What are the three parts of the uveal tract?

The uveal tract consists of the iris (anterior, controls pupil), the ciliary body (middle, produces aqueous humor and controls accommodation), and the choroid (posterior, nourishes the outer retina). They form a continuous vascular middle layer of the eye.

What does the choroid do?

The choroid nourishes the outer retina (photoreceptors and RPE) via the choriocapillaris, absorbs scattered light to reduce glare, regulates temperature through its extremely high blood flow, and provides a secondary pathway for aqueous humor drainage (uveoscleral outflow).

Uveitis is inflammation of the uveal tract. It is classified by location: anterior (iris/ciliary body), intermediate (pars plana region), posterior (choroid), or panuveitis (all structures). Anterior uveitis is the most common form and presents with pain, redness, photophobia, and a small pupil.

Choroid & Uveal Tract for ABO

The Uveal Tract Overview

The uveal tract (also called the uvea) is the eye's vascular middle layer, sandwiched between the outer fibrous tunic (sclera/cornea) and the inner nervous tunic (retina). It is the eye's primary blood supply network and has three continuous components:

Iris (anterior)
Ciliary body (middle)
Choroid (posterior)

All three share a continuous blood supply and are derived from the same embryological tissue, which is why inflammation in one part often affects the others (uveitis).

The Choroid

The choroid is the largest component of the uveal tract, extending from the ora serrata anteriorly to the optic nerve posteriorly. It lies between the sclera (outside) and the retinal pigment epithelium (inside).

Structure

The choroid has four main layers (outer to inner):

Suprachoroidal space: A potential space between choroid and sclera containing loose connective tissue
Large vessel layer (Haller's layer): Contains large arteries and veins
Medium vessel layer (Sattler's layer): Contains medium-sized blood vessels
Choriocapillaris: A dense capillary network immediately adjacent to the RPE

Beneath the choriocapillaris lies Bruch's membrane, a thin layer separating the choroidal vasculature from the RPE.

Functions of the Choroid

Nourishes the outer retina: The choriocapillaris provides oxygen and nutrients to the photoreceptors and RPE (the inner retina is supplied by the central retinal artery)
Absorbs scattered light: Melanocytes in the choroid absorb stray light, reducing reflections that would degrade image quality
Temperature regulation: The extremely high blood flow (the highest per gram of any tissue in the body) helps dissipate heat from light absorption by the RPE
Uveoscleral outflow: Provides a secondary pathway for aqueous humor drainage

The choroid has the HIGHEST blood flow per gram of tissue of any organ in the body. This enormous blood flow is needed to supply the metabolically demanding photoreceptors and to regulate temperature. It also means the choroid is particularly susceptible to metastatic tumors.

The Ciliary Body

The ciliary body extends from the iris root to the ora serrata. It has two main parts:

Pars plicata: The anterior, folded portion containing the ciliary processes that produce aqueous humor
Pars plana: The posterior, flat portion. A common surgical entry point because it lacks critical structures

Functions

Aqueous humor production by the ciliary processes
Accommodation via the ciliary muscle, which controls zonular tension on the crystalline lens
Zonular fiber anchor: The zonules of Zinn extend from the ciliary body to the lens equator

The Iris

The iris is the most anterior component of the uveal tract. It is the colored, visible diaphragm that controls the pupil aperture. Its anatomy and function are covered in detail in the Iris & Pupil Function topic.

Uveitis: When Things Go Wrong

Inflammation of the uveal tract is called uveitis, and it is classified by location:

Type	Location	Common Signs
Anterior uveitis (iritis)	Iris and anterior ciliary body	Pain, redness, photophobia, miosis
Intermediate uveitis	Ciliary body and peripheral retina	Floaters, blurred vision
Posterior uveitis (choroiditis)	Choroid	Blurred vision, floaters, visual field loss
Panuveitis	All three structures	Combined symptoms

As an optician, if a patient presents with a painful, red eye with photophobia (especially if one-sided), do not fit glasses or contacts. Refer them to their eye care provider promptly, as anterior uveitis requires immediate medical treatment to prevent complications like synechiae (iris adhesions).

Thinking the choroid only supplies the retina. While its primary role is nourishing the outer retina via the choriocapillaris, the choroid also absorbs scattered light (reducing glare), regulates temperature, and provides a pathway for uveoscleral aqueous outflow.

Key Takeaways

The uveal tract has three parts: iris, ciliary body, and choroid
The choroid nourishes the outer retina and has the highest blood flow per tissue weight
The ciliary body produces aqueous humor and controls accommodation
All three components share blood supply, so inflammation can spread (uveitis)
The choriocapillaris and Bruch's membrane are the interface between choroid and RPE