Introduction
The digestive system, which is also known as the gastrointestinal tract, is a process where food is taken into the body through the mouth, which is then chewed and swallowed with the help of the mouths saliva glands. It then travels down the oesophagus into the stomach where there is gastric liquid to help break down the bolus so it ends up like a soup consistency, this then passes through the duodenum and carries on through the small and large intestine’s, and finally is dispersed into the blood system. Any substance that is left will be passed through the alimentary canal and will be eliminated as faeces.
Mouth, teeth, tongue
There are five main areas of the digestive system, these are ingestion, propulsion, digestion, absorption and elimination. This starts in the mouth where the chemical digestion process begins. When food and drink is taken into the mouth, three sets of salivary glands start the process of breaking down the food, these glands are called the parotid gland ( which is just under the ear ), The submandibular gland and the sublingual gland ( which are both under the tongue).
Saliva is made up of mucus, water and an enzyme called salivary amylase. The saliva allows the food to be swallowed as it is a wet consistency, and it also keeps the mouth clean and away from any risk of infection. Food can be kept in the mouth for different periods of time, this is due to how much food is in the mouth at one time, and it can also depend on the different types of food, as some food is harder to chew which takes longer, for example, meat which is a form of protein, can take longer than vegetables or potato’s, which are a form of carbohydrates.
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The enzymes are a clever part of the chemical digestion, as they break down the food so it becomes small enough to pass down the oesophagus, which is the hose pipe like tube which leads from the mouth down into the stomach. The tongue is made of muscle which is covered with membrane. It is used to push the food down from the teeth to the pharynx, and down to the oesophagus, when the food reaches here and is covered with saliva it becomes a bolus. This process is called mastication. When the bolus is ready to be swallowed the cardiac sphincter is relaxed enough to let the bolus pass down into the stomach. The epiglottis is part of the larynx and helps by preventing food from going down the wrong way.
Oesophagus
The oesophagus is on average 25 cm’s long. It has muscular contractions, and produces more mucus to help the chewed food to pass through to the stomach by movement called peristalsis. It is a pipe that travels from the pharynx to the stomach. Food is passed through the oesophagus with the help of a wet lining, enabling it to pass with ease. The oesophagus starts and ends with sphincters which deters air from getting in.
Stomach
The stomach is almost in the shape of the letter j, which contains several layers of tissue, these layers are the mucosa, the sub mucosa, the muscularis and the peritoneum. The stomach stomach has three different areas, these are called the fundus, the body and the antrum. When the stomach has nothing in it the pyloric sphincter does not do anything, therefore leaving it open, but when the stomach is full of food the sphincter shuts. The inside of the stomach contains three different muscle layers, these are the outer layer of longitudinal fibres, the middle layer of circular fibres and an inner layer of oblique fibres. The pyloric sphincter and the pyloric antrum contain the most powerful muscle, it is called the circular muscle.
When the bolus enters the stomach it mixes with gastric juices to stop the salivary amylase working, and a peristaltic motion pushes the bolus in the direction of the pylorus. The peristaltic motion causes the bolus to turn into chyme which is passed into the duodenum. The stomach contains pepsin which digests protein, and it also contains gastric lipase which breaks down the fat. The stomach does not absorb a lot of alcoholic products, and also some drugs and water, it also produces the instrinct factor which is required to absorb the vitamin b12. There are lots of gastric glands which can be found in the mucous membrane, the mucous membrane lining is called a rugae when there is nothing in the stomach, and it folds up into a pattern (almost like a vienetta slice), and then back to normal which is like a silky plane. The blood supply from the stomach travels through different named arteries which are attached to the main vain. This is the main way that the vitamins, minerals and all the goodness from food gets into the system. The gastric juice is made up of water, mineral salts, instrinct factor, mucus, hydrochloric acid and active enzyme precursors. The hydrochloric acid is responsible for attacking the food and stopping the salivary amylase from working, and also helps the pepsins. Pepsins break down the protein into little molecules, the molecules work the best when they are at a p.H level of 1.5 to 3.5. The water is responsible for diluting the food further, to ensure that it turns into a runny liquid substance. Gastric juice can be found in the stomach at all times, as a little bit will stay there even when it is empty, as it acts as a fasting juice. The cephalic phase, the gastric phase and the intestinal phase, are the three different phases of the gastric juice secretions.
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Small Intestin
The small intestine is just over five metres long, and is located in the abdominal cavity. It is in the middle of the large intestine, and is where the chemical digestion of food ends. It is divided into three parts, these parts are called the duodenum, the jejunum and the ileum. The duodenum is around 25 cm long and the hepatopancreatic sphincter surrounds it. The jejunum is around two meters long and the ileum is around three metres long. The digestion and absorption take place by peristaltic movements, which combine bile and pancreatic fluid. The enzymes which are in the pancreatic juices are called trypsin (which is an amino acid), lipase (which converts fats to glycerol and fatty acids) and amylase (which converts polysaccharides to disaccharides).
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The amino acids end up in the liver through the hepatic portal vein. Intestinal secretions are made up of mucus, mineral salts and water. There are lots of enzymes in the small intestines which can be found in the villi. The enzymes that end the chemical digestion are peptidases, lipase, sucrose,maltase and lactase.
Large intestine
The large intestine is full of bacteria but this isn’t dangerous, in fact some bacteria can sometimes be good bacteria. The large intestine is around one and a half metres long and starts at the caecum and ends at the anal canal, which is found way down in the pelvis. The vermiform appendix leads from the caecum and is around eight and a half centimetres long. The large intestine surrounds the small intestine and is wider. The large intestine is also made up of the colon, rectum, anal canal and the anus. The large intestines main role is to absorb water and get all the goodness from the digested food, and then get rid of the waste by a big wave of peristaltic movement. This then ends up as faeces and is passed through to the anus.
The digestive system is a very important role of the whole body. It is very clever how it extracts all the goodness out of the protein, fat, carbohdrates,vitamins and minerals that we eat. If we didn’t take in all the requirements for a healthy and balanced diet, then our body would cease to exist.
Reference List
Waugh, A., Grant, A. (2006) Ross and Wilson Anatomy And Physiology in health and fitness. Tenth Edition. Philadelphia: Elsevier.
Tucker, L. (2000) An Introductory Guide to Anatomy and Physiology. Revised Edition. Cambridge: Ruben Publishing.
