THE DIGESTIVE SYSTEM

Transcription

1 THE DIGESTIVE SYSTEM What is digestion? Digestion is the process of breaking down food so that it's small enough to be absorbed and used by the body for energy or in other bodily functions. Digestion involves a number of different stages. The first phase is known as the cephalic (head) phase. It starts before food has even entered your mouth. The sight, smell, taste or even the thought of food will activate saliva in the mouth as well as digestive juices, which contain enzymes to break down food. In the mouth Once food is in the mouth, the taste buds begin determining the chemicals within the food via their nerve endings, in order to give us the taste sensations of salt, sweet, sour or bitter. As your teeth chew and grind the food, breaking it down, it's mixed with saliva. This comprises many enzymes including salivary amylase, which begins to break down the long chains of starch found in foods such as bread, cereals, potatoes and pasta. Saliva also contains mucin, which moistens the food so it can pass easily through the digestive (gastrointestinal) tract. The esophagus After the food has been swallowed, it's carried down the esophagus (a muscular tube) towards the stomach. The esophagus can contract and relax in order to propel the food onwards, and each mouthful of food takes about six seconds to reach the stomach after it has been swallowed. The stomach The stomach is a sack made of muscle and, when it's empty, it has a volume of only 50ml but this can expand to hold up to 1.5 litres or more after a meal. The walls of the stomach are made of three different layers of muscle that allow it to churn food around and make sure it's mixed with the stomach's acidic digestive juices. The presence of hydrochloric acid in the stomach prevents the action of salivary amylase and helps to kill bacteria that might be present. The stomach also produces the enzyme pepsin, which breaks down proteins (mostly found in meat, fish, eggs and dairy products). The hormone ghrelin is produced by cells lining the stomach. Ghrelin stimulates hunger and tends to increase before a meal and decrease after eating. This hormone forms part of the communication system between the gut and the part of the brain that controls hunger and satiety (how full you feel). Food can stay in the stomach for a few minutes or several hours in the gastric phase where numerous acids and enzymes are released, including the hormone gastrin. When the food has been churned into a creamy mixture known as chyme, the pyloric sphincter (an opening controlled by muscle) opens and chyme passes gradually into the small intestine.

2 The small intestine Prebiotics are mainly indigestible carbohydrates called oligosaccharides. On reaching the large intestine, they selectively stimulate the growth and/or activity of beneficial microorganisms already in the colon, such as bifidobacteria and lactobacilli. About 3ml of chyme is squirted into the small intestine at short intervals as the pyloric sphincter opens. This is known as the intestinal phase and causes the secretion of many hormones, which all aid the digestive process. The sphincter is designed to open partially so that large particles are kept in the stomach for further mixing and breaking down. Digestion and absorption of fats, protein and carbohydrates occurs in the small intestine. Three important organs are involved: 1. The gall bladder provides bile salts that help to make fats easier to absorb. 2. The pancreas provides bicarbonate to neutralise the acidic chyme from the stomach, and also produces further digestive enzymes. 3. The intestinal wall contains cells that make up the wall of the small intestine. These cells help to neutralise the acid and also produce enzymes to digest food. The inner surface of the small intestine is folded into finger-like structures called villi, which greatly increase the surface area available for absorption - in fact the surface area of the villi is equivalent to that of a tennis court! Blood vessels receive the digested food from the villi where it's then transported through the blood stream to the liver via the hepatic portal vein. Probiotics are live bacteria similar to the bacterial micro-organisms that live in the large intestine. They are often referred to as 'friendly bacteria', and come from food sources or dietary supplements. The mix of these 'friendly' bacteria and other gut microorganisms is important for good health, and many factors can alter this delicate balance, such as infection or use of antibiotics. Friendly bacteria are vital for proper development of the immune system, to protect against micro-organisms that could cause disease, and to aid the digestion and absorption of food and nutrients. Fat can take much longer to be broken down, with the process of fat digestion and absorption taking between three and five hours. The unabsorbed residue of this process finally reaches the end of the small intestine and enters the large intestine. The large intestine This is one of the most metabolically active organs in the body. It measures about 1.5 metres and contains over 400 different species of bacteria that break down and utilize the undigested residues of our food, mostly dietary fibres. As the watery contents move along the large intestine, water is absorbed and the final product - faeces - is formed, which is stored in the rectum before excretion from the body.

3 A. Reading Comprehension.. 1. Digestion is the process of breaking down food so that it's liquid enough to be absorbed. T/F? 2. How many stages does digestion involve? 3. The cephalic phase starts after the food has entered the mouth. T/F? 4. Saliva and digestive juices are the same. T/F? 5. Chemicals in the mouth determine the taste of food. T/F? 6. Amylase and mucin are enzymes found in saliva. T/F? 7. Amylase helps to break down the long chains of starch which is found in all foods. T/F? 8. Mucin wets the food so as to help it pass easily through gastrointestinal tract. T/F? 9. The esophagus can open and close in order to push the food towards the stomach. T/F? 10. Each mouthful of food reaches the stomach takes about six seconds in less than a second. T/F? 11. The stomach is a sack made of tissue. T/F? 12. Although the stomach has a volume of only 50ml it can expand to contain up to 1.5 litres or more after a meal. T/F? 13. Food is turned around and mixed in the stomach with acidic digestive fluids. T/F? 14. Bacteria that might be present are destroyed by hydrochloric acid in the stomach. T/F? 15.Meat, fish, eggs and dairy products contain protein which is broken down by pepsin. T/F? 16. The sensation of hunger is communicated by Ghrelin before a meal. T/F? 17. Through the gastric phase, food stays in the stomach for several hours. T/F? 18. Oligosaccharides are carbohydrates. T/F? 19. Indigestible means that it can not de broken down. 20.What other organs are involved in the digestion process? 21. How do the villi look like? What is their function? 22. The surface area of the villi looks like a tennis court. T/F?

4 23. The hepatic portal vein takes digested food to the liver through the blood stream. T/F? 24. The bacteria that live in the large intestine are called probiotics. T/F? 25. Where do probiotics they come from? 26. The use of antibiotics may damage the balance between prebiotocs and other usefull intestinal microorganisms. T/F? 27. The so called Friendly bacteria are important only for the immune system. T/F? 28. Fat is digested and absorbed much slower. T/F? 29. The function of the large intestine is to process the matter that has not been digested. T/F? 30. The large intestine cannot break dietary fibres. T/F? 31. The final product of the large intestine is urine. T/F? B. Scan the text and find the word which means: 1. Substance that is found in foods like pasta, bread, potatoes etc 2. Make or keep something a little wet so that it doesn t dry 3. A tube like a pipe 4. Products made of milk: cheese, butter etc 5. A bitter brownish-yellow or greenish-yellow secretion produced by the liver, stored in the gall bladder, and discharged into the duodenum where it aids the process of digestion. 7. Intestine 8. To set free or let loose

5 9. Something that is or causes good 10. A pear-shaped organ that stores bile from the liver, until the body needs it for the digestion and absorption of fats in the duodenum. 11.Short breaks in a period of time 12. The thick semi fluid mass of partly digested food that is passed from the stomach to the duodenum 13. To cause something to work, function or operate 14. Watery substance in the mouth 15. Very small pumps on the surface of the tongue that identify taste 16. To arouse an organism to functional activity 17. Through 18. To feel something 19. To smash something and make it soft or powdery 20. Include, involve, or have as parts or components 21. Of the liver 22. Very small bodies or parts of sthg 23. A substance that is given as nutritional support 24. Someone or sthg that cannot catch disease 25. Digested waste material discharged from the bowels 26. Whatever remains after something else has been removed. 27. A peptide hormone, secreted in the stomach that increases appetite. 28. The process through which cream is separated into butter and buttermilk through repeated stirring. 29. Single elongated piece of material 30. A globular protein that catalyses a biological chemical reaction.

6 The Digestive System Vocabulary Practice Lab WORD MATCH Match the following words so as to make meaningful phrases 1. gastrointestinal a. products 2. indigestible b. walls 3. bodily c. carbohydrates 4. metabolically d. sensations 5. salivary e. microorganisms 6. dietary f. tract 7. stomach g. stream 8. digestive h. fibres 9. at short i. starch 10. immune j. juices 11. taste k. acid 12. hydrochloric l. supplements 13. chains of m. system 14. dairy n. active 15. taste o. intervals 16. beneficial p. intestine 17. dietary q. buds 18. large r. amylase 19. blood s. food 20. a mouthful of t. functions 1

7 The Digestive System Vocabulary Practice Lab Give a synonym for the following words 1. break down (food) 2. absorb 3. function 4. determine 5. feeling 6. comprise 7. volume 8. hold up 9. expand 10. prevent 11. stimulate 12. increase 13. decrease 14. release 15. gradually 16. growth 17. beneficial 18. particles 19. occur 20. make up 21. equivalent 22. receive 23. outer 24. vital 25. utilize 2

8 The Digestive System Vocabulary Practice Lab WORD PRODUCTION OR DERIVATIVES VERB NOUN ADJECTIVE ADVERB 1. prevent 2. function 3. increase 4. contract 5. receive 6. DIGESTIVE 7. secretion 8. growth 9. expand 10. infect 11. measure 12. metabolism 13. supplement 14. release 15. stimulate 16. absorption 17. occur 18. immune 19. decrease 20. excrete 3