Pulmonary capillary filtration and reflection coefficients in the newborn rabbit

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 σ_inulin = 0.04 and σ_albumin = 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.