True or False: The aqueous humor within the eye is replaced every 90 seconds.

The assertion that the aqueous humor within the eye is replaced every 90 seconds is based on the continuous production and drainage of this fluid. Aqueous humor is produced by the ciliary body and plays a crucial role in maintaining intraocular pressure, providing nutrients to the avascular structures of the eye (such as the lens and cornea), and removing metabolic waste.

This fluid circulates through the anterior chamber of the eye and is drained through the trabecular meshwork and Schlemm's canal. The process of production and drainage is dynamic, allowing the aqueous humor to refresh and replace itself approximately every 90 seconds, which helps to maintain optimal conditions for vision. This rapid turnover is vital for eye health, ensuring that the eye is properly nourished and free from excessive pressure that could lead to conditions like glaucoma.

The other options, such as replacement during blinking or only when the eye is closed, do not accurately represent the continuous nature of aqueous humor turnover, which is not linked to the mechanical actions of blinking or eyelid closure.