Pulmonary interstitial compliance: a function of the osmotic constituents of the interstitium

We have attempted to dehydrate the lung interstitium to determine the nature of forces holding water in that compartment. We administered furosemide with and without bovine albumin intravenously to rabbits (n = 21) 18-24 h before they were anesthetized with pentobarbital sodium. Renal pedicels were ligated and 51Cr-labeled EDTA was injected to estimate lung interstitial water volume. After a period of equilibration the thorax was rapidly opened, and left atrial pressure was measured by direct puncture. 125I-labeled albumin was injected to label the lung vascular volume, and the rabbits were killed 3 min later. Lungs were removed and drained of blood, and extravascular water volume, interstitial volume, and dry weight were determined. Results from these rabbits were compared with a group of normal (n = 4) and overhydrated (n = 6) rabbits. We have found that lung interstitial water is removed in proportion to the change in intravascular forces. We estimate interstitial compliance to be 1.76% cmH2O-1. Our results are compatible with the hypothesis that removal of water is opposed by an increase in interstitial colloid osmotic pressure and not by a fall of hydrostatic pressure. This implies that in the normally hydrated state interstitial hydrostatic pressure is ambient.