The uvea is the middle layer of the eye wall, situated between the outer fibrous layer (sclera and cornea) and the inner nervous layer (retina). This highly vascularized layer is often called the eye’s vascular coat, providing the globe with necessary blood supply, nutrients, and structural support. It is also a pigmented layer that helps absorb stray light, preventing internal reflections that could interfere with sharp vision. The uvea is composed of three continuous anatomical parts: the iris, the ciliary body, and the choroid.
The Iris and Light Regulation
The iris is the most anterior part of the uvea and is the colored, doughnut-shaped structure visible from the outside of the eye. It functions as a diaphragm, controlling the amount of light that enters the eye through the central aperture known as the pupil. The iris is composed of a pigmented layer, which gives the eye its color, and two sets of smooth muscles that work in opposition to one another.
The sphincter pupillae muscle is arranged circularly around the pupil margin. Its contraction causes the pupil to constrict (miosis), acting like a drawstring. This action is controlled by the parasympathetic nervous system and occurs automatically in response to bright light, known as the pupillary light reflex.
The dilator pupillae muscle has fibers that radiate outward. When these fibers contract, they pull the iris outward, enlarging the pupil (mydriasis). This dilation is controlled by the sympathetic nervous system and is triggered in low-light conditions to maximize the light reaching the retina. These opposing muscles constantly adjust pupil size, regulating light intake and increasing the depth of field for clear focus.
The Ciliary Body and Accommodation
The ciliary body is a ring-shaped structure posterior to the iris that serves two primary functions. The first involves the ciliary muscle, a band of smooth muscle fibers responsible for changing the shape of the lens to achieve clear focus at various distances. This process is known as accommodation.
When focusing on a near object, the ciliary muscle contracts, releasing tension on the zonular fibers that suspend the lens. The naturally elastic lens then becomes thicker and more convex, increasing its refractive power to focus the image onto the retina.
The second function of the ciliary body is the production of aqueous humor, a clear fluid that fills the anterior chambers of the eye. This fluid is secreted by the ciliary processes, which are finger-like projections lining the inner surface of the ciliary body. Aqueous humor production is an active secretory process, primarily driven by the Na+K+ATPase enzyme system within the ciliary epithelium.
This fluid provides oxygen and nutrients to the avascular structures of the eye, specifically the lens and the cornea. The continuous production and drainage of aqueous humor maintain a stable intraocular pressure, which is necessary to preserve the globe’s shape.
The Choroid and Vascular Supply
The choroid forms the posterior and largest part of the uvea, extending from the ciliary body to the optic nerve. It is a thin, spongy layer situated between the outer sclera and the inner retina. Its primary role is to act as a vascular bed, supplying the outer layers of the retina with oxygen and nutrients.
The choroidal circulation accounts for up to 85% of the total blood flow to the eye and is arranged in multiple distinct layers. The innermost layer is the choriocapillaris, consisting of a dense layer of wide capillaries adjacent to the retina. These highly permeable vessels are responsible for nourishing the photoreceptors and the retinal pigment epithelium. Larger vessels are found in the outer layers (Sattler’s layer and Haller’s layer), which drain into the vortex veins. The choroid also contains the suprachoroidal space, which allows for the passage of nerves and vessels.
Collective Functions and Clinical Significance
The collective functions of the uvea—nourishment, light control, and focus adjustment—are necessary for visual health. The vascular network of the choroid ensures the survival of the outer retina, while the aqueous humor produced by the ciliary body feeds the anterior structures. The coordinated muscle action of the iris and ciliary body allows for automatic adjustments to light and distance.
A major clinical concern related to this structure is Uveitis, which is the term for inflammation in any part of the uvea. Uveitis can manifest as anterior (involving the iris and ciliary body), intermediate, or posterior (involving the choroid) inflammation, or it can affect all parts in a form called panuveitis. This condition is often painful and can cause symptoms like blurred vision, light sensitivity, and eye redness.
Inflammation can lead to complications such as the development of cataracts or the swelling of the central retina (macular edema). It can also disrupt fluid dynamics, causing an increase in intraocular pressure that may result in secondary glaucoma. Because Uveitis can cause damage to eye structures, it is a significant cause of vision loss and requires prompt medical attention.