Final answer:
The absorption of nutrients mainly begins in the duodenum, the first section of the small intestine, where iron is particularly absorbed. The jejunum continues the process by absorbing most nutrients, and the ileum absorbs specific nutrients like vitamin B12.
Step-by-step explanation:
Absorption of nutrients primarily begins in the duodenum, which is the first part of the small intestine. Although a few substances such as water and some minerals and vitamins are absorbed in the stomach and large intestine, the bulk of nutrient absorption occurs in the small intestine. Specifically, the duodenum is where iron is absorbed. The subsequent parts of the small intestine, the jejunum and ileum, play significant roles in the absorption of nutrients as well. The jejunum, for instance, is responsible for absorbing around 95 percent of nutrient molecules into the bloodstream through the epithelial cells that line the mucosa.
As food moves into the duodenum, it mixes with digestive juices, which include enzymes that break down complex molecules in food, facilitating nutrient absorption. Chyme, the mixture of partially digested food, moves from the stomach to the duodenum where digestion continues and absorption begins. After being absorbed, nutrients are then transported to other parts of the body for storage or further chemical modification.