Final answer:
The duodenum has modifications such as circular folds, villi, and microvilli that collectively increase its surface area by 600-fold, facilitating efficient nutrient absorption.
Step-by-step explanation:
The modifications of the duodenum that increase its area 600-fold are crucial for nutrient absorption. The hierarchical modifications begin with circular folds, which are deep ridges in the mucosa and submucosa that cause the chyme to spiral and slow its movement. This allows more time for nutrients to be absorbed. Next, there are finger-like projections called villi, which further increase the surface area. Lastly, on the surface of these villi are tiny projections called microvilli, also known as the brush border, which enormously increase the surface area and host enzymes for digestion.
These structures are most abundant in the proximal two-thirds of the small intestine, where the majority of nutrient absorption occurs. Because of these modifications, if we were to unfold the entire absorptive area of the small intestine, it would cover a massive surface area, equivalent to setting 600 strips side by side, forming a rectangle about 5 meters by 38 meters.