Aqueous humor
What is the aqueous humor?
Aqueous humor is the clear, watery fluid inside the front of the eye. It works to bring nutrients to the structures inside your eye and to maintain the eye’s normal pressure and shape.
The eye is constantly producing new aqueous, while old fluid drains from the eye through the trabecular meshwork. Having equal amounts of aqueous humor flowing in and out of the eye keeps the pressure of the eye balanced.
Aqueous humor anatomy
The aqueous humor is found in the anterior and posterior chambers of the eye.
The anterior chamber is located between the cornea (the clear, front dome of the eye) and the iris (the colored part of the eye). The posterior chamber lies between the iris and the front of the lens.
Aqueous humor consists almost entirely of water — 99.9% to be exact. The remaining 0.01% is made up of nutrients such as sugars, proteins and vitamins.
LEARN MORE: Eye anatomy: A closer look at parts of the eye
Aqueous humor function
Several eye structures located in the anterior and posterior chambers don’t have a blood supply. Instead, they rely on aqueous humor for nourishment.
The ciliary body, behind the iris, creates aqueous fluid from blood plasma. The fluid then passes through a layer of cells called the blood-aqueous barrier. It acts as a natural filter to ensure the aqueous only contains what these structures need.
Aqueous fluid also removes waste from the eye with its constant flow through the anterior and posterior chambers.
While providing nourishment and removing waste, aqueous humor is also responsible for maintaining proper, balanced pressure within the eye. This may seem like a simple task, but it’s incredibly important for eye health.
Imbalances in the creation and drainage of aqueous humor can lead to high intraocular pressure (IOP). High eye pressure is a main risk factor for glaucoma, a potentially sight-threatening condition.
For this reason, the aqueous is a key focus in ocular pharmacology (the study of eye medicine).
RELATED READING: Vitreous humor
Aqueous humor flow
The flow of aqueous humor begins in its production by the ciliary body, in the posterior chamber of the eye. From the ciliary body, aqueous humor flows through the pupil and into the anterior chamber.
Aqueous humor then leaves the anterior chamber through a structure called the drainage angle. Once inside the drainage angle, the aqueous fluid leaves the eye through a spongy tissue called the trabecular meshwork.
The trabecular meshwork moves the liquid into a drainage canal called Schlemm’s canal. From there, the aqueous humor then drains into the eye’s circulatory system.
Aqueous humor and glaucoma
Imbalances in the amount of aqueous production and aqueous outflow can lead to ocular hypertension, or high eye pressure. This can occur due to an overproduction of aqueous humor or due to insufficient drainage.
As mentioned earlier, aqueous humor leaves the eye through the drainage angle. It’s possible for the drainage angle to be “open” (unobstructed) or “closed” (narrow or blocked). The narrower the drainage angle becomes, the more difficult it is for the aqueous humor to drain.
When the drainage angle narrows to the point that aqueous can’t drain effectively, it results in higher eye pressure. This can lead to narrow-angle glaucoma. This type of glaucoma is rare and may occur gradually or suddenly (called acute angle closure).
It’s also possible to experience poor aqueous drainage even when the drainage angle is wide open.
This is known as primary open-angle glaucoma (POAG). POAG is caused by a blockage in the drainage canals — like the trabecular meshwork or Schlemm’s canal — rather than the drainage angle. Open-angle glaucoma typically develops slowly. It is the most common type of glaucoma, representing at least 90% of cases.
Both types of glaucoma, if left untreated, can damage vision and eventually cause blindness.
When to see an eye doctor
When glaucoma development is gradual, a person may experience few to no symptoms. Often, people with glaucoma don’t know something is wrong until their vision is affected.
Treatment is available to manage both open and narrow-angle glaucoma. However, any vision loss that occurs cannot be reversed. Having regular eye exams can help reduce your chances of glaucoma-related vision loss.
During a comprehensive eye exam, the eye doctor will check your eye pressure. If you appear to be a glaucoma suspect — meaning you don’t have glaucoma but have higher than normal eye pressure — they may recommend preventive measures to lower your risk.
READ MORE: 3 main causes of glaucoma
