DIGESTIVE SYSTEM

The human body needs energy to function. That energy comes from food, but only after it has been processed into substances that can be assimilated by various parts of the body. Some nutrients, such as minerals, can be absorbed directly along the way down the digestive tract, but substances such as proteins have to be broken down into smaller molecules.

Digestion starts in the mouth, where saliva containing a digestive enzyme called amylase lubricates food as it is chewed. In the stomach proteins and fats are broken down in an acidic environment, and salt, water, and alcohol are absorbed. Beyond the stomach (in the duodenum), acidity is neutralized before food is treated by secretions from associated digestive organs—the liver, gallbladder, and pancreas. Bile from the liver and gallbladder emulsifies fats, and pancreatic enzymes break down proteins, starch, and fats.

The liver is, in effect, the chemical factory of the body. Among many important metabolic functions, it stores glucose, vitamins, and minerals produced by food processing but not immediately usable by the body, and facilitates the breakdown of fats, which is vital for the conversion of food into energy. As well as digestive enzymes, the pancreas produces insulin (a hormone), and glycogen (a starch), both of which regulate sugar levels. Diabetes is the result of diminished insulin action. In the small intestine, the breakdown of food is completed. Nutrients are absorbed into the blood, carried to cells, and used in the release of energy. Undigested food is expelled from the anus.

Digestive ailments such as irritable bowel syndrome or ulcerative colitis are believed to result from, among other factors, an unsuitable diet or chemical imbalances in the metabolic process.