Production and Drainage of Aqueous Humor

Aqueous and Vitreous Humors

The somewhat archaic term "humor" is still used today to describe certain bodily secretions, among them those of the eye. The eye has two "humors," the aqueous and the vitreous humor. (Only the first is truly a liquid; the latter is a gelatinous mass that fills the bulk of the eye and is perhaps better referred to by the term "vitreous body," but the older terminology is still used.)

Aqueous humor and its production are essential to normal function of the eye. Since the eye is an optical device and must obey the laws of optics, it has to maintain a certain size and shape, or the quality of the image produced and focused on the retina will suffer. The balance between production and drainage of the aqueous humor is one of the important ways in which this is accomplished.

Production

The two-layered epithelium covering the finger-like ciliary processes is the site of manufacture of the aqueous humor. These two layers of cells are sealed together by occluding junctions, and form a "tight" barrier between the posterior chamber of the eye and the blood vessels passing through the ciliary body and its processes. This epithelium has total control of the use of raw materials brought to it in the blood, and the release of aqueous humor into the posterior chamber.

The iris is the boundary between the posterior and anterior chambers. The opening through it, the pupil, is the route by which aqueous humor, released into the posterior chamber, can flow to the anterior chamber.

The peripheral part of the anterior chamber, between the cornea and the iris region is referred to as the filtration angle. It is filled with a delicate connective tissue webwork, the trabecular meshwork through which the flowing aqueous humor percolates. Drainage after filtration is via the Canal of Schlemm (Friedrich S. Schlemm, 1795-1858, a German anatomist). This appears in histological sections as a flattened and irregular vessel much like a lymphatic duct, but like other structures in this region of the eye, it runs circumferentially around the entire periphery of the limbus. The Canal of Schlemm eventually leads into the lymphatic drainage of the eyeball, which exits via vessels in the sclera.

The production of the aqueous humor is a constant process; and its removal is vitally important: the balance between production and drainage determines the intraocular pressure. Normal pressures maintain the normal shape and size of the eyeball, fulfilling the mechanical requirements to keep an image in focus. If the balance is disturbed, vision problems are the normal result.

Blurry vision and "seeing stars" are premonitory symptoms of increased intraocular pressure, which can result from overproduction of aqueous humor, or damage to the drainage system, among other causes. This is glaucoma, and uncorrected, it can damage the eye, causing injury to the retina or optic nerve, and eventual blindness.

LEFT: A three-dimensional representation of the trabecular meshwork