Fundamentals of Human Nutrition/Digestion

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3.2 DigestionEdit

Digestion is the process of breaking food into components small enough to be absorbed by the body (Whitney 72).

5.2.1 PhasesEdit

Food enters the mouth where it is tasted and the mechanical breakdown, by saliva, and chemical digestion begins. Secondly, the tongue moves the food back towards the pharynx, which is shared by the digestive and respiratory tracts. Thirdly, a valve-like flap called the epiglottis blocks the air passages during swallowing to allow the food to take the right path down the esophagus and into the stomach. Next, the food enters the stomach by passing through a sphincter, which is a muscle that encircle the tube of the digestive tract and acts like a valve. The food is then mashed up and mixed with acid to turn into a semi-liquid food mass called chime. The chime leaves the stomach and enters into the small intestine, where the main site of digestion and absorption of nutrients occurs. The small intestine also decreases the stomach motility and slows the secretion of gastric juices. From here, any materials that are no absorbed in the small intestine move on to the large intestine via the ileocecal valve. This valve does not let materials to re-enter the small intestine. In the large intestine, most water and vitamins are absorbed. Finally, anything that is not absorbed passes through the anus and exits as feces (Whitney 72, 73).

5.2.2 Mechanical processesEdit

The chewing of food begins the mechanical process of digestion. It makes food easier to swallow and increases the surface area in contact with digestive juices. Moreover, the tongue uses its mechanics to push the food to the back of the throat to the pharynx. Next, the esophagus moves the food down to the stomach through rhythmic contractions of the smooth muscles, which is called peristalsis (Whitney 75). As the food moves through the digestive tract, peristalsis is a significant mechanical process. Digestion: Muscular action Food travels through the gastrointestinal tract through many different muscular actions. “Gut motility is the term given to the stretching and contractions of the muscles in the gastrointestinal tract” (Kellow and Parkman, 2014). When food first enters the mouth, it is chewed up into many small pieces so that swallowing is easy. After food is swallowed, the autonomic muscles of the GI tract take over. The first muscular action in the GI tract is peristalsis. There are circular muscles in the GI tract and longitudinal muscles that surround the circular muscle. These muscles act together to constrict the GI tract and push food through the tract. “Factors such as stress, medicines, and medical conditions may interfere with normal GI tract contractions” (Whitney, 2013, 3.1b).

Once the food reaches the stomach, another muscular action begins. The main function of the stomach is to “churn food into a consistency that is easier for intestines to digest” (“The Stomach and its Role in Digestion”, 2015). To aid in the major breakdown of the food, “the stomach has the thickest and strongest walls of all the GI tract organs” (Whitney, 2013, 3.1b). The stomach has three types of muscles: circular, longitudinal, and diagonal. These three muscles work together to push the food, which becomes chyme in the stomach down throughout the stomach. The chyme is then moved into the intestines to absorb the essential nutrients and then moved to be excreted. Once in the intestines, the muscular action of segmentation is implemented. The circular muscles on the walls of the intestines contract and squeeze and mix the chyme. The chyme is mixed with certain stuff that break down nutrients and absorb them.

The last muscular action the GI tract implements to move food along is sphincter contractions. There are many sphincter muscles throughout the GI tract that are regulated to open and close in order to move food through the GI tract and block movement of food back up the GI tract. The first sphincter is at the top of the esophagus, called the upper esophageal sphincter. This sphincter is activated when you swallow food. When the food gets to the end of swallowing, it hits the lower esophageal sphincter, or cardiac sphincter, allows the food go into the stomach and blocks the food from going back up into the esophagus. The pyloric sphincter, at the bottom of the stomach, allows chyme to go into the intestines and blocks it from going back into the stomach. The ileocecal valve in the small intestine allows chyme to flow into the large intestine. At the end of the tract, the two sphincters of the anus and the tightness of the rectal muscles keeps waste inside the body until it is excreted.

5.2.3 SecretionsEdit SalivaryEdit

The salivary glands moisten food and helps us taste and swallow the food we ingest (Whitney 76).. The enzyme, salivary amylase, that is secreted out of the salivary glands help with the digestion of starch and other carbohydrates, as well as cleanses the mouth, and protects teeth from decay (Whitney 76). Also, it lubricates the upper GI tract. GastricEdit

Gastric acid is secreted in the stomach. It helps in digestion by creating the ideal pH for pepsin and gastric lipase and by stimulating pancreatic bicarbonate secretion. Additionally, the arrival of protein in the stomach further encourages gastric output (DiMarino). PancreaticEdit

Secretes bicarbonate to neutralize intestinal contents. Also produces enzymes that digest carbohydrates.

5.2.4 RegulationEdit HormonesEdit

Released in the blood to regulate activity in the GI tract. They are called enterogastrones. These include, gastrin, which is secreted by stomach, secretin, which is secreted by the duodenum, and the pancreatic secretions vary depending on the food content. These hormones, along with many more, serve to prepare different part of the gut for the arrival of food, in addition to regulation of the digestion of nutrients and the rate at which food moves through the system (Overview of Gastrointestinal Hormones) Nervous systemEdit

Nerve cells help to regulate activity in the GI tract. The sight and smell of food, as well as the presence of food in the gut, stimulates nerves. Nerve signals cause muscle contractions that churn, mix and propel food through the gut at a rate that allows for the absorption of nutrients. Additionally, they stimulate or inhibit digestive secretions (The Enteric Nervous System). 3.3 Absorption Process of taking substances into the interior of the body.