NCLEX Review Guide: Anatomy and Physiology of the Eye

External Eye Structures

• Conjunctiva: Thin, transparent membrane covering the sclera and inner eyelids that produces mucus to lubricate the eye and contains blood vessels for nourishment.
• Cornea: Clear, dome-shaped front layer that provides 65-75% of the eye's focusing power and has no blood supply, receiving oxygen directly from tears and aqueous humor.
• Sclera: White, fibrous outer layer that maintains eye shape and provides attachment points for extraocular muscles responsible for eye movement.
• Iris: Colored portion containing smooth muscle that controls pupil size through constriction (miosis) and dilation (mydriasis) to regulate light entry.

Internal Eye Structures

• Lens: Transparent, biconvex structure that changes shape (accommodation) to focus light on the retina, becoming less flexible with age causing presbyopia.
• Vitreous Humor: Clear, gel-like substance filling the posterior chamber that maintains eye shape and allows light transmission to the retina.
• Retina: Light-sensitive inner layer containing photoreceptors (rods for dim light/peripheral vision, cones for color/central vision) that convert light to electrical impulses.
• Optic Nerve: Cranial nerve II that transmits visual information from retina to brain, with the optic disc being the "blind spot" where it exits the eye.

Vision Process

1. Light enters through cornea and is initially focused
2. Iris adjusts pupil size to control light amount
3. Lens fine-tunes focus through accommodation
4. Light passes through vitreous humor to reach retina
5. Photoreceptors convert light to nerve impulses
6. Optic nerve transmits signals to visual cortex

Aqueous Humor Dynamics

• Production: Continuously produced by ciliary body at 2-3 mL/hour, providing nutrients to avascular structures like lens and cornea.
• Circulation: Flows from posterior chamber through pupil to anterior chamber, then drains through trabecular meshwork into Schlemm's canal.
• Normal intraocular pressure (IOP) ranges 10-21 mmHg - elevated pressure can damage optic nerve causing glaucoma.

Age-Related Changes

• Presbyopia: Lens becomes less flexible after age 40, making near vision difficult and requiring reading glasses or bifocals.
• Decreased tear production: Results in dry eyes, increased infection risk, and corneal damage if severe.
• Pupil changes: Smaller pupils and slower light adaptation make older adults more prone to falls in dim lighting.

Common Pitfalls

• Don't confuse miosis (constriction) with mydriasis (dilation) - remember "My-driasis = MY pupils are BIG"
• Remember the cornea has no blood supply - this affects healing and medication absorption
• IOP above 21 mmHg is abnormal but not all patients with high pressure develop glaucoma