Pulmonary Circulation and Transcapillary Exchange of Electrolytes

¹ From the Radioisotope Service, Veterans Administration Hospital, Bronx, New York

The transcapillary exchange of Na²⁴, K⁴², iodide-¹³¹ and P³²-labeled phosphate during the first circulation through the lungs was studied in normal subjects, in patients with congestive heart failure and in anemic patients. Arterial dilution curves were obtained using P³²-labeled red cells or I¹³¹ albumin as the reference material compared with simultaneously injected tracer electrolytes. Employing labeled red blood cells as a reference standard, there was no evidence for a significant extravascular leak of albumin during the initial passage through the cardiopulmonary pool. However, the mean transit time of P³² red cells through the cardiopulmonary pool was observed consistently to be faster than that for albumin-I¹³¹. In normal subjects, 5–20% of the various tracer electrolytes passes out of the pulmonary circulation during a single transit through the cardiopulmonary pool. Loss of electrolyte from the pulmonary vascular bed is negligible in moderately severe anemia but exaggerated in congestive heart failure. These differences appear to be correlated with differences in the pulmonary circulation times in these conditions. Measurements of the Na²⁴ ‘space’ and sucrose-C¹⁴ ‘space’ of lung tissue indicate that the limited transcapillary exchange of electrolytes in the lung of man is not attributable to a small extracellular volume of distribution. It appears as if the pulmonary capillaries may be less permeable to several small anionic and cationic species than are capillaries of skeletal muscle and certain other viscera.