# 3.1 Gastrointestinal System

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.

## 3.1.1 Structures

Mouth 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 catalyzes the hydrolysis of starch, creating sugars. The production of saliva is triggered before the food even enters your mouth. Once the food enters your mouth, saliva is even further produced. The mouth also mechanically processes your food with your teeth to grind up the food, making it easier to swallow and enter the esophagus. The mouth also contains your tongue, which helps to propel the food down your throat to continue digestion.
Pharynx Once the food is ground up by the mouth, it goes through the pharynx, or your throat – a passageway that allows the 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 that separates the two systems and ensures that air enters one while food enters the other.
Esophagus Once the food reaches the esophagus it becomes a bolus. It moves and enters 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 that allows the food to enter into the esophagus, and the other is the lower esophageal sphincter, or the cardiac sphincter, which goes near the heart, through the diaphragm, and then enters 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.
Stomach Once the bolus enters the stomach it remains in the upper portion and churns. The stomach adds acid to it so that it can become more of a liquid, and then it becomes chyme. Not only does it provide lubrication to the food, but it also releases the proteolytic enzyme pepsin and hydrochloric acid to denature proteins and other components. Once the chyme is made, the stomach keeps moving the food along 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 that 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.
Small intestine The small intestine is the main organ for digestion. It is here where the food is digested and absorbed into the body in order to access the nutrients from your food. The inner walls of the small intestine absorb the nutrients into the blood stream. The absorption is attained through passive diffusion, facilitated diffusion, or active transport. 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 abdomen; 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 pancreatic enzymes and liver bile brought to the small intestine to allow the chyme to break down into attainable nutrients.
Large intestine After the small intestine absorbs all of the nutrients out of the food, it travels into the large intestine, or colon, by ways of the ileocecal valve. The large intestine, although shorter in length than the small intestine, is thicker than the small intestine. It surrounds your body with the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon. The small tube known as the appendix is attached to the cecum. 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 through its abundance of microflora, good bacteria. Additionally, any leftover fluid is absorbed here. The large intestine ends at the rectum, and digestion is finished at the anus.
Anus The anus is the end of digestion where food is to be excreted. The muscles at the rectum are under voluntary contractions, so as to allow a person to control it himself. These muscles include pelvic floor muscles and two anal sphincters. Once your muscles relax, the sphincters on the anus excrete the waste from your body.

## 3.1.2 Accessory organs

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.

## References

Mandal A. What Does the Small Intestine Do? News-Medical.net. http://www.news-medical.net/health/what-does-the-small-intestine-do.aspx. Accessed May 29, 2016.
Pedersen, A., Bardow, A., Jensen, S., & Nauntofte, B. (2002). Saliva and gastrointestinal functions of taste, mastication, swallowing and digestion. Oral Diseases, 117-129. Retrieved November 20, 2015, from http://onlinelibrary.wiley.com/doi/10.1034/j.1601-0825.2002.02851.x/full
Salivary Glands. (2014, April 21). Retrieved November 28, 2015, from http://www.entnet.org/content/salivary-glands
Taylor T. Large Intestine. InnerBody. http://www.innerbody.com/anatomy/digestive/large-intestine. Accessed May 29, 2016.
The Digestive System. WebMD. http://www.webmd.com/digestive-disorders/digestive-system. Accessed May 29, 2016.
The Structure And Function Of The Digestive System. (n.d.). Retrieved November 30, 2015, from https://my.clevelandclinic.org/health/ns_overview/hic-the-structure-and-function-of-the-digestive-system
Your Digestive System and How It Works. (n.d.). Retrieved December 1, 2015, from http://www.niddk.nih.gov/health-information/health-topics/Anatomy/your-digestive-system/Pages/anatomy.aspx