The cornea stays clear by going without blood vessels
The cornea is the transparent, dome-shaped surface at the front of the eye. It bends incoming light so the eye can focus it, and that job depends on keeping the tissue clear. Blood vessels would carry useful oxygen, but they would also scatter light across the viewing surface.
Instead, the cornea has no blood vessels. When your eye is open, oxygen from the air dissolves into the thin tear film covering it, then diffuses through that film into the cornea. Diffusion is the movement of molecules from an area where they are more concentrated to one where they are less concentrated.
A clear surface has to manage a supply tradeoff
Going without vessels means the cornea must get oxygen another way. It also receives nutrients from the fluid inside the eye and from nearby tissues. When the eyelids close, oxygen from the air is cut off, so the cornea gets less oxygen; the eye’s other sources help support it during that time.
Contact lenses introduce another part of the tradeoff. A lens sits over the cornea and can reduce how readily oxygen reaches it, which is why lenses are designed to let oxygen pass through and should be worn as directed. The same surface that makes a sharp view possible is therefore also unusually dependent on the air reaching its tear-covered face.
