When a person eats food, the body has to convert that food into molecule chains that can be converted to energy and raw nutrients. The human digestive system controls the distribution of nutrients and energy in the body by subjecting the food to chemical reactions as it travels through the digestive tract. The structure of the digestive system, also known as the alimentary canal, allows the different organs to sequentially break down and process food. Apart from the mouth and esophagus, there are three main organs in the digestive system, as well as many smaller organs. The process of digestion starts in the mouth, with saliva moistening and breaking down food into manageable chunks.
The stomach is the first part of the structure of the digestive system, consisting of a muscular hollow sac located just below the diaphragm and ribcage. It is separated from the esophagus and the small intestine by the esophageal sphincter and the pyloric sphincter respectively. The structure of the digestive system requires that food be broken down before passing into the rest of the alimentary canal; a film of mucus covers the lining of the stomach to protect the walls of the stomach against hydrochloric acid that breaks the food down. Food remains in the stomach until it undergoes physical digestion to make it more suited for nutrient extraction in the small intestine.
After food is broken down in the stomach, it passes into the small intestine immediately below in the structure of the digestive system; the small intestine is 20 feet (about six meters) long on average for adults. The small intestine does most of the work of chemical digestion via enzymes secreted by the pancreas. Proteins break down into amino acids, fats break down into glycerol, and carbohydrates break down into glucose and other sugars. The interior wall of the small intestine contains finger-like folds of tissue called villi, which catch food molecules and pass them into exterior capillaries and then to larger blood vessels.
The small intestine connects to the large intestine. The large intestine is the final main part of the structure of the digestive system, and is responsible for absorbing excess water, converting the solid waste into stool and excreting it. Bacteria residing in the small intestine, known as gut flora, absorb the solid material remaining after all nutrients and energy have been extracted from food and drink. As a rule, solid waste takes a day to travel through the large intestine.