Effect of concentration and hydration on restriction of albumin by lung interstitium

In previous studies, the flow of albumin solution through hydrated lung interstitial segments was higher than a prior flow of Ringer solution (A. Tajaddinni et al., 1994, J. Appl. Physiol. 76, 578-583). We wondered whether this effect was caused by an increased pore size. We measured the flow of albumin solutions through interstitial segments subjected to a driving pressure of 5 cm H(2)O and various mean interstitial pressures (P(if)). The ratio of albumin concentration (C(alb)) of the output solution to that of the input solution (C(out)/C(in), sieving ratio) was measured using tracer (125)I-albumin. At normal hydration (0 cm H(2)O P(if)), C(out)/C(in) was minimal (0.6) with the flow of Ringer solution, increased to 0.8 with the flow of 5 g/dl albumin solution, and increased to 1 with increased hydration at 15 cm H(2)O P(if). We modeled the interstitium as a membrane subjected to flows of high Peclet numbers. Accordingly, the albumin reflection coefficient [sigma = 1 - (C(out)/C(in))] at 0 cm H(2)O P(if) was 0.4 with the flow of Ringer solution and decreased to 0 at 5 g/dl C(alb) and 15 cm H(2)O P(if). This behavior suggests that the flow of albumin occurred through interstitial pores that increased in size as either C(alb) or hydration increased. We conceive of an interstitium that consists of pores with permeable moveable walls across which osmotic interaction occurs between the pore liquid and the surrounding tissue.