Blood Vessels Could Circle Earth Twice

The sheer scale of the body's internal transportation network (Review) is almost incomprehensible. This vast system of arteries, veins, and capillaries has a singular, vital mission: to deliver oxygen and nutrients to every one of the trillions of cells that make you, you. To ensure no cell is left behind, this vascular highway must be incredibly dense, weaving through every organ and tissue from the surface of your skin to the core of your bones. It's this biological need for total coverage that results in its astonishing total length.

The overwhelming majority of this distance is not from the large arteries or veins we can easily see, but from the countless capillaries. These microscopic vessels are the true workhorses of the circulatory system, connecting the larger arterial roads to the venous ones. Many capillaries are narrower than a single human hair, forcing red blood cells to deform and squeeze through in single file. This intimate, slow-moving procession is by design, as it maximizes the time for oxygen to be delivered and waste products to be removed at the cellular level.

This complete map of our circulation wasn't always known. For centuries, physicians believed blood ebbed and flowed like the tides. It wasn't until the 17th century that William Harvey correctly described the circulatory system as a closed loop, but