The mixture that arrives in the small intestine is the same food that was eaten, (plus some extra enzymes and acids). It has been broken down into digestible pieces (ingestion), but none of the food has entered the bloodstream (digestion). It’s still in the hollow of the series of organs that run from “hole to hole”.
Now that the food has been converted to chyme and is in the small intestine, digestion proper can occur. To achieve this the small intestine performs two roles. Firstly, it breaks the chyme into molecules that can be digested. Secondly, it absorbs those molecules through the intestinal wall. The small intestine is composed of the duodenum, the jejunum and the ileum. The duodenum is where role 1 takes place. Role 2 takes place in the jejunum.
Role 1: Breaking Down
You can tell this stage of the process is important because other major organs get involved. Bile stored in the gallbladder is secreted into the small intestine, while the pancreas sends a combination of enzymes called “pancreatic juice”. The combination of pancreatic juice and bile breaks the chyme down into molecules that our body can use.
Role 2: Absorbing
The small intestine’s second responsibility is absorbing those molecules. Before we discuss that, we need to understand the structure of the small intestine.
The small intestine is a narrow tube about 10 feet long. It’s a jumbled tube packed with ridges and folds. These ridges and folds increase the surface area, which increases the amount of contact the nutrients have with the wall. The intestinal wall is connected to the bloodstream. If we want to absorb the nutrients, they need to pass through this wall.
A jumbled tube packed with corners maximises the opportunities for molecules to come in contact with the wall and be absorbed. The surface of the wall is also covered with villi, which themselves are covered in microvilli. The villi and microvilli are like little hands and fingers which can “catch” the molecules, further improving the chances that they will enter the blood stream.
About 90% of all digestion occurs in the small intestine. In other words, 90% of all nutrition depends on the small intestine (HINT: the digestive system is very important!!).