.6 mm Hg which is exerted chiefly by the colloid osmotic pressure of the tissue fluid (π if ), with the possible contribution of a slightly subatmospheric P if . A reduction of tissue fluid volume of 30 % led to a reduction of Pci of approximately 4 mm Hg, while doubling the volume raised P ci by 3–4 mm Hg. Since these P ci changes must be partly ascribed to alterations in π if , the actual changes in Pif were even smaller. The compliance of the interstitial fluid space in skeletal muscle is therefore high (above 1.4 ml/100 g tissue × mm Hg) within the normal range of P if . The large extravascular fluid dept in skeletal muscle is thus easily mobilized to replenish the circulatory system when capillary pressure falls. as after a blood loss.