Interstitial fluid pressure (Pif) (mmHg)
FIGURE 3 Relation between the interstitial fluid pressure and interstitial fluid volume. Note that small increments in interstitial volume elicit large increases in interstitial pressure in the low-compliance region, whereas much larger increases in volume are required to elevate pressure further when interstitial compliance is high.
subatmospheric values measured in lung, skeletal muscle, and subcutaneous tissue, whereas positive values (1-3 mm Hg) are obtained in liver, kidney, and intestine. These resting values of Pif can increase or decrease dramatically if fluid is added to or removed from the interstitial compartment. The fact that adding small volumes of fluid into the normal interstitium causes large increases in Pif suggests that interstitial compliance is normally low (Fig. 3). This allows for some control of the capillary filtration rate because the sudden increases in Jv that accompany a rapid rise in Pc can be offset by a simultaneous rise in Pif. However, if there is excessive accumulation of fluid in the inter-stitium, the interstitial matrix components are disrupted and interstitial compliance rises. With an elevated compliance in the interstitium, large volumes of fluid can accumulate without major changes in Pif. Thus, the capacity for increases in Pif to oppose further capillary filtration is virtually lost when the compliance of the interstitial compartment increases greatly above normal.
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