Albumin permeability in the pulmonary capillaries

At any given hydrostatic pressure, one of the critical factors controlling water flux across the pulmonary capillary membrane is the permeability of that membrane to serum albumin. Because the adult respiratory distress syndrome (ARDS) may be associated with an abnormality of pulmonary water, a definition of albumin kinetics across the normal pulmonary capillaries and of the normal pulmonary lymph flow is appropriate to a better understanding of the ARDS. In this article the results are reported of a study of the normal pulmonary lymph flow rate and of the normal pulmonary capillary permeability to albumin in vivo. It was found that the value for the apparent volume of pulmonary lymph (V(la)) is only 0.13 that of the interstitial fluid space (V(isp)), measured by indicator dilution curves. Using V(isp) as the pulmonary lymph volume results in a calculated pulmonary lymph flow 4 to 5 times greater than previously reported values. It is possible that this discrepancy is caused by a resorption of water from lymph to blood occurring in the lymph nodes traversed by pulmonary lymph prior to reaching the right thoracic duct (RTD), and that the total pulmonary flow rate (Q(p)) rather than the flow rate in the right thoracic lymph duct (Q(rtd)) approximates the actual value of pulmonary lymph flow. The calculated P value for albumin is approximately 5 fold greater than a reported value of 2.8 x 10^-9 cm/sec for the entire alveolocapillary membrane in vitro. This P value may be too large if the albumin concentration in pulmonary interstitial fluid is actually lower than in RTD lymph, or if there is a significant amount of nonpulmonary lymph, with a higher lymph/plasma Risa 125 (radioiodine¹²⁵ serum albumin) ratio present in RTD lymph.