Health_and_Social_Care_Unit_16_Ao1_Digestive_System

The Digestive System What is digestion' Digestion is the process by which food and drink are broken down into their smallest parts so the body can use them to build and nourish cells and to provide energy. Why is digestion important' When you eat your food, it is not in a form that the body can use as nourishment. Food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion involves mixing food with digestive juices, moving it through the digestive tract, and breaking down large molecules of food into smaller molecules. Digestion begins in the mouth, when you chew and swallow, and is completed in the small intestine. The Mouth Once food is in the mouth, the taste buds begin determining the chemicals within the food thru their nerve endings, in order to give you 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 contains many enzymes including salivary amylase, which begins to break down the long chains of starch found in foods (such as bread, cereals, pasta and potatoes) Saliva also contains mucin, which moistens the food so it can pass easily through the digestive (gastrointestinal) tract. The Oesophagus After the food has been swallowed, it's carried down the oesophagus (a muscular tube) towards the stomach. The oesophagus can contract and relax in order to propel the food onwards. 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 (proteins are 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 how full you feel. When the food has been churned into a creamy mixture known as chyme, an opening controlled by muscle (the pyloric sphincter) opens and the chyme passes gradually into the small intestine. The small intestine Prebiotics are mainly indigestible carbohydrates called oligosaccharides. On reaching the large intestine, they selectively stimulate the growth and activity of beneficial microorganisms already in the colon, such as bifidobacteria and lactobacilli. 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: * The gall bladder provides bile salts that help to make fats easier to absorb. * The pancreas provides bicarbonate to neutralise the acidic chyme from the stomach, and also produces further digestive enzymes. * 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. 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. Pancreas The pancreas secretes digestive enzymes into the duodenum, the first segment of the small intestine. These enzymes break down protein, fats, and carbohydrates. The pancreas also makes insulin, secreting it directly into the bloodstream. Insulin is the chief hormone for metabolizing sugar. Liver The liver has multiple functions, but its main function within the digestive system is to process the nutrients absorbed from the small intestine. Bile from the liver secreted into the small intestine also plays an important role in digesting fat. In addition, the liver is the body’s chemical "factory." It takes the raw materials absorbed by the intestine and makes all the various chemicals the body needs to function. The liver also detoxifies potentially harmful chemicals. It breaks down and secretes many drugs. Gallbladder The gallbladder stores and concentrates bile, and then releases it into the duodenum to help absorb and digest fats. 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 utilise 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. Rectum The rectum is an 8-inch chamber that connects the colon (large intestine) to the anus. It is the rectum's job to receive stool from the colon, to let the person know that there is stool to be evacuated, and to hold the stool until evacuation happens. When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters relax and the rectum contracts, disposing its contents. If the contents cannot be disposed, the sphincter contracts and the rectum accommodates so that the sensation temporarily goes away.