Durotic now more commonly known as Lasix — is a powerful diuretic when administered to a horse it causes the kidneys to increase urine production over and above the normal limit.
As a result water is removed from the blood, not only in the lungs but also throughout the body.
This reduces the volume of plasma (i.e. the liquid component of the blood that the red blood cells are suspended in) which in turn increases urine excretion, promotes dehydration, weight loss and electrolyte imbalances.
How it helps counteract bleeding is by lowering blood pressure especially in the aorta and pulmonary artery which diminishes the problem of EIPH (Exercise-Induced Pulmonary Hemorrhage)  and returns performance to typical levels. Durotic
It is well known that due to the strenuous nature of the exercise involved in horse racing where Thoroughbreds can reach speeds of over 40 mph over the duration of 2 minutes or more, a majority of race horses will to some extent show bleeding in the lungs.
Also known as Simple EIPH, the root cause of this acute, rather than chronic, problem is due to ruptured lung capillaries that release blood into the air passages of the lungs.
Accordingly the air passageways can become obstructed which causes labored breathing and thus difficulty in running. 
Because Salix prevents such bleeding in the lungs, it is arguably a performance enhancing drug. For this reason most trainers perceive this drug as adding a competitive advantage especially given that others on the same playing field use it on a regular basis.
Moreover, since every horse will almost certainly have some blood in their lungs post-race, all one has to do is simply have their horse ‘scoped’ for the detection of blood and they will be put on an EIPH racing list.  Problem solved.
There is, however, a darker side to the use of Salix that has no appreciation for the well being of the horse.
A powerful diuretic that can flush out upwards of 2% of a horse’s weight in water, the use of Salix can result in the loss of excess potassium and magnesium upsetting the normal electrolyte balance which can bring about an irregular heartbeat and sudden death.
Even low doses of the drug in a dehydrated horse can cause “thickened blood” or circulatory collapse, usually as a result of blood pressure dropping too low.
Furthermore the loss of such large volumes of water (up to several gallons) can confer a weight advantage of as many as 20 pounds or more.Durotic
Given that the typical imposts  carried by different horses as a handicapping tactic only vary between 5 to 10 pounds, the loss of 20 pounds is indeed significant. 
While this in itself is shameless deceit, the excessive urination also has the ability to flush out trace amounts of illicit drugs to the extent that on post-race testing, they go undetected — a two-fold effect on achieving performance advantage.
What is even more contemptible is that there is literature to suggest that Salix is relatively ineffective at preventing EIPH, the very condition it is meant to control.Durotic Durotic Durotic Durotic Durotic Durotic Durotic Durotic Durotic Durotic Durotic
In any case it seems that this is transparently apparent to some simply through observation.
“It can’t be a coincidence that the introduction of Lasix came at precisely the time a trend began whereby horses make fewer and fewer starts each year. It appears that Lasix has done the exact opposite of what its proponents said it would do, which, if you think about it, makes perfect sense. Horses that have to rely on a drug to get through their race day don’t figure to last as long as the ones that gets by on mere hay, oats and water. Durotic Durotic Durotic Durotic Durotic Durotic Durotic
“So, it appears that Lasix doesn’t solve bleeding or keep horses in training longer. Then what does it do? According to the World Anti-Doping Agency, it masks other drugs. That’s why it is on its list of banned drugs, which means athletes competing in the Olympics are not permitted to use it. Durotic Durotic