Why do horses bleed?

Racehorses are unique, in that they are one of very few animals known to suffer Exercise Induced Pulmonary Hemorrhage (EIPH) (more commonly referred to as bleeding), Professor Ken Hinchcliff, dean of veterinary and agricultural sciences at the University of Melbourne and co-author of a series of comprehensive studies in recent years looking into the effects of Lasix on racehorses, told the Guardian. One of the causes of EIPH is a four-fold increase in pulmonary blood pressure when horses exercise or compete.
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“If you take a horse out and exercise it, then the blood pressure leading from the artery on the right side of the heart to the lungs increases from about 25mm of mercury [pressure] to about 100mm of pressure,” he said. “You don’t see anything like this in humans, for example.”
-“A horse can pass between 10 to 15 liters of urine in the first hour after Furosemide is administered,” said Hinchcliff. “Whereas a horse throughout the normal course of the day would only produce about 10 to 15 liters of urine. So it markedly increases the rate of urine production.”
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I'm no doctor but you would think after one hour it would create a Hydrate problem,  Do they still have  to wait 4 hours
before a race with no water ???
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