Digestive System PDF

## The structure, function and main disorders of the digestive system **Alimentary canal** The alimentary canal is a tube that extends from the mouth to the anus. It is dilated, folded and puckered in various places along its length. Many glands are associated with the alimentary canal and they ha...

## The structure, function and main disorders of the digestive system **Alimentary canal** The alimentary canal is a tube that extends from the mouth to the anus. It is dilated, folded and puckered in various places along its length. Many glands are associated with the alimentary canal and they have important roles to play in digestion. **When food is taken into the mouth it is mixed with saliva, chewed (or masticated) by the action of the tongue and teeth, rolled into a small ball known as a bolus, and swallowed. This process is called mechanical digestion and it is an important part of physically breaking the food down at an early stage.** **The salivary glands** There are three pairs of salivary glands that pour their secretions, known as saliva, into the mouth. Saliva contains an enzyme known as salivary amylase which begins the digestion of carbohydrates, as well as lubricating the mouth and helping bolus formation. **The oesophagus ** The oesophagus (or gullet) transports the food bolus from the back of the mouth (the pharynx) to the stomach in the abdomen. The swallowed bolus is in the oesophagus for a few seconds only and no enzymes are secreted here, although salivary amylase will continue to act during this brief journey. The oesophagus is mainly a transit for food boluses, which it moves by muscular contractions known as peristalsis. A moveable flap called the epiglottis covers the opening of the trachea during swallowing to prevent food entering the respiratory system. **The stomach** The stomach is the widest part of the alimentary canal, tucked mainly behind the rib cage under the diaphragm on the left side and receives food from the mouth by way of the oesophagus. During this time, the strong stomach walls roll and churn the food around and pour on secretions from the gastric glands. The resulting paste-like material is called chyme. **Gastric glands produce gastric juice that contains the enzyme gastric protease and hydrochloric acid. The gastric juice works on proteins. In babies, another enzyme, rennin, solidifies and digests milk protein.** **The pH of the stomach is 1-2 which is strongly acidic. The epithelial lining of the stomach contains goblet cells which produce thick mucus to protect the stomach lining from acid erosion. The stomach empties the chyme in spurts into the duodenum through the pyloric sphincter, a thick ring of muscle that alternately contracts and relaxes.** **The duodenum** The next part of the alimentary canal is the small intestine, so-called because of its small diameter - not its length, for it is around 6 metres long! The first C-shaped part, and the shortest, is called the duodenum. The duodenum is mainly concerned with digestion and is helped by two large glands, the liver and the pancreas, that pour their secretions or juices into this area. The duodenal wall also contains glands which secrete enzyme-rich juices that continue the digestive process on carbohydrates and lipids (or fats). **The ileum** The remainder of the small intestine, known as the ileum, is mainly concerned with the absorption of the now fully digested food. It is specially adapted for this by its: - long length - folded interior - lining covered in many thousands of tiny projections called villi - epithelial cell lining being covered in microvilli, projections so small that they can only be detected using an electron microscope. These adaptations enormously increase the surface area for absorption of nutrients from digested food. **The colon** In the right-hand lower corner of the abdomen, the small intestine meets the large intestine. There are two biological remnants at this point, the caecum and the appendix. The colon runs up the right side of the abdomen and turns to travel across to the left side before ending at the anus. There are no enzymatic juices in the large intestine. The colon has a puckered appearance because the outer longitudinal muscle coat splits into three bands and the circular muscle bulges out between the bands. **Faeces contain:** - cellulose (fibre or roughage) from plant cell walls left after digesting fruit and vegetables - dead bacteria, including the usually harmless bacteria living in the large intestine that have died a natural death, and other bacteria, which are often killed by the hydrochloric acid in the stomach - cells scraped off from the gut lining. Mucus, secreted by enormous numbers of goblet cells in the gut lining, reduces friction as chyme and waste are moved along by peristalsis. **Key Term:** **Emulsification - occurs when an emulsifier causes oil or lipids to be suspended as a large number of tiny globules in water.** **The liver** The liver is a large, dark-red organ occupying the top right half of the abdomen and partly overlapping the stomach. It has many vital functions in the body, one of which is to produce bile. Bile flows down the bile duct into the duodenum, after temporary storage in the gall bladder on the under surface of the liver. The liver also removes glucose and other sugars from the blood coming from the small intestine and converts them into glycogen for storage. Surplus amino acids that are not required for manufacturing cell proteins are broken down in the liver to form glycogen and urea. **The pancreas** The pancreas is a slim, leaf-shaped gland located between the intestines and the stomach, close to the duodenum. It secretes enzyme-rich pancreatic juice as well as alkaline salts needed to neutralise the acidic secretions from the stomach. Pancreatic enzymes go to work on all three macronutrients (protein, fat and carbohydrate) and are important agents for the complete breakdown of complex food molecules into amino acids, glucose and similar simple sugars, fatty acids and glycerol. The pancreas also makes the hormones insulin, required to metabolise glucose, and glucagon, required to release stored glucose, in the islets of Langerhans. **Breakdown and absorption of food materials** **Ingestion, digestion, absorption, egestion** It is vital to understand that without the organs and glands of the digestive system, you would be unable to use the substances collectively called food by means of digestion. Taking food in through the mouth (what is called 'eating') is known technically as ingestion. **Key Terms:** - **Digestion-** conversion of food into simple, soluble chemicals capable of being absorbed through the intestinal lining into the blood to be used by body cells. - **Ingestion-** taking in food, drink and drugs by the mouth. - **Absorption-** taking up of substances to be used by the body cells and tissues. - **Egestion-** process involved in eliminating waste material from the body as faeces **Peristalsis** Food and chyme move down the alimentary canal by a process known as peristalsis. Note that in there are two sheets of muscle surrounding the tube - one sheet runs in a circular fashion around the tube while the other runs down the tube. **The role of enzymes in digestion** To break down large complex molecules in the laboratory you would use heat (as in cooking) or add chemicals such as acids or alkalis. Body cells are able to produce substances called enzymes that can alter the rate of chemical reactions to build up or break down other molecules, without using heat or harmful chemicals. Enzymes are biological catalysts. This means that they are substances that can act within living organisms to enable the breakdown or building-up of other chemicals, but they remain unchanged themselves at the end of the reactions or tasks. **Major products of digestion** **Roles in the body, storage and deamination** - **Peptides and amino acids** are nitrogenous compounds. They travel via the bloodstream to areas of need in body cells. They are important in making enzymes, some hormones, plasma proteins, new cells (growth) and in repair processes. Surplus amino acids are broken down in the liver as they cannot be stored. Some parts of the molecules are used for energy but the nitrogen-containing part is converted into urea in the liver by a process called deamination, and excreted by the kidneys in urine. - **Sugars, chiefly glucose** are transported to cells to be broken down in internal respiration to release energy. Excess carbohydrate is stored in liver and muscles as glycogen or converted into fat to be stored around organs or under the skin. Glycogen is converted back to glucose when energy is required to top up the blood glucose supply to cells, or for muscle contraction. - **Glycerol** is used for energy, or converting fatty acids into a form of fat that can be stored. - **Fatty acids** travel from the lacteals through the lymphatic system into the main veins of the neck. This circuitous route enables smaller quantities of potentially harmful lipids to enter the circulation gradually. Fatty acids are used in internal respiration to release energy to drive metabolic processes. - **Fat** is stored under the skin and around organs, where it forms a long-term energy store to be used after glycogen stores are depleted. **The absorption of food** The ileum is mainly concerned with the absorption of the now fully digested food. **Disorders of the digestive system** There are many disorders that can affect the digestive system, ranging from mouth ulcers to various types of cancer. **Ulcers** Ulcers can occur singly or in groups and occur mainly in the mouth, stomach and duodenum. An ulcer is a localised inflammation that has destroyed a small area of skin or mucous membrane, producing an open sore. **Hepatitis** This is inflammation of the liver and can be caused by viruses or chemical substances, including alcohol. The symptoms of hepatitis may include nausea, vomiting, lack of appetite and jaundice - a yellowing of the skin and white of the eyes, and passing of dark-brown urine. The condition can be acute or chronic (long-term). **Coeliac disease** Coeliac disease is an autoimmune disease that runs in families and so is thought to have a genetic base. An intolerance of gluten, a protein found in wheat and other cereals, triggers an immune response that can lead to loss of weight, some vitamin deficiencies and often diarrhoea.