Which of the following is found in higher concentrations in aqueous humor than in plasma:
**Core Concept**
The aqueous humor is a dynamic fluid produced by the ciliary body in the eye, playing a crucial role in maintaining intraocular pressure and providing nutrients to the avascular cornea and lens. Its composition is distinct from plasma due to the unique physiological processes occurring within the eye.
**Why the Correct Answer is Right**
The correct answer is **a. Urea**. Urea is found in higher concentrations in aqueous humor than in plasma due to the active transport mechanisms of the ciliary body. The high concentration of urea in aqueous humor helps to maintain its osmotic balance and prevent excessive loss of fluid from the eye. This is achieved through the action of urea transporters, which are specifically expressed in the ciliary epithelium. The increased concentration of urea in aqueous humor also plays a role in regulating intraocular pressure.
**Why Each Wrong Option is Incorrect**
**Option B:** Glucose is actually found in lower concentrations in aqueous humor than in plasma. This is due to the consumption of glucose by the lens and cornea, as well as the active transport of glucose out of the aqueous humor by the ciliary body.
**Option C:** Sodium is present in higher concentrations in aqueous humor than in plasma, but not to the same extent as urea. The high concentration of sodium in aqueous humor is due to the active transport of sodium ions into the aqueous humor by the ciliary body.
**Option D:** Potassium is actually found in lower concentrations in aqueous humor than in plasma. This is due to the active transport of potassium ions out of the aqueous humor by the ciliary body.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the composition of aqueous humor is distinct from plasma due to the unique physiological processes occurring within the eye. Understanding the mechanisms of aqueous humor production and composition is crucial for managing various eye disorders, such as glaucoma and cataracts.
**Correct Answer:** A. Urea.