Editor’s Note: This excerpt is from Understanding Equine First Aid by Michael Ball, DVM. 

Blood is an essential component of the mechanism whereby oxygen is transferred from the lungs to all of the body’s organs and tissues. So just how much blood does a horse have anyway? It varies some from breed to breed, but an average value is 80 ml (cc) per kilogram of body weight (100 ml/kg for “hot bloods” such as the Thoroughbred and 65 ml/kg for “cold bloods” such as a Percheron). So, the average 1,200-pound horse (545.5 kilograms at 2.2 kilograms per pound) has about 54.5 liters of blood, which is approximately 12.3 gallons of blood.

Now that we know that the average horse has about 12 gallons of blood, how much can be lost before the danger of shock becomes significant? The general rule of thumb is that an animal will start to show signs of shock from blood loss when 10% of its blood volume has been lost. Based on the averages, the adult 1,200-pound horse can lose up to two gallons of blood before serious concern. Most bleeding from wounds appears much more copious than it actually is, but any time there is hemorrhage there should be an attempt to control it.

The use of pressure bandages and their application already has been discussed, but will be briefly reviewed here. Remember that the first aid kit should contain leg wrap material and elastic bandage material. If the wrap is too thick, it will decrease the ability to apply an adequate amount of pressure to control the bleeding effectively. The bandage should be applied in a tight, smooth manner and in such a way as to apply significant pressure directly over the wound (if possible). The signs of blood loss shock include weakness, whole body sweating, colic, progressively elevated heart rate, and pale/white mucous membranes