"Blood shift" is a term used when blood flow to the extremities is redistributed to the head and torso during a breath-hold dive. Peripheral vasoconstriction occurs during submersion by resistance vessels limiting blood flow to muscles, skin, and viscera, regions which are "hypoxia-tolerant", thereby preserving oxygenated blood for the heart, lungs, and brain. The increased resistance to peripheral blood flow raises the blood pressure, which is compensated by bradycardia, conditions which are accentuated by cold water. Aquatic mammals have blood volume that is some three times larger per mass than in humans, a difference augmented by considerably more oxygen bound to hemoglobin and myoglobin of diving mammals, enabling prolongation of submersion after capillary blood flow in peripheral organs is minimized.