Giraffes have an extremely powerful heart which is needed to pump the blood all the way up its long neck and legs. The very high blood pressure is up to double that of a human, around 280/180 mm Hg. The heart has evolved to have a small radius and thick muscle walls, allowing it to pump at about 170 beats a minute. A series of valves located in the blood vessels that lead up the neck prevent the blood from flowing back to the heart in between beats. The walls of the blood vessels also thicken as the neck grows longer with age, to avoid rupturing under pressure.