What structures in the small intestine increase its surface area for absorption?

The structures in the small intestine that significantly increase its surface area for absorption are villi. Villi are small, finger-like projections that extend into the lumen of the intestine. This increase in surface area is crucial because it allows for a more efficient absorption of nutrients, as a larger surface area means more space for absorption to occur.

Along with villi, microvilli—tiny hair-like structures on the surface of epithelial cells—also play an essential role in increasing the surface area even further. Together, the villi and microvilli maximize the absorptive capacity of the small intestine.

Folds, or circular folds (plicae circulares), also contribute to the surface area but are less effective than the presence of villi and microvilli. Cilia, on the other hand, are not found in the small intestine; they are involved in other types of cellular movement or fluid movement in different tissues.

Thus, the primary structures responsible for increasing the absorption surface area in the small intestine are indeed villi, making that the most accurate answer.