However, due to the shape of the lung and reflections off the chest wall, the wave of expansion and compression becomes focussed and amplified in the dorso-caudal lung.[31] The alternate expansion and compression at the microscopic level in adjacent areas of lung tissue creates shear stress and capillary disruption. The theory predicts that hemorrhage would be more severe on hard track surfaces, but it does not explain why EIPH can occur in horses during swimming exercise.