The effect of donor age on the flow properties of blood. Part I: Plasma and whole blood viscosity in adult males.

The relationship between blood viscosity and age was studied using heparinized blood samples obtained from 50 normal male blood donors between the ages of 20 and 65 years. There was a slight but significant decline in packed cell volume (hematocrit) with age. Plasma viscosity showed no significant variation with donor age, but the viscosity of blood samples standardized to a packed cell volume of 45 ml/dl showed an increase as the age of the donor rose. The age-related trend to a higher viscosity was present at shear rates below 46 s-1, but not at higher shear rates. This shows that with age there is a rising trend in the red cell contribution to blood viscosity, which is not dependent on the packed red cell volume; this component increases across a wide age range in early and middle adult life, and is not a characteristic appearing exclusively in later age periods. The tendency for the viscosity trend to be greater at lower shear rates indicates increased shear thinning in blood obtained from older subjects, the cause of which may be either diminished red cell deformability in these subjects, or an increased tendency to form aggregates at low shear rates.