Holes Anatomy and Physiology Practice Test

Nutrients are absorbed best where the lining has a huge surface area and specialized cells ready to move digested molecules into the body. The small intestine fits that role because its mucosa is folded into villi and microvilli, creating a vast surface for contact with digested food. Enterocytes along this surface have transporters and brush-border enzymes that actively take up sugars, amino acids, and fats, sending them into the blood or lymphatic system. The duodenum and jejunum do most of the work for carbohydrates, proteins, and fats, while the ileum handles bile acids and some vitamins.

In contrast, the stomach mainly digests and begins nutrient breakdown, with only limited absorption (and some substances like alcohol can pass through). The large intestine mainly reabsorbs water and electrolytes and houses microbial activity that produces some vitamins, rather than serving as the primary site of nutrient uptake. So, the small intestine is the main site where most nutrients are absorbed.