Blood Viscosity and Bleeds in Racehorses

In 2007, veterinary researchers published a scientific study on the role of blood viscosity in EIPH (exercise-induced pulmonary hemorrhage or “bleeds”). They wrote:

During acute and intense training, a series of cardiovascular modifications arise, leading the system to the edge of its response capacity and a significant percentage of horses shows different levels of pulmonary bleeding (produced by exercise [EIPH]), which would be due to two factors: high blood pressure and viscosity. The rise in blood pressure can lead, in some animals, to rupture of lung capillaries.

As it has been previously stated, during exercise, splenic contraction triggered an outstanding raise in hematocrit, which in some animals surpassed the 60%, increasing blood viscosity over that proportion.

…these results can be analyzed in relation to the oxygen transport capacity of blood, which rises by the increase in hematocrit during exercise. However, at higher hematocrit, higher as well is blood viscosity, that augments flow resistance, thus hindering oxygen transportation.

Catalani G, Dottavio ME, Rasia M. Acute training in racing horses at two different levels of effort: A haemorheological analysis. Clin Hemorheol Microcirc 2007; 37:245-52.