After food particles leave the stomach, they enter the small intestine. The small intestine has three parts: the duodenum, the jejunum, and the ileum. The small intestine's main job is to digest and absorb the nutrients from the food so the body can use them.

The first part of the small intestine is the duodenum. It makes mucous that helps protect the small intestines from the stomach acid. As food particles enter the duodenum, they are mixed with different fluids from the pancreas, liver, and gallbladder. This mixture is called chyme. The liver makes a yellow-colored fluid called bile, which is stored in the gallbladder. Bile is important for digesting food. The gallbladder squeezes bile through the bile duct into the small intestine. The pancreas releases fluid with digestive enzymes into the small intestine to help digest proteins, carbohydrates, and fats so they can be absorbed. The mixture is more liquid at this point.

The wall of the small intestine is lined with villi, which are shaped like fingers. The villi allow the intestine to absorb nutrients more completely. The small intestine contracts in a specific rhythm, called peristalsis. This rhythm moves the mixture to the second part of the small intestine, the jejunum, which further digests the food particles.

The ileum has two jobs. First, the ileum continues to absorb and carry the nutrients to the blood where they can be stored and used by the body. By the time the food reaches the end of the ileum, it is only water, electrolytes, and waste. The second job of the ileum is to recycle the bile. The ileum reabsorbs most of the bile that was released to digest nutrients. The bile goes back to the liver and is released when a person eats or drinks again.