In an isolated, Ringer-perfused lung preparation, we measured values of the filtration coefficient and reflection coefficients of NaCl, mannitol, sucrose, raffinose, inulin, and albumin for the pulmonary capillaries of newborn rabbits. We found the capillary filtration coefficient value to be 8.3 × 10-5 cm3 sec-1 cm H2O-1, which, when normalized for the estimated exchange area, is at least a factor of 2 larger than the value found for adult rabbit lungs. Using the osmotic transient technique we measured the osmotic flow induced by adding test molecules to the perfusate. NaCl, mannitol, sucrose, and raffinose were found to have equivalent effects. Assuming these molecules only draw fluid from cells, and not across the capillary endothelium (capillary σ = 0) we estimate that σinulin = 0.04 and σalbumin = 0.07. These results indicate that the pulmonary capillaries of newborn rabbits are less selective ("leakier") than those of adult rabbits. We estimate an equivalent pore radius of 150-300 Å for the newborn pulmonary capillary endothelium, although the data are not consistent with a uniformly sized population of pores.
Bibliographical noteFunding Information:
We wish to acknowledget he valuable technicala ssistanceo f Randy Ewald, Martin Vick, and Paul Savaryn. This work was supportedi n part by a Grant-in-Aid from the University of Minnesota Graduate School, and in part by the U.S. Public Health Service (NIH Grant HD 08981).
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