Mean red cell volume as a correlate of blood pressure. 
Clinical studies suggest that hypertensives have lower mean corpuscular volume (MCVs) than do normotensives.  Epidemiological studies show no relation or higher MCVs.  In the present study of elderly men (71 to 93 years of age) of the Honolulu Heart Program, elements of both findings are confirmed.  Three groups are identified: (1) those receiving no hypertension treatment, (2) those receiving treatment with any diuretic, and (3) those receiving treatment with nondiuretics only.  MCV is lower in group 3 than in group 1 (-0.85 fL, P<.001) but the same in groups 1 and 2.  Within groups 1 and 3, inverse relations of -0.22 and -0.09 mm Hg/fL (P<.05) are noted for systolic (SBP) and diastolic (DBP) blood pressures.  No relations are observed in group 2.  MCV and red blood cell count (RBC) are inversely correlated (r=-.45).  In group 2, adjustment for RBC unmasks a direct relation between MCV and SBP (0.5 mm Hg/fL, P=.02) and DBP (0.3 mm Hg/fL, P=.02).  In groups 1 and 3, relations between SBP and MCV are lost after adjustment for RBC (0.005 mm Hg/fL).  For DBP, adding RBC plus an MCV x RBC interaction is significant (P<.001).  DBP is 5 mm Hg greater in the highest RBC quartile than in the lowest.  A +3 mm Hg difference between extreme MCV quartiles is noted only at high RBC levels.  The relation between blood pressure and red cell measures is probably mediated by whole blood viscosity.  Hematocrit is a determinant of whole blood viscosity.  Viscosity affects peripheral resistance to blood flow, and peripheral resistance affects DBP.  At high RBC levels, MCV may be "downregulated."  This may lower whole blood viscosity and partially reduce DBP without compromising flow.