Distribution of red and white blood cells in alveolar walls

Dog lungs were perfused with blood and rapidly frozen with liquid Freon gas at various pulmonary artery and venous pressures. The numbers of red and white blood cells per mm-2 of alveolar wall were counted in lung sections and, in addition, the proportion (by area) of the wall occupied by the cells was measured by point counting. The number and proportional area of the red blood cells rapidly increased as perfusing pressure was raised. These findings are consistent with earlier observations of capillary recruitment and distension. An unexpected observation was the large number of leukocytes in the capillaries especially at low perfusing pressures. For example when arterial exceeded alveolar pressure by 5 cmH2O (as occurs near the apex of the upright human lung), there were about 5,000 red cells and 4,000 white cells per mm-2 of alveolar wall. As perfusing pressure was increased, the number of leukocytes paradoxically decreased in zone 3 but remained constant in zone 2. Most of the white cells were mononuclear cells. These results suggest that the lung behaves as a mechanical sieve for large cells and that the number of trapped cells depends on the capillary pressure.