The Only Part of Your Body That Breathes Air Directly

The cornea is the only part of the human body that does not receive oxygen from the blood. To keep your vision perfectly clear, this tissue must remain completely transparent, which means it cannot have any blood vessels blocking the view.

Instead, the cornea breathes. It absorbs oxygen directly from the air that touches your eyes. The oxygen dissolves in your tears and then diffuses into the corneal cells to keep them alive. When you sleep, your eyes get less oxygen from the thin layer of blood vessels on the inside of your eyelids, which is why your eyes might feel 'puffy' or dry in the morning.

This lack of blood flow makes the cornea incredibly fast at absorbing certain nutrients but very slow to heal from deep injuries. It is packed with more nerve endings than any other part of your body—nearly 400 times the density of your skin—which is why even a tiny speck of dust feels like a massive intrusion.

Contact lenses can actually 'su f focate' the eye if they aren't breathable. Modern lenses are designed with tiny pores to allow oxygen to pass through, but wearing old-style lenses for too long can cause the eye to grow emergency blood vessels into the cornea, which can eventually ruin your vision.

Because there are no blood vessels, the cornea is also one of the easiest organs to transplant; the body is less likely to 'see' the new tissue and reject it because the immune system's traditional pathways aren't there.