Fundamentals of Human Nutrition/Gastrointestinal system

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3.1 Gastrointestinal SystemEdit

The gastrointestinal tract consists, put simply, of a hollow tube passing through the body. It is an external part of the body, responsible for processing and filtering ingested material. It includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. The Gastrointestinal (GI) tract is a duct that expands through the body and it allows food to enter from one end to the other. The inside of the GI tract is known as the lumen, it extends from one end to another.

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3.1.1 StructuresEdit

The mouth is where the process of digestion all begins. When you put food into your mouth you chemically and mechanically process the food. Chemically your mouth secretes saliva which contains salivary amylase, a main enzyme that is able to catalyze the hydrolysis of starch and make them into sugars. The mouth also mechanically processes your food with your teeth to grind up the food and make it easier to swallow to enter the esophagus. The mouth also contains your tongue which helps to propel the food down your throat to continue with digestion.

Once your food is grinded up by the mouth, it goes through the pharynx which is mainly just a passageway allowing your food to move out of the mouth and into the esophagus. According to Whitney/Rolfes (3.1a) the pharynx shares its functions with both the digestive system and the respiratory system. Since the pharynx is involved with both of these systems, it contains a flap of skin called the epiglottis so it can separate these two systems and to make sure that air gets in one and that food gets in the other one.

Once the food hits the esophagus it becomes a bolus and the food is then moved and able to enter the esophagus through sphincters that open and close in order for the food to get through. There are two sphincters in the esophagus, one is the upper esophageal sphincter to allow the food in the esophagus and then once it is finished it goes through the lower esophageal sphincter or also called the cardiac sphincter because it goes near the heart and through the diaphragm and then is able to enter the stomach. An interesting fact about the esophagus is that it also prevents the stomach contents from refluxing upward according to Mayo Clinic and Oxford Medicine.

Once the bolus enters the stomach it remains in the upper portion and it churns. The stomach adds juice in it in order for it to become more liquid and then it becomes chyme. Not only does it provide lubrication to the food, it also releases enzymes like pepsin as well as a protein to denature proteins with hydrochloric acid. Once the chyme is made, the stomach keeps moving the food through the stomach by ways of peristalsis according to Dr. Gillaspy a professor at the University of Phoenix. Peristalsis is a way that the muscles of the stomach move in order to move and churn the food in the stomach. It is able to churn the food because it contains an extra layer of muscle which is diagonal. The stomach’s main action is to move food not to absorb nutrients, yet. Once the chyme is ready to go into the small intestine, it moves through the pyloric sphincter and out of the stomach it goes.

Small intestine
The small Intestine is the main organ for digestion, it is here that all of the food is digested and absorbed into the body in order to access the nutrients from your food. There are three parts of the small intestine, the duodenum, the jejunum, and the ileum. According to “Understanding Nutrition” by Whitney/Rolfes, the small intestine…” is almost 10 feet of tubing coiled within the abdomen.” (3.1a) The small intestine is very large and folded around the center of your stomach, this is because it needs to have a maximal amount of surface area in order for the food and nutrients to be absorbed optimally. The way the small intestine increases its surface area is by ways of microvilli. There are also juices present in the small intestine that are secreted from the pancreas and the gall bladder that allow the chyme to break down and into attainable nutrients.

Large intestine
After the small intestine absorbs all of the nutrients out of the food you ate, it travels into the large intestine or colon by ways of the ileocecal valve. The large intestine surrounds your body by an ascending colon, transverse colon, descending colon, and lastly the sigmoid colon. The large intestine is also responsible for breaking down fibers into fatty acids and breaking down vitamins. The way that the large intestine achieves this is by having a lot of microflora which are good bacteria. The appendix is also attached to the large intestine. The large intestine ends at the rectum and is then finished at the anus.

The anus is the end of digestion where your food will now be excreted. The muscles at the rectum are under voluntary contractions so that you are able to control it yourself, so once your muscles relax then there are 2 sphincters on the anus that then excrete the waste out of your body.

3.1.2 Accessory organsEdit

Salivary Glands The salivary glands are located in the mouth and throat and can be split into major salivary glands and minor salivary glands. The major salivary glands are known as the parotid, submandibular, and sublingual glands and are located near the upper teeth, under the tongue, and on the floor of the mouth, respectively. There are many minor glands as well located throughout the mouth. The function of these salivary glands is to secrete saliva into the mouth. Saliva has a vast multitude of functions from beginning to breaking down carbohydrates while still in the mouth to protecting teeth by neutralizing acidic substances to assisting in chewing and swallowing. Therefore, saliva plays a vital role in the gastrointestinal system and digestion process.

Liver The liver is located superiorly and to the right of the stomach. It has multiple functions, including several in the digestive system. Primarily, the liver aids in digestion by processing nutrients that the small intestine has absorbed. Using these nutrients, the liver then makes compounds that the body needs to function properly. Another function of the liver in the digestive system is to create bile for secretion into the small intestine where is assists in fat digestion by breaking fats down into liquid form so that the intestinal enzymes can continue in digestion. Bile is stored in the gallbladder and is secreted through the bile ducts when a person eats.

Pancreas The pancreas is located under the stomach and is almost surrounded by it as well. This gland’s main function in the digestive process is to create a pancreatic juice that contains digestive enzymes to break down macromolecules such as fats, carbohydrates, and proteins. These digestive enzymes are then secreted through ducts into the duodenum, which is the first portion of the small intestine. Once there, the pancreatic enzymes are then able to aid in the digestion of carbohydrates, fats, and proteins. Another function of the pancreas is to secrete insulin, which is a hormone that assists in the metabolism of sugars.

Gallbladder The gallbladder is an accessory organ to the gastrointestinal system that is located under the liver and is slightly covered by it. The function of the gallbladder is to store the bile that the liver creates between meals. Once a person begins to eat again, the gallbladder’s role is to squeeze the bile out so that it is secreted through the bile ducts into the small intestine. Once the bile is in the small intestine, it can begin to assist with the digestion of fats in the small intestine.


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