# 3.2 Digestion

Digestion is the process of breaking food into components small enough to be absorbed by the body (Whitney 72). Digestion is the body’s way of getting ready for absorption by obtaining nutrients from food sources. There are bacteria located in the digestive tract that are apart of the human normal flora that help aid in many digestive processes. Multiple enzymes, hormones, and organs work together that makes digestion a daily process. Digestion is important to the human body because the body uses the nutrients from foods for body functions such as cell repair, growth, and energy to keep us going (National Digestive Diseases Information Clearinghouse, 2013). As digestion starts from beginning to end, large food particles are broken down into smaller particles by many gastric secretions to be absorbed by the small intestine. The waste product of digestion results in feces (National Digestive Diseases Information Clearinghouse, 2013).
The digestive process faces many obstacles during its duration such as food and liquid must be separated from the passageway of breathing so that we don’t choke and food nor liquid goes into the lungs. The body must have an entrance into the stomach passing through the diaphragm. The food taken in through the digestive system needs to be kept moving toward the small intestine at a steady pace so that all processes can take place properly (Whitney & Rolfes, 2013).

## 3.2.1 Phases

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).

## 3.2.2 Mechanical processes

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.
Chewing
Chewing begins the mechanical process during digestion. Food is broken into smaller pieces that are easier to swallow by the chewing of the teeth, saliva, and the actions of the tongue (Whitney, 75). After swallowing, the rest of the mechanical processes no longer require much conscious thought and the body handles the nutrients as needed.
Peristalsis
Longitudinal muscles surround circular muscle rings, which line the entire GI tract. The tube becomes constricted when the long muscles relax and the rings tighten. Peristalsis is when the long muscles tighten and the rings relax, creating a bulge (Turley, 101). Peristalsis is always occurring and pushes the contents in the intestines through.
Stomach Action
The stomach has both the circular and longitudinal muscles that are seen in the other GI tract organs. However, the stomach also has diagonal muscles. These muscles also contract and relax, like the others. These three muscles push the chyme downward towards the pyloric sphincter. Chyme is a predigested that is formed by mixing chemicals in the stomach (Tortora, 966). The pyloric sphincter prevents chyme from getting into the duodenum of the small intestine. While the chyme is at the bottom of the stomach waiting for the pyloric sphincter to open, the stomach wall will release gastric juices. These juices will liquefy the chyme completely. Once this happens, the pyloric sphincter will open. It opens around three times per minute and only lets a small portion of chyme into the intestine at a time (Whitney 75).
Segmentation
The intestines have circular muscles, which contract and squeeze the contents within them. Segmentation is the contractions that occur in the intestines. These contractions mix the chyme with gastric juices, as well as allowing the absorbing cells of the wall to meet with the chyme. (Whitney 75)
Sphincter Contractions
There are several sphincters located along the digestive tract. They open and close to allow the GI contents to move along at a controlled pace. First, there is the esophageal sphincter, which opens when someone swallows. Then, there is the lower esophageal sphincter that prevents reflux. It is also called the cardiac sphincter since it is close to the heart, which is why the burning in the esophagus from reflux is commonly known as “heartburn.” The pyloric sphincter keeps the chyme in the stomach long enough to be completely liquefied while also preventing the intestinal contents from backing up into the stomach. The ileocecal valve is located at the end of the small intestine. Its purpose is to allow contents to move from the small intestine to the large intestine. The rectal muscle and the two sphincters of the anus prevent the continuous elimination of waste (Whitney 75).

## 3.2.3 Secretions

There are five organs that contribute secretions during digestion: Salivary glands, the stomach, pancreas, liver, and the small intestine (Whitney & Rolfes, 2013).

### 3.2.3.1 Salivary

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.Salivary glands break down starches and make swallowing easier by secreting saliva. Saliva has a combination of water, salt, mucus, and different enzymes that aid in carbohydrate digestion. Also, saliva protects the teeth and linings of many digestive organs from damaging particles (Whitney & Rolfes, 2013). Salivary Amylase starts digestion in the mouth of starch and glycogen, and then pancreatic amylase continues the rest of this digestive process. Salivary amylase hydrolyzes these carbohydrates to simple sugars such as galactose, glucose, and fructose. Glucose is then absorbed by our body in the small intestine (Pandol, 2010).

### 3.2.3.2 Gastric

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). In the stomach, gastric juice is present and contains the main protein digestive secretion, hydrochloric acid. Sometimes this acid can cause heartburn if it goes back up into the esophagus. Mucus is also secreted from the walls of the stomach to protect cells from the strong acid present here (Whitney & Rolfes, 2013).

### 3.2.3.3 Pancreatic

Secretes bicarbonate to neutralize intestinal contents. Also produces enzymes that digest carbohydrates.The pancreas secretes enzymes that break down carbohydrates, lipids, and proteins and then delivers to small intestine for absorption (Whitney & Rolfes, 2013). One pancreatic enzyme that aids in lipid digestion is pancreatic lipase. Pancreatic lipase hydrolyzes triglycerides (Pandol, 2010). Another enzyme called protease is apart of protein digestion. Proteases are stored and deposited from the pancreas in an inactive form and then later on in the duodenum, trypsin activates them (Pandol, 2010).

### 3.2.3.4 Liver

The liver produces bile, which is stored in the gallbladder until needed. The gallbladder will then release bile into the duodenum when it is time. Bile is an emulsifier and is mixed with lipids to help absorption of fats in the small intestine. It brings the fat into water so enzymes can break them down (Whitney & Rolfes, 2013).

### 3.2.3.5 Small Intestine

The small intestine has some bacteria that produce enzymes to complete digestion processes (Whitney & Rolfes, 2013).

## 3.2.4 Regulation
### 3.2.4.1 Hormones

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)

### 3.2.4.2 Nervous system

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). Extrinsic and Intrinsic nerves are the two nerve types that control the processes of the digestive system. Extrinsic nerves link the CNS to the organs of digestion. This type of nerve releases chemicals that will either make the GI tract contract or relax. Intrinsic nerves are located within the GI tract and are activated when food particles causes the walls of the digestive organs to expand. These nerves distribute many substances that will either quicken or slow down the movement of food through the GI tract (National Digestive Diseases Information Clearinghouse, 2013). 3.3 Absorption Process of taking substances into the interior of the body.