...e.combiosciapdynamichuman2contentgifs0123.gif t MERGEFORMATINET the alimentary canal, but it alays contains cells hich secrete mucus.Beneath it is a layer of connective tissue called lamina propria, hich 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 to 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 ith a single layer of thin, smooth closely fitting cells. INCLUDEPICTURE http.mhhe.combiosciapdynamichuman2contentgifs0120.
gif t MERGEFORMATINET e can observe in detail each part of the human alimentary canal structure in this diagram.The mouthTaking food into the mouth is called ingestion. e use lips, tongue and teeth. The tongue is also important in tasting food, to tell you hether it is good to eat if not it ill be ejected from the mouth rather than salloed.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 cheing, or mastication. Strong muscle is the jas move the loer ja up and don from side to side, grinding the teeth in the loer ja against those in the upper ja.The premolar and molar teeth have ridges and grooves, hich trap food beteen them and crush it as che. Mastication greatly increases the surface area of the food, bringing more of it into direct contact ith enzymes in the digestive juice and so speeding up chemical digestion.Three pairs of salivary glands secrete atery liquid saliva, hich pours along ducts into the mouth. Like all secretions along the alimentary canal, saliva is mostly ater. It contains mucus, hich mixes ith the food as it is cheed, helping to glue it loosely together into a ball called a bolus. The mucus also makes the bolus slippery, so that is easier to sallo.Saliva contains the enzyme amylase, hich 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, hich is also found in tears, can destroy several types of bacteria hich can cause infection in the mouth and throat, including Staphylococcus and Streptococcus. The lysozyme,together ith a general ashing action of saliva, and a small amount of hydrogen carbonate ions in it hich partly neutralizes acids on teeth appear to help reduce the incidence of tooth decay.The stomachhen a bolus of food is salloed ,it is moved siftly don to the esophagus by peristalsis and into stomach.The stomach is a muscular sac, ith 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 hen there is food in the stomach.This not only mixes the food ith the juices secreted in the stomach, but also helps to continue the process of mechanical breakdon begun by cheing in the mouth.The inner layer of the stomach all, the mucosa, is specialized to produce large quantities of gastric juice, up to 2 dm each day. Gastric juice contains protease and lipase, as ell as hydrochloric acid HCL.To protect the cells in the all from damage by the acid and proteases, they are covered ith a slimy coat of mucus containing hydrogen carbonate ions hich 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 hich 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 hen it is exposed to the acidic conditions inside the stomach. It is also achieved by pepsin molecules hich have already been activated they digest pepsinogen molecules to convert them into more pepsin.Pepsin catalyses the hydrolysis of peptide bonds ithin protein molecules it does not break the bonds holding the end amino acids of the polypeptide chains. Proteases hich do this are called endopeptidases endo means ithin.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 hich is found in the stomach hen gastric juice has been secreted because this juice contains large amounts of hydrochloric acid. Hydrochloric acid helps to destroy many potentially harmful microorganisms hich 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. Hoever, 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 hich 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 all, hich adsorb it. People ho dont secrete intrinsic factor cannot absorb vitamin B ,hoever much the eat in their diet. They suffer from pernicious anaemia, an illness in hich 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 ater, called chyme, cannot pass on the next part of the alimentary canal, the duodenum, until a band muscle called the pyloric sphincter relaxes. hen this happens depends on many factors hich seem to relate to ho quickly the duodenum ill be able to deal ith hat is being sent into it. For example if there is a lot of fat chyme ,it ill be alloed into the duodenum only in small amounts at a time, to give the duodenum a chance to deal ith it. The small intestine The duodenum and the ileum together make up the small intestine. The first part of the ileum is sometimes knon as the jejunum. The overall length of the small intestine is about 5m, of hich the duodenum makes up the first 25m. It is ithin the duodenum and the ileum that most digestion and absorption occurs.The muc...
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