 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 vessel walls.