Ensuring blood circulation to a brain 20ft (6m) above the ground requires a particular adaptation of the giraffe, including an extremely powerful heart and a blood pressure twice higher than in humans. When the giraffe lowers its head to drink, its head, and so its brain, falls of 13 to 16ft (4 to 5 m). The blood flow is regulated. A network of blood capillaries, assisted by valves in the giraffe's neck, constricts and dilates to let go through the necessary blood to the brain only.