or quantify hemorrhage is not useful due to general sequestration of labeled RBC by the lung, even in the absence of hemorrhage. Post-mortem examination EIPH seldom causes the death of a horse, but if an affected horse dies and undergoes a post-mortem examination, repeated episodes of EIPH will have caused a characteristic blue-gray-brown staining in the lungs. The staining is due to the presence of hemosiderin. The staining is usually most intense in the dorso-caudal region of the left and right diaphragmatic lobes which often progresses cranioventral with repetitive damage. There are often distinct borders between healthy lung tissue and those parts of the lungs that have been affected by EIPH. Other histopathologic findings include fibrosis, bronchial artery neovascularization, venous remodeling, bronchiolitis, hemosiderin accumulation, increased tissue cellularity (i.e. hemosiderophages), multifocal areas of inflammation, and increased thickness of vascular and airway walls. Causes EIPH occurs when blood enters the air passages of a horse's lung, due to fractured capillaries. 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. EIPH begins in the dorso-caudal region of the lung and progresses in a cranioventral direction over time. High pulmonary blood pressures The most widely accepted theory is that high transmural pressures lead to pulmonary capillary stress failure. There may be contributions from the bronchial circulation. Pulmonary capillary transmural pressure is determined by pulmonary capillary pressure and airway pressure. The horse has very high pulmonary vascular pressures during intense exercise, exceeding 100 mmHg in the pulmonary artery during intense exercise. During expiration the high positive pressures in the pulmonary blood vessels pushing out are opposed by high positive airway pressures pushing back and this does not place undue stress on the thin blood