Blood flow is introduced to the foot through the constant flexing of the hoof. When the leg flexes to take a step, veins are compressed at the back of the pastern. As the motion progresses and the strides completes, the vein is released. The pressure shoots the blood through the vein and through the foot. When a horse is on stall rest, laminitis is extremely common due to the decreased circulation through the foot.
I have a disection video from the Institute of Equine Podiatry which shows this very clearly. The disected leg is skinned and as the foot is physically bent back and forth to emulate a stride, you can clearly see veins and arteries being compressed and released accordingly. It’s long been thought that the frog is a blood pump, but this is not true. The frog is a mass of fatty tissue which has no signifigant veins or arteries. But rather the veins and arteries feeding the foot are located at the back of the pastern and travel down into the hoof capsule.
As the bloods flows into and out of the foot, toxins are cleaned from the tissues. When movement, and therefore circulation, are decreased, the toxins are allowed to build in the foot to a higher level. When a horse is on stall rest, the circulation to the hooves are minimal.