The cornea cells of the eye are the only living cells in the body that are not directly supplied by blood vessels. Instead, oxygen and nutrients diffuse directly from the tear fluid on the outside and the aqueous fluid (the thick, watery material inside the eye between the lens and the cornea) together with the nerve fibres that are attached to the cornea.

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The reduction in external oxygen supply caused by contact lenses is one of its issues. This can result in corneal neovascularization, when blood vessels sprout into the cornea, even with gas-permeable lenses. The most recent silicone hydrogel lenses reduce or completely remove this issue. In addition to the cornea, the exterior layers of the skin, hair, nails, and tooth enamel are other bodily parts without blood veins.

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