Why is the lens considered metabolically active despite being avascular?

Why is the lens considered metabolically active despite being avascular?

• A. It relies on aqueous humor for nutrient exchange and waste removal
• B. It uses the vitreous humor for nutrients
• C. It has inherent metabolic activity independent of fluids
• D. It receives nutrients directly from retinal vessels

Answer: (A)

Metabolism can occur in tissues without their own blood vessels when they can obtain nutrients from surrounding fluids. For the lens, that surrounding fluid is the aqueous humor, which bathes the anterior surface and surrounds the posterior aspect. The aqueous humor delivers glucose and oxygen and carries away wastes, providing the source of energy for the lens’s cells. The anterior lens epithelium and the deeper fiber cells actively use these nutrients to power ion transport and other metabolic processes that keep the lens clear. Because nutrients diffuse from the aqueous humor into the lens, and wastes diffuse back out, the lens can stay metabolically active even though it has no internal blood supply. The vitreous humor isn’t the primary nutrient source for the lens, and nutrients aren’t delivered directly from retinal vessels due to barriers, so diffusion from the aqueous humor best explains the lens’s metabolic activity.

Metabolism can occur in tissues without their own blood vessels when they can obtain nutrients from surrounding fluids. For the lens, that surrounding fluid is the aqueous humor, which bathes the anterior surface and surrounds the posterior aspect. The aqueous humor delivers glucose and oxygen and carries away wastes, providing the source of energy for the lens’s cells. The anterior lens epithelium and the deeper fiber cells actively use these nutrients to power ion transport and other metabolic processes that keep the lens clear. Because nutrients diffuse from the aqueous humor into the lens, and wastes diffuse back out, the lens can stay metabolically active even though it has no internal blood supply. The vitreous humor isn’t the primary nutrient source for the lens, and nutrients aren’t delivered directly from retinal vessels due to barriers, so diffusion from the aqueous humor best explains the lens’s metabolic activity.