Mechanism of Aqueous Humor Secretion, Its Regulation and Relevance to Glaucoma

Abstract

The principal function of the eye is to receive the light signal from the environment and transmit it to the brain to create vision. This requires that the structures of the eye involved in light transmission, such as the cornea and the lens, must be transparent. Unlike other tissues of the body, nutrients and oxygen supply to these structures must be accomplished without blood interference in transmission of light, i.e., these structures must be avascular. Apart from this, the eye must also maintain optimum pressure within the globe, which is important to give the rigidity necessary for optical alignment of the cornea, lens and the retina. The eye achieves this by the production, accumulation and circulation of a clear fluid called aqueous humor (AH). AH is produced by selective transfer of solutes and water from the blood plasma by a complex ocular epithelium, the ciliary epithelium. The fluid then accumulates in the two special compartments within the eye, and the excess leaves the globe to enter the blood through complex pressure dependent outflow pathways. This complex arrangement of production, accumulation and drainage of aqueous humor contributes to the nutrition and survival of the avascular tissues as well as to good optical properties of the eye. However, this also poses a problem in that any imbalance in the production and drainage of aqueous humor inevitably causes abnormal intraocular pressure (IOP). For example, in case of insufficient drainage, excess accumulation of fluid within the globe will raise the IOP to higher than the physiological level; a clinical condition called glaucoma. Abnormally high pressure causes death or degeneration of the light sensitive and signal transmitting tissues of the eye leading progressively to partial or complete blindness. Understanding the mechanism of AH secretion and its regulation is important to develop rational and target specific drugs for the treatment of glaucoma. In this chapter we will try to present the readers with a comprehensive and up to date description of the relevant ocular anatomy, physiology, biochemistry and pharmacology of AH secretion and its relevance to pathophysiology of ocular hypertension or glaucoma.