Blood rheology and aging

Abstract

The flow properties of blood play significant roles in tissue perfusion by contributing to hydrodynamic resistance in blood vessels. These properties are influenced by pathophysiological processes, thereby increasing the clinical relevance of blood rheology information. There is well-established clinical evidence for impaired blood fluidity in humans of advanced age, including enhanced plasma and whole blood viscosity, impaired red blood cell (RBC) deformability and enhanced RBC aggregation. Increased plasma fibrinogen concentration is a common finding in many studies owing to the pro-inflammatory condition of aged individuals; this finding of increased fibrinogen concentration explains the higher plasma viscosity and RBC aggregation in elderly subjects. Enhanced oxidant stress in advanced age is also known to contribute to altered blood fluidity, with RBC deformability being an important determinant of blood viscosity. Several studies have shown that physical activity may improve the hemorheological picture in elderly subjects, yet well-designed observational and mechanistic studies are required to determine the specific effects of regular exercise on hemorheological parameters in healthy and older individuals.

Keywords: Aggregation; Deformability; Elderly; Erythrocyte; Red blood cell; Viscosity.

Figure 1.. The effect of shear rate on whole blood viscosity.

Shear rate is the velocity gradient between adjacent layers of blood, is expressed as 1/s and is proportional to flow rate in a tube (e.g., blood vessel). The lower shear rates correspond to the circulatory conditions in the venous circulation while higher shear rates characterize the arterial vessels. The unit of viscosity is mPa.s, numerically equal to centiPoise (cP); water has a viscosity of about 1 mPa.s at 25°C and is independent of shear rate.

Figure 2.. Red blood cell aggregates in autologous plasma.

Small round particles are blood platelets.

Figure 3.. The relationship between plasma fibrinogen concentration and the half-time for red blood cell (RBC) aggregation.

A decreased half-time means a faster rate of RBC aggregation. Data are values determined with blood from 25 donors aged 23–79 years. The solid line represents linear regression and the dashed lines indicate the associated 95% confidence intervals; the slope of the line is highly significant (P = 0.003).