Before the food leaves the stomach for the small intestine it passes through another sphincter, called the pylorus, which acts like a policeman directing rush-hour traffic down to a single-lane road. This powerful ring-like muscle is critical in the digestive process because it joins two organs that are very different in terms of size, shape, purpose and chemical environment.

The stomach is really a big storage bag but the small intestine is a narrow tube in which the major part of the digestive process takes place. The pylorus ensures that the small intestine is not over-filled by too much food entering all at once and that there is enough time for the digestive enzymes in the small intestine to break down the food chemically.

The Small Intestine
The contractions of the small intestine continue to break down the food physically, but more importantly, the small intestine secretes enzymes that break down the food chemically and absorbs the resulting nutrients. In the duodenum, the first part of the small intestine, the food is mixed with large amounts of digestive enzymes secreted by the pancreas, a nearby gland, and bile manufactured by the liver.

The chemical activity of digestion is critical to our ability to use the food we eat. Protein, for example, is a long chain of building blocks called amino acids. Although the body cannot absorb the entire chain, it can absorb amino acids, either singly or in pairs. Similarly, the complex fat molecules that we eat are broken down into fatty acids, and complex carbohydrates, long chains of sugars, are broken down into absorbable single sugars.

The body produces different enzymes to digest each type of nutrient. Some require intermediate steps for successful breakdown. Fats, for example, cannot mix with the watery contents of the duodenum, which contains the fat-breaking enzymes. The bile secreted by the liver first emulsifies the fat, breaking it down into small droplets that the enzymes can then break down into fatty acids that can be absorbed.

Some easily digested foods are completely broken down in the upper small intestine and other components of the food that don't need to be digested, such as iron, calcium, or zinc, are directly absorbed here. The rest of the absorption takes place in the remainder of the small intestine, the jejunum (about 40 percent of the small intestine) and the ileum (about 60 percent of the small intestine), which links to the large intestine.

Absorption of Nutrients
Despite its name, the small intestine stretches to about fifteen feet, depending on a person's size. While an impressive length, the intestine's surface area would be insufficient to absorb what we eat each day if it were simply a length of hose. Instead, it is structured to increase its surface area tremendously through fingerlike projections along the lining called villi that are in turn covered with hairlike projections called microvilli.