Circulatory System
The horse's circulatory system includes the four-chambered heart, averaging 8.5 lb (3.9 kg) in weight, as well as the blood and blood vessels. Its main purpose is to circulate blood throughout the body to deliver oxygen and nutrients to tissues, and to remove waste from these tissues. The hoof (including the frog - the V shaped part on the bottom of the horses hoof) is a very important part of the circulatory system. As the horse puts weight onto the hoof, the hoof wall is pushed outwards and the frog compressed, driving blood out of the frog, the digital pad, and the laminae of the hoof. When weight is removed from the hoof, the release of pressure pulls blood back down into the foot again. This effectively creates an auxiliary blood-pumping system at the end of each leg. Some of this effect is lost when a horse is shod (eliminating the expansion and contraction of the hoof wall and raising the frog higher from the ground).
Read more about this topic: Equine Anatomy
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