Capillary filtration measurement by strain gauge. II. Effects of mannitol infusion.

Maintenance of normal partition of extracellular fluid into vascular and interstitial compartments is a tightly controlled system. One means of studying control of this system is to induce filtration by fluid loading, and to measure associated changes capillary filtration coefficients (CFC) and Starling pressures in a particular vascular bed. We therefore measured responses of the dog hindlimb to isotonic mannitol infusions. Using strain-gauge plethysmography to measure CFC, the Johnson zero-flow technique to measure isogravimetric capillary hydrostatic pressure, and chronically implanted capsules to measure muscular interstitial hydrostatic pressure, we infused isotonic mannitol into anesthetized dogs for 30 min at 20 ml/min and 30 ml/min. There was a sustained 75% increase in CFC in intact hindlimbs, no change in hydrostatic capillary or interstitial pressure, and a 4-5 mmHg rise in filtration pressure due to dilution of serum protein. The most likely mechanism for increased CFC is capillary recruitment, since distension of capillaries requires large increases in hydrostatic pressure. Interstitial fluid compliance appears high. These studies indicate capillary intermittency may be an important mechanism in the disposition of fluid loads.