The choroid lies between the sclera and the cornea and extends across the entire posterior part of the eyeball. It has a high density of blood vessels and supplies the retina with nutrients and oxygen.
In the anterior part of the eye, the choroid continues into the ciliary body. In addition, the choroid is responsible for regulating the retina’s temperature and absorbs stray light that would otherwise impair vision. The anterior part of the choroid is even involved in accommodation of the lens.
The choroid is the most richly perfused tissue in the eye and is therefore responsible for the red pupils that appear in flash photographs.