Final answer:
The small intestine is the primary site of nutrient absorption in the human digestive system, facilitated by the villi and microvilli that increase its surface area.
Step-by-step explanation:
This is where the digestion of protein, fats, and carbohydrates is completed. The small intestine is structured to maximize absorption efficiency. It features a highly folded surface with finger-like projections known as villi, and on the top of these villi are microscopic projections called microvilli. These increase the surface area available for nutrient absorption.
The majority of nutrient molecules are absorbed in the second part of the small intestine, called the jejunum, though there are specific sites for certain nutrients, such as iron in the duodenum and vitamin B12 in the ileum. After absorption, nutrients are transported to other parts of the body for use or storage.