All animals need to engage in DIGESTION and are equipped with the means to do so. Animals are in a sense hollow, the ALIMENTARY CANAL (or digestive tract) being a tube that extends from the MOUTH to the anus. The digestive system of MAMMALS contains specialized organs to achieve specific goals. It includes the alimentary canal (MOUTH, PHARYNX, OESOPHAGOUS, STOMACH, SMALL INTESTINE, LARGE INTESTINE, ANUS) and related organs such as the salivary glands, PANCREAS, LIVER and GALLBLADDER, The principal functions of each major section are as follows. (1) MOUTH and pharynx: three main processes occur here: salivary glands secrete saliva to begin digestion of CARBOHYDRATES; a food bolus for swallowing is created; and taste processing occurs (see CEPHALIC REFLEXES OF DIGESTION; GUSTATION). (2) OESOPHAGOUS: this is a passage taking food from the pharynx to the stomach; it transmits a bolus using PERISTALSIS; enzymes in saliva continue to work while the bolus is in transit. (3) STOMACH: this is a large organ capable of storing and releasing food in small quantities such that there is no need to eat constantly. Enzymes begin digestion of PROTEINS: the stomach lining (epithelium) secretes GASTRIC ACID (which is highly acidic [pH 2] to degrade cells) and PEPSIN (an enzyme that degrades proteins -it is secreted in an inactive form, pepsinogen, which gastric acid converts to pepsin). The stomach is protected from gastric acid by a coating of mucus and constant regeneration of its lining. The stomach mixes food (SMOOTH MUSCLES move it) and the product released at the pyloric sphincter into the small intestine is known as acid chyme. Note that in some mammals there are multiple stomachs—ruminants such as cattle have three stomachs to allow breakdown of complex plant cellulose; other animals (birds for instance) have a crop before the stomach and a gizzard after it to allow for breakdown of foods. Many INVERTEBRATES also have a crop and gizzard. (4) SMALL INTESTINE: this is the principal organ of digestion and the longest part of the human alimentary canal (at 6 metres); the initial 25 cm is known as the DUODENUM, followed by the JEJUNUM and then ILEUM, which connects to the large intestine.
In the duodenum, fluids from the pancreas (including ALKALI solutions and enzymes) and the gallbladder (bile-produced in the liver but stored in and released from the gallbladder) are mixed with acid chyme. Enzymes such asCHOLECYSTOKININ, SECRETIN and ENTEROGASTRONES are secreted by the duodenum to affect peristalsis and stimulate the activity of the pancreas. Digestion proceeds in the jejunum and ileum: the lumen of the small intestine and the epithelium (or brush border, so-called because of the microvilli—small finger-like projectionspresent there) are differentially involved. In the lumen, pancreatic amylases degrade carbo hydrates; trypsin and chymotrypsin degrade complex proteins and aminopeptidases and carboxypeptidases break down smaller proteins to amino acids; nucleic acids (DNA and RNA) are broken down by nucleases to NUCLEOTIDES; and FATS are degraded by bile salts and lipases to GLYCEROL, FATTY ACIDS and glycerides (see TRIGLYCERIDES). In the epithelium, disaccharidases degrade DISACCHARIDES to MONOSACCHARIDES; PEPTIDES are degraded to AMINO ACIDS by dipeptidases; nucleotides are reduced to NUCLEOSIDES by nucleotidases and nucleosidases degrade these to even simpler elements. ABSORPTION of NUTRIENTS also occurs: villi and microvilli give the epithelium of the small intestine a huge surface area. Small vessels of the LYMPHATIC SYSTEM (a part known as the lacteal system), BLOOD capillaries and veins transport nutrients away, sometimes by passive diffusion, sometimes with the aid of membrane pumps. The capillaries and veins drain into the hepatic portal vessel, a major blood vessel leading directly to the liver. (5) LARGE INTESTINE: this is also known as the colon: it is connected to the ileum. At this point there is a three-way connection—the CAECUM is also connected. The main component of the caecum is the appendix. Caecum and appendix are involved in cellulose digestion and are relatively small in humans (to the extent that the appendix is removable). The principal purpose of the colon is the reabsorption of water left after digestion in the small intestine. Movement through the colon is relatively slow (12–24 h): the colon terminates at the RECTUM where faecal matter is stored before evacuation via the anus. (The anus and rectum are both sphincters: control over the rectal sphincter is INVOLUNTARY, while control of the anal sphincter is VOLUNTARY.)
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