The aqueous humor (aqueous) is a transparent fluid that fills the anterior and posterior chambers of the eye. It is the major nutrient source for the avascular lens and cornea and also serves as a medium for removal of waste products.
Ocular fluids are separated from blood by barriers formed by the tight junctions of epithelial cells and those of endothelial cells. These barriers are called either blood–aqueous or blood–retina barriers, depending on their location in the eye. Because of these barriers, the composition and amounts of all materials entering and leaving the eye can be carefully controlled. Perturbations of these blood–ocular barriers cause blood constituents to mix with ocular fluids; this mixing leads to plasmoid aqueous, retinal exudates, or retinal edema.
The blood–aqueous barrier is composed of the tight junctions of the following:
This barrier restricts plasma proteins from entering the aqueous. Consequently, aqueous is essentially protein-free, which gives it a refractive index of 1.336 and allows optical clarity for transmission of light along the visual pathway. The blood–aqueous barrier, along with active transport systems, also allows increased levels of ascorbate and some amino acids in aqueous compared to levels in blood plasma. Breakdown of this barrier is discussed later in the chapter.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.