Transcapillary Pulmonary Exchange of Water in the Dog

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Abstract
An exptl. procedure was devised for the calculation of the pulmonary transcapillary exchange of the anaesthetized dog. Instantaneous injn. is made into the jugular vein of a soln. containing a reference substance (e.g., T.1824), assumed not to leave the blood stream, and the test substance (deuterium or tritium oxides). Blood is obtained from a catheter in the brachial or carotid artery at intervals of 0.5 to 2.0 seconds. There is negligible displacement of such substances as Na, p-aminohip-purate, and thiocyanate ions, urea and inulin with respect to the reference substance. With deuterium and tritium oxides there is significant displacement but no significant net loss. The results are interpreted as representing loss and return of the labelled water from the capillaries of the lungs. Calculations indicate that 50% of the water passing through the lungs crosses the capillary walls and returns. It is pointed out that conventional sampling techniques for the calculation of exchange rates cannot take this rapid exchange into account and that the results are compatible with the hypothesis that passage across capillary walls takes place by diffusion rather than by filtration.