In the body, all the organs and tissues involved in the digestion of food. In animals, these consist of the mouth, stomach, intestines, and their associated glands. The process of digestion breaks down the food by physical and chemical means into the different elements that are needed by the body for energy and tissue building and repair. Digestion begins in the mouth and continues in the stomach; from there most nutrients enter the small intestine from where they pass through the intestinal wall into the bloodstream; what remains is stored and concentrated into faeces in the large intestine. Birds have two additional digestive organs – the crop and gizzard. In smaller, simpler animals such as jellyfish, the digestive system is simply a cavity (coelenteron or enteric cavity) with a ‘mouth’ into which food is taken; the digestible portion is dissolved and absorbed in this cavity, and the remains are ejected back through the mouth.
The digestive system of humans consists primarily of the alimentary canal, a tube which starts at the mouth, continues with the pharynx, oesophagus (or gullet), stomach, large and small intestines, and rectum, and ends at the anus. The food moves through this canal by peristalsis whereby waves of involuntary muscular contraction and relaxation produced by the muscles in the wall of the gut cause the food to be ground and mixed with various digestive juices. Most of these juices contain digestive enzymes, chemicals that speed up reactions involved in the breakdown of food. Other digestive juices empty into the alimentary canal from the salivary glands, gall bladder, and pancreas, which are also part of the digestive system.
The fats, proteins, and carbohydrates (starches and sugars) in foods contain very complex molecules that are broken down (see diet; nutrition) for absorption into the bloodstream: starches and complex sugars are converted to simple sugars; fats are converted to fatty acids and glycerol; and proteins are converted to amino acids and peptides. Foods such as vitamins, minerals, and water do not need to undergo digestion prior to absorption into the bloodstream. The small intestine, which is the main site of digestion and absorption, is subdivided into the duodenum, jejunum, and ileum. Covering the surface of its mucous membrane lining are a large number of small prominences called villi which increase the surface for absorption and allow the digested nutrients to diffuse into small blood-vessels lying immediately under the epithelium.
Mouth, pharynx, and oesophagus The digestive process begins in the mouth with mastication and salivation. The purpose of mastication is to crush and grind the food into small particles. Saliva is poured into the mouth from three pairs of glands named parotid, submandibular, and sublingual. The parotid gland secretes a clear saliva, the other two a sticky saliva containing mucin. Saliva has an important chemical action which, by means of an enzyme called ptyalin, converts cooked starch in the food into maltose, a kind of sugar. The saliva also moistens the food so that it can be rolled by the tongue and palate into a soft bolus. When the food is sufficiently masticated, the bolus is pushed backwards by the tongue into the pharynx. Here the swallowing reflex causes a series of muscular movements which propel the food into the gullet, or oesophagus, and from there into the stomach.
Stomach The stomach is entered by the cardiac orifice which relaxes to admit the food and then closes. The mucous membrane of the stomach is lined with columnar epithelium, in which are embedded little pits called the gastric glands. Gastric juice pours from these glands when they are stimulated by the approach of food. The muscular coat of the stomach produces movements which churn the food and tend to urge it towards the intestine, but the pylorus (opening from the stomach) opens only in response to an acid stimulus. The food received from the gullet is alkaline, owing to the presence of salivary secretions, and so it stays in the stomach until thorough mixing with the hydrochloric acid in the gastric juice has rendered it acid. Gastric juice also contains an enzyme called pepsin, which is released as an inactive precursor called pepsinogen; this is a protease that converts the proteins in the food into polypeptides. In babies, there is an enzyme called rennin that coagulates milk. About 30 minutes after the start of a meal, the stomach begins to discharge the semi-solid and partly digested food, termed ‘chyme’, into the intestine, although it takes about three hours to empty the contents of the stomach completely.
Small and large intestine The small intestine secretes succus entericus (intestinal juice); two other secretions, pancreatic juice and bile, enter by their ducts, which open into the duodenum, or the first part of the small intestine. Pancreatic juice contains three enzymes: trypsin (released as a precursor, trypsinogen) which degrades proteins more completely than gastric juice, converting them into amino acids; amylase, which converts starch into maltose, thus taking over the function of salivary juice whose activity is stopped in the stomach; and lipase, which splits the fats into glycerin and fatty acids. Bile by itself has no digestive action, but it aids the action of lipase. The intestinal juice contains the enzymes enterokinase, which is concerned in the production of trypsin, and aminopeptidases, which aid trypsin in the breaking up of polypeptides; it also contains enzymes that convert maltose and other sugars into glucose.
The glycerin and fatty acids are carried into the central lacteal or lymphatic vessel and are again united into small globules of fat. The amino acids from the proteins are carried in the bloodstream and used to repair and build up the tissues; any excess is converted by the liver into urea, which then passes to the kidneys and excreted via the bladder as urine. The tiny globules of fat pass into the thoracic duct; from there they pass into the bloodstream and ultimately into the tissues, where they produce heat by oxidation or are stored in the form of adipose tissue. The sugar is temporarily stored in the liver as glycogen and converted back to glucose as and when required.
The small intestine is about 6.9 m/23 ft long and it takes about 4 hours for the food materials to pass through it. These materials then travel more slowly through the 1.2 m/4 ft of large intestine, taking from 12 to 18 hours to reach the rectum. During this time water is absorbed and the waste residue is gradually compressed as a compact mass into the rectum, which is eventually egested, or expelled, by the anus.
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