The structure of the human alimentary canal The alimentary canal is a long hollow tube which runs from the mouth to the anus .Together with several other organs, including the liver and the pancreas, it makes up the digestive system. The total length of the human alimentary canal is between 5 and 6 m, from anus to mouth. To fit this considerable length into body, parts of the canal are folded and coiled inside the abdomen .The mucus is a substance secreted along the tube by cells lining its walls .Mucus helps food to slide through the canal without doing too much damage to the lining. It also forms a protective covering which keeps the digestive juices, which are inside the lumen of the canal, from coming into contact with the living cells of the walls. Along the whole length of the alimentary canal there are muscles in the walls. These produce waves, of Contraction and relaxation called peristaltic waves, which move food along the alimentary canal and help to mix the contents. Each region of the alimentary canal has it own function and different structure. There are 4 basic layers in the wall of the alimentary canal. Working from the inside these are: a) the mucosa b) the submucosa c) the muscularis externa d) the serosa. Many of this names came from Latin origin. The mucosa is made up of 3 layers. The innermost layer is the epithelium. The structure of the epithelium varies in different parts of the alimentary canal, but it always contains cells which secrete mucus. 42833iem24lnq9k Beneath it is a layer of connective tissue called lamina propria, which means ‘closest layer’. And beneath that is a layer of smooth muscle called the muscular is mucosa. The sub mucosa is made up of areole connective tissue. This is an open-textured stretchy tissue, containing many elastic fibred and collagen fibres. Running through it are numerous blood vessels and nerves. The muscularis externa ia made of two layers of muscle. The innermost layer has fibres running around the tube, and is called circular muscle. The outermost layer has fibres running along the tube and it is called longitudinal muscle. The serosa is a very thin layer, made up of connective tissue covered with a single layer of thin, smooth closely fitting cells. en833i2424lnnq We can observe in detail each part of the human alimentary canal structure in this diagram. The mouth: Taking food into the mouth is called ingestion. We use lips, tongue and teeth. The tongue is also important in tasting food, to tell you whether it is good to eat; if not it will be ejected from the mouth rather than swallowed. The main purpose of the human teeth is to break up large pieces of food, thus beginning the process of the mechanical digestion. This is done by chewing, or mastication. Strong muscle is the jaws move the lower jaw up and down from side to side, grinding the teeth in the lower jaw against those in the upper jaw. The premolar and molar teeth have ridges and grooves, which trap food between them and crush it as chew. Mastication greatly increases the surface area of the food, bringing more of it into direct contact with enzymes in the digestive juice and so speeding up chemical digestion. Three pairs of salivary glands secrete watery liquid saliva, which pours along ducts into the mouth. Like all secretions along the alimentary canal, saliva is mostly water. It contains mucus, which mixes with the food as it is chewed, helping to glue it loosely together into a ball called a bolus. The mucus also makes the bolus slippery, so that is easier to swallow. Saliva contains the enzyme amylase, which catalyses the hydrolysis of starch. . Thus, digestion by amylase produces maltose and small chain made up of three, four or more glucose molecules on the end of a chain . Thus , digestion by amylase produces maltose and small chains made up three, four or more glucose molecules linked together, but it does not produce individual glucose molecules. Saliva also contains an enzyme called lysozyme. This enzyme, which is also found in tears, can destroy several types of bacteria which can cause infection in the mouth and throat, including Staphylococcus and Streptococcus. The lysozyme, together with a general ‘washing’ action of saliva, and a small amount of hydrogen carbonate ions in it ( which partly neutralizes acids on teeth ) appear to help reduce the incidence of tooth decay. The stomach: When a bolus of food is swallowed ,it is moved swiftly down to the esophagus by peristalsis and into stomach. The stomach is a muscular sac, with a capacity of up 5 dm.. In some parts of the stomach the muscle layers of the muscularis externa are thicker than in most other parts of the alimentary canal. They produce strong, rhythmic, churning movements when there is food in the stomach. This not only mixes the food with the juices secreted in the stomach, but also helps to continue the process of mechanical breakdown begun by chewing in the mouth. The inner layer of the stomach wall, the mucosa, is specialized to produce large quantities of gastric juice, up to 2 dm each day. Gastric juice contains protease and lipase, as well as hydrochloric acid (HCL).To protect the cells in the wall from damage by the acid and proteases, they are covered with a slimy coat of mucus containing hydrogen carbonate ions which neutralize the acid. The protease secrete in gastric juice is pepsin. Pepsin is secreted from large cells in the gastric glands called chief cells .It is secreted in an inactive form, as pepsinogen ,to prevent it from digesting proteins in the cells which produce it. Pepsinogen is a larger molecule than pespsin, and it is activated by removing a strip of several amino acids from it. This happens automatically when it is exposed to the acidic conditions inside the stomach. It is also achieved by pepsin molecules which have already been activated; they ‘digest’ pepsinogen molecules to convert them into more pepsin. Pepsin catalyses the hydrolysis of peptide bonds within protein molecules; it does not break the bonds holding the ‘end’ amino acids of the polypeptide chains. Proteases which do this are called endopeptidases (‘endo’ means ‘within’).Pepsin therefore breaks protein molecules into short chains of amino acids called peptides, but produces almost no individual amino acid molecules. Pepsin molecules are unusual proteins in that they are only stable in acidic conditions. The optimum pH for the pepsin found in the human stomach is about 2 or 3.This is, of course the pH which is found in the stomach when gastric juice has been secreted because this juice contains large amounts of hydrochloric acid. Hydrochloric acid helps to destroy many potentially harmful microorganisms which might be present in food. It is secreted from parietal cells in the gastric glands. The lipase in gastric juice begins to hydrolyze triglycerides into fatty acids and glycerol. However, the majority of the digestion of triglycerides and other lipids happens later, in the small intestine. Gastric juice also contains a substance called intrinsic factor. This is a glycoprotein which binds to vitamin B and protects it from begin digested. Later, in the ileum, the intrinsic factor-vitamin B complex sticks to the surfaces of the cells of the ileum wall, which adsorb it. People who don’t secrete intrinsic factor cannot absorb vitamin B ,however much the eat in their diet. They suffer from pernicious anaemia, an illness in which not enough red blood cells are formed. Food may be kept in the stomach for several hours. The acidic mixture of partly digested food and water, called chyme, cannot pass on the next part of the alimentary canal, the duodenum, until a band muscle called the pyloric sphincter relaxes. When this happens depends on many factors which seem to relate to how quickly the duodenum will be able to deal with what is being sent into it. For example; if there is a lot of fat chyme ,it will be allowed into the duodenum only in small amounts at a time, to give the duodenum a chance to deal with it. The small intestine: The duodenum and the ileum together make up the small intestine. (The first part of the ileum is sometimes known as the jejunum.) The overall length of the small intestine is about 5m, of which the duodenum makes up the first 25m. It is within the duodenum and the ileum that most digestion and absorption occurs. The mucosa of the whole of the small intestine is greatly folded, forming tiny projections called villi. In the duodenum, these are flattened with a rather leaf-like shape, while in the ileum the are more finger-like. A villus is about 0.5mm to 1.0mm long; villi are very thin and make the inner surface of the small intestine look rather like velvet. As in the mucosa layer of all parts of the alimentary canal, this mucosa is made up of three layers an epithelium, a layer of connective tissue and the muscular is mucosa. .The muscles of the muscular is mucosa contract and relax, so that the villi sway about. This helps to bring their surfaces into contact with more of the contents of the small intestine than if they remained still. The cells which make up the epithelium of the villi have a very folded cell surface membrane on the side nearest to the lumen of the small intestine; these little folds are called microvilli. Seen under the microscope, the surface of the cells looks like the bristles of a burst and it is called a bursh border. The villi and the microvilli produce an enormous surface area within the small intestine, which greatly increase the rate at which absorption can take place. In the ‘troughs’ between the villi in the duodenum are glands, known as crypts of Lieberkuhn which secrete mucus. The crypts also constantly produce new cells, which move up the villi until they ‘fall off’ at the top. This constant replacement of the surface cells is essential, as individual cells do not last long. Deeper in the walls of the duodenum, in the submucosa , Brunner’s glands are found. These glands secrete a watery mucus that contains hydrogencarbonate ions to help neutralize the acidic chyme flowing into the duodenum from the stomach. Digestion in the small intestine: Digestion in the small intestine is brought about by enzymes from 2 sources. One of these is the cells which cover the surface of the vili, and the other is the pancreas. The pancreas secretes pancreatic juice. This flows into the duodenum along the pancreatic duct. The pancreas has another role as part of the endocrine system, where it helps in the regulation of blood glucose levels. Pancreatic juice contains hydrogencarbonate ions and a number of enzymes, especially amylase, the three proteases trypsin, chymotrypsin and carboxypeptidase and lipase. The enzyme in pancreatic juice continue to digest the partly digested substances which flow into the small intestine from the stomach. Digestion is completed by enzymes which are produced by the cells on the surface of the villi and remain on their surfaces. Indeed , some of the pancreatic enzymes become absorbed onto these surfaces, so that much of the digestion in the small intestine takes place on the brush border of the villi. This is useful because it means that the products of digestion are right next to the surface across which they can be absorbed, which probably increases the speed at which they are taken up into the cells. Absorption in the small intestine: The small intestine is the area of the alimentary canal in which all absorption of nutrients occurs. The very large surface area provided by the villi, and the microvilli in the surfaces of the cells which cover them, greatly speed up absorption. Inside each villus is blood capillary, which can transfer absorbed nutrients to a branch of the hepatic portal vein. There is also a lymph vessel, called a lacteal, which you will see is important in the adsorption of lipids. To get into either the blood capillary or the lacteal, nutrient molecules must first cross the cells surface membrane on the ‘outer’ surface of the one of the cells on the surface of villus. Then they must cross the cell and leave it across the cell surface membrane on the side furthest away from the lumen. Then they have to across either the wall of the blood capillary or the wall of the lacteal. This last part of the journey does not cause too many difficulties, as these walls are adapted to allow various substance to pass in and out. molecules across the cells surface membranes of the villus cells by diffusion and endocytosis. Glucose is absorbed into the cells by indirect active transport, involving the co-transport of sodium ions. Then the glucose moves out of the opposite side of the cells by facilitated diffusion and simple diffusion ,into the tissue fluid inside the villus and then into blood capillary. Amino-acids are absorbed into the villus cells by active transport and pass out of the opposite side by diffusion. In a fetus a newly born baby , some entire undigested proteins can be absorbed by pinocytosis. This is how babies are able to absorb some of their mother’s antibodies from the milk. This can also happen to a small extent adults. Fatty acids and glycerol are easily absorbed across the cell surface membrane of the villus cells because they are lipid-soluble; they move across by simple diffusion. Once inside the cells, they are taken to the smooth endoplasmic reticulum where some are reconverted to triglycerides. They are moved to the Golgi apparatus, where they are surrounded in a coat of protein, phospholipids and cholesterol to from chylomicrons. These tiny structures ranging from 100 to 600nm in diameter ,are moved out of the far side of the cell into the tissue fluid in the villus by exocytosis. Although they are very small, they are too big to get through the even smaller holes in the walls of the blood capillaries, and so they do not enter the blood. They can, however, readily enter the lacteals .The chylomicrons suspended in the lymph inside the lacteals from a milky emulsion, which is what gives these structures their name. (‘Lact’ means ‘to do with milk’). Considerable amounts of water and inorganic ions, such as sodium, chloride, calcium, and irons are also adsorbed in small intestine .The absorption of calcium is helped by the presence of vitamin D. The adsorption of irons is helped by citrate ions and ascorbic acid, both of which is found in citrus fruits. This is probably why fresh fruits and vegetables in the diet can help to prevent anaemia. On the other hand, drinking to much tea can hinder irons absorption, because tannins in the tea react with irons to produce compounds which cannot be adsorbed. Vitamins are also absorbed in the small intestine. The fat-soluble vitamins A, D, and E can simply cross the cell surface membranes by diffusion; you have seen how bile salts help to bring them to the surface of the villi along with fatty acid. The water –soluble vitamins, such as vitamin C and the many types of B vitamins are moved across the cells surface membranes by specific transport. Vitamin B can only be adsorbed in combination with intrinsic factor. The colon and rectum: By the time the food has reached the end of the small intestine, virtually everything which could be absorbed has entered the villi. What is left? The undigested, unabsorbed remains are mostly fibre; humans cannot digest cellulose or lignin. At the entrance of the colon from the small intestine, there is a blind-ending side branch-the caecum and appendix. The appendix has no function in humans. The colon however is very important indeed as it is here that much of the remaining water is adsorbed into the blood,together with sodium and chloride ions. These processes also occur in the caecum. The colon has no villi but it does have a large surface area produced by many folds in its wall, to increase the efficiency of a absorption. The rectum is a short straight section of the alimentary canal, which leads from the colon to the anus and thus to the outside world. It is usually empty only receiving the contents of the colon now called faeces when they are ready to be passed out of the anus. The control of secretions in the alimentary canal: As food passed along the alimentary canal, numerous sections are produced to help to digest it. It is important that this secretions are only produced when needed. Various mechanism are used to ensure that this happens. Saliva is produced by a reflex actions resulting from a stimulus of the thought sight smell or taste of food. Gastric juice like saliva beings to be secreted even before anything has been eaten just at the smelling food. This impulses came from brain and along a branch of the vagus nerve to the gastric glands. In animals such as dog this impulses also cause the release of a hormone called gastrin. Like all hormones gastrin is secreted into the blood and it is carrying in the blood to the gastric glands. The secretion to the pancreatic juice into the duodenum is controlled in similar way. Most secretions happens when chyme from the stomach enters in duodenum. Acid entering the duodenum causes cells in it wall secrete a hormone called secretin, which is carried in the blood to the pancreas and increases the production and release of pancreatic juice especially rich in ions. Another hormone with a similar effect to secret in is called cholecystokinin or CCK and old name pancreozynim. This hormone it is found in brain and it is secreted by the walls of the duodenum when chyme enters from the stomach. CCK stimulates the secretion of bile, walls of the gall bladder. It is also stimulates the production of pancreatic juice especially rich in enzymes.
