Allometric scaling of vascular resistance, vessel dimensions, and blood volume based on body surface area and height accounts for age-dependent shifts in circulation, including a transition from upper-body-dominant blood flow in infancy to lower-body-dominant blood flow in adulthood.

As a person grows from infancy to adulthood, their blood vessels get longer and wider in proportion to their height and body size, while the heart becomes stronger and larger. Blood volume is higher per pound in babies, and more of it flows to the head and arms. As the body grows, the legs and trunk develop more muscle and tissue that need more blood, so the vessels in the lower body widen more than those in the upper body, and the heart pumps more blood downward. At the same time, arteries stiffen with age, which changes how blood moves through the body and helps sustain higher pressure in the lower half.