Your blood vessel system is enormous. Arteries are the big highways carrying blood away from your heart. Veins are the return highways bringing it back. Both are visible - you’ve seen them. But between the two, your body has a vast network of microscopic vessels called capillaries, where the actual exchange of oxygen and waste happens between blood and your cells.
Capillaries are tiny. The smallest are about 5 micrometers wide - thinner than a single human hair. They’re so narrow that red blood cells, normally round and floppy, have to squish into a long flexible shape to slip through one at a time. The single-cell-thick walls of capillaries let oxygen and nutrients pass out into the surrounding tissue, and let waste products pass back in.
You have an estimated 25 billion capillaries in your body. Laid end to end, they’d stretch around 60,000 miles - long enough to wrap around the Earth two and a half times. They reach almost every cell in your body. No matter where you point, you’re never more than a few cells away from a capillary delivering you fresh oxygen.
