system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some parts of the eyes not need blood vessels?
gpt
The cornea and lens both lack blood vessels, since they need to be clear for good vision.

The cornea (front window of your eye) is only about 0.5 mm thick, enabling it to get the oxygen it needs via diffusion (the front half gets oxygen from the outside air, and dissolved O2 in the tear film. The back half gets its O2 from dissolved O2 in the aqueous humor).

The lens is not nearly as metabolically active as the cornea, so its oxygen needs are much lower. The oxygen it needs, it gets via diffusion from the aqueous humor (eye fluid).