The human digestive system is an open-ended tube that starts at the mouth and ends at the anus. The primary purpose of the digestive system is to break down foods and absorb the nutrients into the blood. The mouth and stomach play small roles in breaking down and absorbing nutrients, but the majority of the process occurs in the small intestine. The small intestine is part of a set of structures known as the intestinal tract.
The Intestinal Tract
The intestinal tract is made up of the small intestine and the large intestine. Although the small intestine is several times longer than the large intestine – the large intestine is approximately 5 feet long while the small intestine is at least 20 feet – it gets its name because it is smaller in diameter. The interior walls of the small intestine have grooves called rugae, which contain structures called villi and microvilli that increase the surface area of the intestine for more efficient absorption. The small intestine is also divided into three sections -- the duodenum, jejunum and ileum – and each has a different role in breaking down and absorbing food.
Duodenum
The duodenum is the section of the intestine closest to the stomach and is connected to the pancreas and gallbladder through a series of ducts. When you eat, the stomach processes the food into a substance called chyme, which slowly drips into the duodenum. The gallbladder releases bile, which breaks large globules of fat into smaller, easier to manage particles. The pancreas releases the digestive enzymes lipase, protease and amylase.
Jejunum
The jejunum is the second and longest section of the intestines where most of the food is broken down and absorbed. Here, the digestive enzymes break down the whole nutrients into their chemical components. Lipase breaks down fats, such as the oil in salmon skin, into fatty acids such as omega-3 and omega-6. Protease breaks down proteins, such as the salmon flesh, into amino acids such as lysine and arginine. Amylase breaks down starches and leafy vegetables, such as bread and lettuce, into simple and complex sugar chains. The jejunum simultaneously breaks foods down and absorbs the nutrients.
Ileum
The ileum is the last part of the small intestine and the part that connects to the large intestine. As the food travels into this part, most of the digestive enzymes are gone and all that is left are a few nutrients to be absorbed and waste products. The ileum absorbs whatever nutrients are left and moves the waste into the large intestine, which absorbs the excess water and minerals into the blood and turns the liquid waste into solid feces.
References
- “Principles of Anatomy and Physiology”; Gerard J. Tortora et al.; 2011
- Cleveland Clinic: Structure and Function of the Digestive System
