What happens in the small intestine?

The small intestine contains a complex lining, which has an extremely large surface area capable of absorbing nutrients. This lining is often called the brush border. Sodium absorption drives the absorption of water, some amino acids, and glucose. Other nutrients are absorbed through specialized areas in the brush border.

Structure
The small intestine itself is an amazing and complex structure, largely because it manages to absorb thousands of tiny nutrients within a very short period of time. The small intestine therefore has a complex inner surface, consisting of folds and projections, which dramatically increase surface area. Instead of being flat, the intestinal wall rises and falls in mucosal folds. On these folds are finger-like projections called villi, which form peaks and valleys. These villi are covered in millions of tiny microvilli, also with peaks and valleys. Because the undulating appearance of the intestinal lining resembles a brush, it is often called the brush border.

Arteries, veins, and lymph ducts extend from the intestinal wall through the peaks of the villi, where absorption occurs.

Nutrient absorption
The lining of the small intestine consists of billions of cells, which pump sodium from the intestine to the bloodstream. The sodium travels with glucose and several amino acids, a process known as co-transporting. This creates a concentration gradient across the membrane, and water osmotically follows sodium into the bloodstream.

While glucose and some amino acids enter the blood with sodium, other minerals, fats and vitamins enter the blood through specialized places in the small intestine. Integrity of the intestinal lining is vital for proper absorption of these nutrients.