BY: BRIAN LIU Digestion in Humans

Human Digestive System The human digestive system does activities such as: ingestion, digestion, absorption, respiration, movement, circulation, co-ordination, secretion, excretion and reproduction. When we are eating, the food are being chewed then are broken down into small pieces which are acted upon a variety of enzymes secreted into the mouth. Saliva moistens the food and causes chemical digestion. The food then passes through the oesophagus then reaches the stomach. In the stomach, the food are broken down into even smaller pieces which then becomes enzymes. These enzymes break down the protein into smaller molecules, then they are pass onto small intestine.

Mouth & Teeth Humans are omnivores, they have teeth such as: Incisors, Canine, Premolars and Molars. Having these teeth allows them to have a variety of diets. The mouth’s job is to allow the food to be inserted into our body, as well as using the teeth to chew the food, breaking it down into smaller pieces. As the food is breaking down into smaller pieces, the saliva is released from the salivary glands and mixes together with the food. Doing this allows the food to be become soft, and breaks down the mass of the food so its easier to swallow. Saliva contains the enzymes amylase. This enzyme can digest starch molecules and break them down into shorter chain such as dextrins and disaccharides. At the start of the swallowing process, a bolus is forced to the back of the mouth. Once the swallowed process has started, the rest of the process is done automatically.

Oesophagus The oesophagus is a soft, muscular tube. The length of the oesophagus is about 20-23cm long in a fully grown size human. It secretes mucus and transports food from the mouth to the stomach. Muscles in the wall of oesophagus moves the food from the mouth to the stomach by peristalsis. It takes the food about 5 second for the food to be moved from the mouth to the stomach. Peristalsis involves a series of muscular contractions and relaxation, which allows the food to move forward until it reaches the sphincter. The opening to the oesophagus and the trachea are very close to each other. When the food is swallowed, the epiglottis closes the trachea so that food goes down the oesophagus. If the food blocks the trachea, the process of the oesophagus may not function properly, and choking can occur.

The Stomach The stomach is located at the bottom of the chest, which is also just right below the eosophagus. First thing the stomach needs to do when the food travels down from the eosophagus, is to accept and store the food that enters into the stomach. When the food arrives into the stomach, the glands in the lining of the stomach produces gastric acid. The muscles in the stomach helps to move the food and acid around so that they are well mixed. The gastric acid helps to break the food down into even smaller pieces so it is easier to digest. The acid also helps to kill bacteria which are contained in the swallowed food or saliva.

The Small Intestine The small intestine is the longest portion of the digestive system. It is 6.2 meters long, 2.5 centimeters in diameter. The small intestine is split into 3 section: Duodenum, Jejunum and Ileum. When the food enters into the stomach, it is like a thick mixture of soupy mixture. The food remains in the small intestine for about 3 – 6 hours. The small intestine is the digestion of fat, proteins and carbohydrates contained in the food we eat, all of these are done within the small intestine. Nutrients are produced through the digestion, and are absorbed through the lining of the small intestine and transferred to the bloodstream.

Small intestine - Duodenum Duodenum is the link of the stomach and small intestine, it is the first part of the small intestine. It is C-shaped and about 25cm long. Thick liquid mixture of partly digested food and acid from the stomach then enters the duodenum. The acid is neutralised in the alkaline environment. The duodenum also receives bile duct from the gall bladder, and other digestive enzymes from the pancreas. These enter the duodenum through small ducts or tubes. The bulk of the digestion of proteins, fat and carbohydrates takes place in the duodenum before they travel further into the small intestine.

Small intestine - Jejunum and Ileum The Jejunum is the second part/mid section of the small intestine. The Ileum is the final section of the small intestine, which then leads into the large intestine. The ileum is about 2-4 metres in length. The villi which is also known as the inner lining of the jejunum and ileum which contains tiny bumps on the inside of the small intestine. The point of these bumps on the inside of the small intestine means that the surface area is much larger than the lining. The jejunum increase the surface area which then improves the small intestine’s ability to absorb nutrient. The final stage of digestion are completed in small intestine, where foods and liquids are fully broken down into their nutrient components such as: smaller molecules, vitamins, minerals, salt, water etc.

Large intestine The large intestine is the stage where waste products from the food we eat are collected and processed faeces (poo). The large intestine is about 1.5m long and consist of 4 section: Caecum, appendix, colon and rectum. The Large intestines job: - reabsorb water and maintains fluid balance of the body. -absorb certain vitamins - processes undigested material(fibre) -stores waste before it is eliminated

Large intestine - Caecum Caecum is the first section of the large intestine. It has a shape similar to a small pouch and it located in the right lower abdomen. Caecum is the connection between the small intestine and the colon. Cacum jobs is to accept and store the processed material from small intestine and moves it towards the colon. As the food reaches the end of the small intestine, a valve that seperates the small and large intestine opens, the caecum then expands and the materials enter. This stage normally contains mixture such as: -Undigested food -A little bit of water -Some vitamins -Some minerals or salts

Large intestine – Appendix The appendix is a small section that is merged from the caecum. The appendix has no known function and humans thinks that this was a remaining piece from a previous time in human evolution. In some people, the appendix becomes infected or inflamed. The appendix is then needed to be removed surgically.

Large intestine - Colon Colon is the stage where all the mixture of fibres, small amount of water and vitamins etc, start to mix with mucus and with bacteria that lives in the large intestine, then starts to form faeces. As the faeces travels down the colon, the lining of the colon absorbs most of the water and some vitamins and minerals. The bacteria in the colon chemically breaks down some of the fibre in order to produce nutrients for their own survival and to also nourish the cells lining the colon. Therefore, fibre in our diet is extremely important to maintain for long term health of the colon. Faeces is pushed through muscular movements of the colon until it reaches the walls of the sigmoid colon contact, causing the faeces to move into the rectum.

Large intestine – rectum The rectum is the final part of the large intestine. It is where faeces is stored before being passed as a bowel motion.

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