Effect of osmolality on red blood cell viscosity and transit through the lung

The mean transit times of labeled red blood cells and albumin were compared in eight isolated rabbit lungs perfused with physiological albumin solutions. The osmolality of these solutions was adjusted by altering the concentration of sodium chloride. The ratios of the mean transit times of injected red blood cells to those of albumin increased as perfusion osmolality increased from hypotonic to isotonic and from isotonic to hypertonic levels. This change occurred despite a decline in pulmonary vascular resistance and red blood cell size as osmolality was increased. Red blood cell viscosity (determined with a cone-plate viscometer) increased with osmolality and it was concluded that this change of viscosity impaired the relative rate of red blood cell transit through the lungs. Passage of red blood cells through rigid homoporous membranes appeared to be related primarily to red cell size rather than vascosity. These observations suggest that both red blood cell viscosity and capillary distensibility play an important role in determining the velocity of red blood cells through the capillaries.