The digestive system
The digestive system is a tube open at both ends, with accessory organs beside it, that breaks food down so that it can be absorbed through the tube's wall. Food passes through the organs of the tract, whose wall has four layers, from the mouth to the large intestine. The liver and pancreas serve digestion from outside the tube, and each digestive juice supplies enzymes that digest particular substances.
The tract and what the system does
The organs of the digestive system form an irregularly shaped tube open at both ends, the mouth and the anus, called the alimentary canal or gastrointestinal (GI) tract. Food in that tube is not "inside" the body. It must be broken down and then absorbed through the walls of the tract before cells can metabolise and use it. Digestion is both mechanical and chemical.
The digestive system includes the elongate digestive tract and its associated accessory organs. Its functions include ingestion, mechanical processing, secretion, digestion, absorption, compaction and defecation.
Organs and the layers of the tract wall
The main organs are the mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum and ileum) and large intestine (caecum; the ascending, transverse, descending and sigmoid colon; rectum; and anal canal). The accessory organs include the teeth, tongue, salivary glands (parotid, submandibular and sublingual), liver, gallbladder, pancreas and vermiform appendix.
Although the tract is described as a tube, its wall has four tissue layers, from the lumen outward: mucosa, submucosa, muscularis, and adventitia or serosa. The same four coats form the various organs of the GI tract, but their structure varies with the organ's function. The stomach, for example, has a thicker muscularis to aid digestion.
The path of food
Digestion begins in the mouth, which performs mechanical processing, lubrication and the initiation of digestion by salivary enzymes. The pharynx is a muscular tube belonging to both the digestive and respiratory systems. Food passes from the pharynx into the oesophagus, another muscular tube, which is simply a passageway to the stomach and has no digestive function.
The stomach is an expanded part of the tube, no larger than a small sausage when empty. Contractions of its muscular walls mix food with hydrochloric acid and gastric enzymes and break it into smaller molecules. After about three hours in the stomach, food passes into the small intestine.
The small intestine is almost 20 feet long, and thousands of microscopic intestinal glands in it secrete the intestinal enzymes that complete the breakdown of food. Its lining is folded into multiple pleats called plicae, which are covered with thousands of villi. A plica is a ridge or fold of peritoneum or other membrane. A villus is a minute fingerlike projection of the mucosa into the lumen of the intestine. The plicae and villi enormously increase the area available for absorbing digested nutrients to an estimated 100 square feet.
Undigested and unabsorbed material enters the large intestine, where water and salts are resorbed as faeces form. Faeces are the excrement discharged from the intestines, and consist of bacteria, cells exfoliated from the intestines, secretions chiefly of the liver, and a small amount of food residue. The bacteria that live in the large intestine can digest materials that escaped digestion in the small intestine, and this breakdown may release additional nutrients. The bacteria also synthesise the vitamin K the body needs.
The liver and pancreas
The liver, gallbladder and pancreas are not part of the digestive tube, but they play important roles in digestion.
The liver is the largest gland in the body and its most important biochemical-processing organ. It produces bile, which is stored in the gallbladder and is an emulsifying agent, not an enzyme. The liver also plays a major role in maintaining normal blood glucose concentration, carries out the first steps of protein and fat metabolism, and synthesises several proteins, including the prothrombin and fibrinogen needed for blood clotting. Liver cells detoxify various poisonous substances and store several substances, primarily iron and vitamins A, B and D.
The pancreas has both exocrine and endocrine functions. Its exocrine product, pancreatic juice, is one of the most important digestive juices. Pancreatic juice contains enzymes that digest proteins, carbohydrates and lipids, and sodium bicarbonate, an alkaline substance that neutralises the hydrochloric acid arriving from the stomach.
Enzymes of the digestive juices
Each digestive juice supplies enzymes that digest, or hydrolyse, particular substances. Saliva supplies amylase, which digests starch. Gastric juice supplies protease plus hydrochloric acid, which act on proteins, and lipase, which acts on emulsified fats. Pancreatic juice supplies protease for proteins, lipase for bile-emulsified fats and amylase for starch. Intestinal juice supplies peptidases for peptides, and sucrase, lactase and maltase for sucrose, lactose and maltose respectively.