Anesthetized rats were infused with lactated Ringer solution (LR) at constant rate for 30 or 60 min; delivered volume loads ranged from 0.03 to 0.08 ml/g body wt. Controls were given only a sustaining infusion of saline at 0.002 ml.g-1.h-1. Only 7-14% of the LR remained in the plasma at the end of the infusion; 76-88% entered the interstitial compartment, and 7-17% was excreted. The amount of plasma protein lost from the circulation with the extravasated fluid was studied simultaneously by two methods: 1) material balance in the whole animal and 2) changes in 131I-labeled albumin uptake (VA) and water content (VW) in individual tissues. The extravasation of 0.03-0.06 ml fluid/g body wt (75-160% initial plasma volume) did not significantly increase plasma protein extravasation in the whole rat. Nearly all of the sampled tissues of LR-infused rats had higher VW than controls. Tissue VA tended to increase with VW, but the regression slopes (delta VA/delta VW), a measure of the tracer albumin concentration of capillary filtrate relative to plasma, were low; skin, 0.006; paw, 0.018; skeletal muscles, 0.007; heart, 0.057; jejunum, 0.095; ileum, 0.045; cecum, 0.026; and colon, 0.027. These ratios are consistent with the very small loss of total plasma protein observed and attest to high solvent-drag reflection coefficients (sigma approximately equal to 1 - delta VA/delta VW): greater than 0.98 in capillaries of skeletal muscles, skin, and paw and 0.91-0.97 in heart and intestine.
Extravasation of plasma proteins is increased after volume expansion with whole blood or plasma. To investigate the mechanisms responsible for this phenomenon, we measured extravascular accumulation of exogenous 131I-labeled bovine serum albumin in several tissues and organs of anesthetized rats. Plasma volume was increased acutely by infusion of isoncotic albumin or polyvinylpyrrolidone, with or without subsequent infusion of a 1:10 dilution of the colloid to induce blood-to-tissue fluid movement. Controls were given only a slow sustaining infusion of saline. The amounts of fluid and plasma protein lost from the circulation were followed simultaneously by two methods: 1) material balance in the whole animal, and 2) changes in 131I-labeled albumin uptake (VA) and water content (VW) in the individual tissues. Plasma volume expansion of 80-90% increased plasma protein extravasation in the whole rat by an average of 2.7-fold over a 30-min period. Of the protein extravasated, 42% entered the abdominal cavity. The rest was distributed in the interstitial compartment of various tissues and organs. Tracer albumin accumulation (averaged over 30 min) was increased 38-82% in skin and paw, 40-59% in skeletal muscles, 131% in hearts, and 167-230% in different parts of the intestine. Increased convective transport does not appear to be a major factor. There was little or no relation of albumin transport increase to the magnitude or direction of net fluid transfer. Coupling of albumin transport to volume flow was not greater than previously reported for saline infusion or venous congestion. Convective redistribution (convective transport without net fluid transfer, "volume recirculation") is estimated to increase albumin transport no more than 10% under the conditions of our experiments. The greater part of the increase is thus dissipative, i.e., attributable to increased diffusion or increased vesicular exchange. Control of dissipative transport of albumin may play an important role in regulating plasma volume.
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