Final answer:
Microvilli are the primary structures responsible for the vast surface area of the small intestine's lining, which aids in nutrient absorption.
Step-by-step explanation:
The structures in the small intestine that are most directly responsible for the huge amount of surface area of its lining are the microvilli. These finger-like projections on the absorptive cells substantially increase the surface area and are essential in aiding the absorption of nutrients from food. The villi, which are larger folds on the mucosa of the small intestine, also contribute to increasing the surface area available for nutrient absorption.
Each villus is covered in microvilli, making the inner lining akin to a velvety texture which greatly enhances the capacity for efficient nutrient exchange. This adaptation allows for the inner surface area of the small intestine to be as expansive as a tennis court, facilitating the effective absorption of nutrients into the bloodstream.