Massive hemorrhage causes changes in morphometric parameters of lung capillaries and concentration of leukocytes in microvasculature.

The study examines the changes in morphometric lung parameters caused by a massive acute blood loss. Three groups of rats were compared: In one group, massive hemorrhage was elicited prior to instillation of the fixative; in the first reference group, standard instillation fixation was performed, and in the second, the circulation was abruptly stopped before instillation of the fixative by contracting a snare around the atrioventricular sulcus of the heart. The two reference groups showed identical morphometric data except for the capillary erythrocyte volume density and its derived parameters, since the standard lung fixation procedure leads to a certain hemoconcentration in the lung microvasculature, as described previously. In the hemorrhage group the capillary volume was reduced to 43% of the value of the snare group. Together with a decrease in morphometric hematocrit (from 0.43 to 0.29), this led to a reduction of the erythrocyte volume to 27%. The alveolar surface area remained unchanged, the capillary surface area showed a tendency to be smaller in the hemorrhage group. As a consequence, the morphometric estimate for the pulmonary oxygen diffusing capacity DLO2 was reduced to about 40% of the controls. A surprising finding was the high leukocyte content in the capillaries after hemorrhage, suggesting that the leukocytes are retained in the capillaries even after severe blood loss.