Effects of racing and gender on viscoelastic properties of horse blood.

Splenic contraction in racing horses increases the hematocrit (hct), thereby increasing blood viscosity. We tested as to whether racing also affects the elastic properties of blood. Mares and geldings were studied for thus purpose. After racing, there was: (i) an increased erythrocyte count independent of gender and race distance (0.32 to 1.7 km): (ii) an increased mean erythrocyte volume in both sexes; (iii) an increased heterogeneity of RBC size in both sexes; (iv) an increased plasma fibrinogen concentration and erythrocyte sedimentation rate in both sexes; and (v) an increased elastic yield stress (EYS). When corrected to a constant hct of 65%, the blood of mares, but not geldings, had increased EYS after racing. Gender differences in fibrinogen response (p = 0.72) did not account for this and the mechanism is not known. Since EYS is analogous to the point at which ketchup starts to flow from a bottle, its increase could be deleterious in vascular beds characterized by pulsatile flow, e.g. the coronary circulation.