Where does most absorption occur via diffusion in the small intestine?

The jejunum is the section of the small intestine where the majority of nutrient absorption occurs through diffusion. This middle segment is specifically designed for optimal absorption due to its large surface area, which is enhanced by the presence of numerous villi and microvilli. These structures increase the absorptive surface, allowing for efficient movement of nutrients from the intestinal lumen into the bloodstream.

In the jejunum, various nutrients, including carbohydrates, proteins, and lipids, are primarily absorbed. The mechanisms of diffusion and facilitated diffusion play critical roles in this process, allowing for the gradual movement of these substances down their concentration gradients.

While the duodenum and ileum also participate in absorption, their functions differ; the duodenum is primarily involved in the initial digestion and mixing of food with bile and pancreatic juices, while the ileum focuses more on the absorption of vitamin B12 and bile salts. The stomach is primarily responsible for digestion rather than absorption, which is why it is not the correct answer in this context.