Aqueous humor is a clear, water-like fluid located in the front of the eye, between the lens and cornea. It is similar to blood plasma but contains lower protein concentrations. This fluid plays an important role in maintaining the eye’s internal pressure and overall structure. Continuous production and drainage ensure a balanced internal environment.
The Ciliary Body: The Source of Aqueous Humor
The ciliary body is the primary structure responsible for producing aqueous humor. This ring-shaped tissue is located directly behind the iris, encircling the lens. It forms part of the uveal tract, which also includes the iris and choroid. The ciliary body contains specialized non-pigmented ciliary epithelial cells within its ciliary processes that form this fluid.
How Aqueous Humor is Formed
The formation of aqueous humor by the ciliary body involves three mechanisms: active secretion, ultrafiltration, and diffusion. Active secretion is the most significant contributor, accounting for 80% to 90% of the aqueous humor produced. This energy-dependent process involves the active transport of ions like sodium, chloride, and bicarbonate across the non-pigmented ciliary epithelial cells. Water then passively follows these ions due to osmotic gradients, moving into the posterior chamber.
Ultrafiltration also plays a role, involving the pressure-driven movement of fluid from the capillaries within the ciliary processes into the surrounding tissue. This passive process contributes a smaller percentage to the overall fluid volume. Diffusion allows for the passive movement of some substances down their concentration gradients across the ciliary epithelium. These combined mechanisms ensure the unique composition of aqueous humor, which is distinct from plasma.
Journey and Exit: Aqueous Humor’s Circulation and Drainage
Once produced, aqueous humor circulates within the eye. It initially flows from the ciliary body into the posterior chamber, the narrow space between the iris and the lens. From there, the fluid moves through the pupil, the opening in the center of the iris, to enter the anterior chamber. This larger space is located between the iris and the inner surface of the cornea.
The primary drainage pathway for aqueous humor is through the trabecular meshwork, a spongy tissue at the angle where the iris and cornea meet. This meshwork acts as a filter, allowing the fluid to pass into Schlemm’s canal, a circular vessel that encircles the cornea. From Schlemm’s canal, the aqueous humor drains into a network of collector channels and eventually into the episcleral veins, rejoining the bloodstream. A secondary drainage route, the uveoscleral pathway, allows a small amount of fluid to pass through the ciliary muscle and into the suprachoroidal space.
Aqueous Humor’s Important Role in Eye Health
Aqueous humor performs several important functions for eye health. It helps maintain intraocular pressure (IOP). The continuous balance between its production and drainage is necessary for preserving the eye’s spherical shape and ensuring proper optical function. Normal intraocular pressure ranges between 10 to 21 mmHg.
The fluid also serves as a transport medium, delivering nutrients like glucose, amino acids, and oxygen to avascular structures of the eye, such as the cornea and lens, which lack their own blood supply. Aqueous humor efficiently removes metabolic waste products from these tissues. An imbalance in its production or drainage can lead to elevated intraocular pressure, which can damage the optic nerve and contribute to conditions like glaucoma.