 A variety of causes have been proposed, but EIPH is most likely a multi-factorial condition, involving airway, vascular, inflammatory, blood, cardiac, locomotory, and remodelling components.The primary mechanism is likely to be high pulmonary vascular pressures with concurrent negative airway pressures, causing extreme stress across the pulmonary capillary membrane (the fragile membrane separating blood in the pulmonary capillaries from the air-filled alveoli) and consequent hemorrhage into the air spaces of the lung.Other contributing factors may include upper airway obstruction, increased blood viscosity, abnormalities of cardiac origin (small cross-sectional area of atrioventricular valves, stiff valves, slow left ventricular relaxation time, right tricuspid valve regurgitation), preferential distribution of blood flow to the dorsocaudal lung regions, mechanical trauma, lower airway obstruction, inflammation, abnormalities of blood coagulation, inhomogeneity of ventilation and locomotory trauma.