What is the main process responsible for the slightly hyperosmotic state of aqueous compared to plasma?

The slightly hyperosmotic state of aqueous humor compared to plasma is primarily due to active secretion. In the ciliary processes, the aqueous humor is produced through a combination of mechanisms, predominantly involving active transport processes. Specific ion pumps, such as the Na+/K+ ATPase and the Na+/HCO3- cotransporter, actively move ions into the posterior chamber of the eye, creating an osmotic gradient that draws water into the aqueous humor through osmosis. As a result, the concentration of solutes, including proteins like lactate, is higher in the aqueous humor than in plasma, leading to the hyperosmotic state.

This active secretion process ensures that the necessary nutrients and waste products are maintained in the aqueous humor for corneal nutrition and intraocular pressure regulation. In contrast, ultrafiltration, which relies on pressure differences to guide fluid movement, does not primarily contribute to the osmotic state in this context. Other options, such as diffusion and excretion, involve processes that would not primarily establish the hyperosmotic condition but instead relate to fluid movement or exit mechanisms rather than the creation of the osmotic gradient within the eye.