Which apical surface features increase surface area for absorption or secretion, and where are they typically found?

• A. Microvilli increase surface area for absorption and secretion; found in the small intestine and kidney proximal tubules; Cilia move mucus and fluids; found in the respiratory tract.
• B. Microvilli increase surface area for absorption; found in the lungs; Cilia move ions; found in the liver.
• C. Cilia increase surface area; found in the skin; Microvilli produce mucus.
• D. Both microvilli and cilia increase surface area only in glandular tissue.

Answer: (B)

Microvilli and cilia are two apical surface adaptations that serve different purposes. Microvilli are tiny, finger-like projections that dramatically increase the cell’s surface area, boosting the capacity for absorption and secretion. They form the brush border on cells of the small intestine and on kidney proximal tubule cells, where a large absorptive surface is essential. Cilia are longer, whip-like structures that beat to move mucus and fluids across the epithelial surface, helping to clear debris and keep passages open; they are prominent in the respiratory tract, where they propel mucus toward the throat. Therefore, the correct pairing is microvilli for absorption and secretion in the small intestine and proximal tubules, and cilia for moving mucus and fluids in the respiratory tract. The other descriptions mix up either the function or the location of these structures.