The horses hooves are extremely complex structures, very sensitive to stress and pressure and with an excellent blood and nerve supply. On the outside and underneath, they are protected by horn (a form of modified, hardened skin) which grows down from the coronet band, a fleshy ridge around the top of the hoof, equivalent to the cuticle on human nails. Inside the hoof, the horny outer structures are tightly bonded to the sensitive ones by means of leaves of horn and flesh (called laminae) which interlock around the wall of the hoof. The sensitive structures themselves surround the bones of the foot.
When weight is put on the foot it flattens and expands slightly, squashing the sensitive tissues and their blood vessels between the horn outside and the bones inside. The blood is squeezed up the leg into the veins, which have valves stopping the blood running back again. When the weight is removed, fresh blood rushes back into the tiny vessels (called capillaries) and so the process goes on.
It was thought until very recently that it was almost entirely pressure on the frog which pumped the blood around like this, but recent research has shown that, although the frog plays a part, it is the expansion of the whole foot which is important. The frog, together with the plantar cushion inside the heels, mainly helps reduce concussion on the foot.
The Need For Shoes
The hoof horn grows all the time but is worn away very quickly in a horse working on a hard surface. Horses are shod with metal shoes to prevent them from becoming footsore, but this prevents the horn from being worn down, so the farrier has to trim away excess horn at each shoeing before refitting or replacing the shoes (approximately every 4-8 weeks, depending on the rate of wear or growth).
It takes a horse an average of six months to grow a complete new hoof. Existing horn quality cannot be improved. However, a new horn can be improved by diet containing methionine, biotin, and other substances of which your vet can advise you on.