Aqueous humor structure

Structures of the anterior chamber of the eye. Tissues with significant autonomic functions and the associated ANS receptors are shown in this schematic diagram. Aqueous humor is secreted by the epithelium of the ciliary body, flows into the space in front of the iris, flows through the trabecular meshwork, and exits via the canal of Schlemm arrow). Blockade of the 13 adrenoceptors associated with the ciliary epithelium causes decreased secretion of aqueous. Blood vessels (not shown) in the sclera are also under autonomic control and influence aqueous drainage. 

Fig. 12.6. Raman detection of ocular drugs in the anterior chamber. Top panel a molecular structure b Raman spectrum of aqueous solution of antibiotic drug amphotericin B (adapted from [19] c Raman spectra from aqueous humor of excised porcine eye obtained for decreasing levels of injected drug, adapted from [19] d in vivo Raman spectra of vitreous humor of rabbit eye measured through all anterior optical media before and after injection with drug, adapted from [20]...

The sclera and the cornea are the toughest and outermost layers of the eye and resist the normal internal pressure of 13 to 19 mmHg. This intraocular pressure (IOP) gives the eye its shape and maintains its dimensions that are necessary for sharp vision. The sclera covers 5/6 of the eye s surface and the cornea the remaining 1/6. Although the principal structural element of both tissues comprises of type 1 collagen fibers, differences in size and orientation of the fibers, degree of hydration, and presence of mucopolysaccharides are responsible for differences in transparency. The avascular cornea receives nourishment from the tear film, the aqueous humor, and the limbal vessels. In contrast, the sclera is vascularized and is supplied by several blood vessels, particularly in the uppermost layers (episclera). 

The eye globe is a hollow structure filled anteriorly by the aqueous humor and posteriorly by the vitreous. The aqueous humor (Figure 24.1) is continuously formed by the ciliary body. It is secreted into the posterior chamber from which it passes through the pupil into the anterior chamber and is drained at the anterior chamber periphery. Due to the blood aqueous barrier formed by zonulae occludentes of the nonpigmentary ciliary epithelium, macromolecules such as proteins can pass to the aqueous in very small quantities, regardless of their plasma concentrations. In humans, the aqueous humor protein level is around 20 mg/100 mL, less than 0.5% of the normal plasma total protein levels (Table 24.1) [22]. In the human eye, the rate of aqueous formation is around 2.5 pL/min, whereas in the case of rabbit eye it is... 

Lens The lens is the transparent biconvex structure situated behind the iris and in front of the vitreous. It plays an important role in the visual function of the eye and also enables accommodation together with the ciliary muscle. The lens is made up of slightly more than 30% protein (water-soluble crystallins) and therefore has the highest protein content of all tissues in the body [20], The lens receives its nutrients from the aqueous humor and its transparency depends on the geometry of the lens fibres. 

The eye is a unique structure, because several of its fluids and tissues—tear film, cornea, aqueous humor, lens, and vitreous humor—are almost completely transparent. These components of the ocular system have no direct blood supply in the healthy state. Each can be considered a separate chamber or compartment. A compartment is defined here as a region of tissue or fluid through which a drug can diffuse and equihbrate with relative freedom. Each compartment is generally separated by a barrier from other compartments, so that flow between adjacent compartments requires more time than does diffusion within each compartment. 

A variety of clinical situations can give rise to corneal edema (Box 15-1). Because the endothelium is the main structure involved in maintaining normal corneal deturges-cence, it plays a role in stromal hydration and compensates for the driving force of intraocular pressure. Also, the active transport system involved in the movement of water and electrolytes from the cornea to the aqueous humor must be maintained to prevent fluid retention. Endothelial feilure, a frequent cause of corneal edema, can occur due to defects in the transport system or stromal compression resulting from elevation of intraocular pressure, which can induce water movement toward the epithelium. 

The cornea serves as the front part of the AC of the eye. Its exterior is covered by the precorneal tear film, which lubricates, nourishes and protects the corneal surface. The iris and the pupil represent the posterior border of the AC. The AC angle is an important structure which is comprised of Schwalbe s line, Schlemm s canal and trabecular mesh-work, scleral spur, anterior border of the ciliary body and the iris (Figure 5.2). Aqueous humor that fills the AC is produced by the ciliary epithelium located in the posterior chamber. The fluid flows through the pupil and is drained by the trabecular meshwork into Schlemm s canal and subsequently into the episcleral vessels. This passage is named the conventional pathway. Aqueous humor is also drained by a uveoscleral pathway across the ciliary body into the supraciliary space. 

Echothiophate is a cholinesterase inhibitor that causes miosis, increase in facility of outflow of aqueous humor, a fall in lOP, and potentiation of accommodation by enhancing the effect of endogenously liberated acetylchohne in the iris, ciliary muscle, and other parasympathetic innervated structures of the eye. It is indicated in the treatment of chronic open-angle glaucoma and treatment of accommodative esotropia (see also Figure 12). 

Figure 6-5. Some pharmacologic targets in the eye. The diagram illustrates clinically important structures and their receptors. The heavy arrow (color) illustrates the flow of aqueous humor from its secretion by the ciliary epithelium to its drainage through the canal of Schlemm. (M, muscarinic a, alpha receptor p, beta receptor.)...

As an organ, the eye is constituted by various structures each formed by a specifically differentiated cellular tissue the transparent cornea, the aqueous humor... 

The vitreous is a composite gel structure, which consists of collagen fibers and hyaluronic acid. The water content is as high as around 99%. It maintains the shape of the eyeball, absorbs impact, maintains the balance of the aqueous humor in the eyeball, and regulates eye pressure. However, details are not known. 

Most biological systems are predominantly water, with other components conferring important structural and mechanical properties. The complexity of the fluid can have a substantial impact on rates of diffusional transport. For example. Chapter 5 discusses the consequences of having self-organized phospholipid phases (i.e., membrane bilayers) in systems that are primarily composed of water. Membranes separate the medium into smaller aqueous compartments, which remain distinct because the membrane permits the diffusion of only certain types of molecules between the compartments. Complex fluid phases have diverse roles in biological systems hyaluronic acid forms a viscoelastic gel within the eye (vitreous humor) that provides both mechanical structure and transparency actin monomers and polymers within the cytoplasm control cell shape and internal architecture. Drug molecules often must diffuse through these complex fluids in order to reach their site of action. 

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