Demonstration of pulmonary vascular perfusion by electron and light microscopy

To estimate the fraction of dense pulmonary capillary network that is perfused under physiological conditions, we developed a new method for the demonstration of in vivo capillary perfusion by light and electron microscopy. Blood plasma was labeled by 8-nm colloidal gold particles coated with rabbit serum albumin. In anesthetized rabbits, 4-5 ml of this tracer were injected into the right atrium. Two and 15 min later, the circulation was interrupted by a snare around the heart, and the lung was fixed by instillation with glutaraldehyde. Gold particles were found in the plasma space of alveolar capillaries as well as in other organs. A random sample of thin sections studied by electron microscopy revealed that the entire capillary bed of the lung was perfused at least with plasma within 2 min after tracer infusion. Light microscopy of silver-enhanced sections showed areas with different staining intensities but no obviously unperfused capillaries. The concept of capillary recruitment, which would require a significant fraction of capillaries unperfused at rest, may have to be reassessed to consider time factors as well as the two-phase nature of blood; red blood cells and plasma may take different paths.