An Introduction 

Digestive system has specialized organs in the breakdown of food into smaller particles and in the use of nutrients present in them. This system is also responsible for eliminating material that has not been digested. 

The human digestive system is formed by a kind of alimentary canal, which communicates with several accessory glands that release essential substances for the digestion process. Below we will learn more about this important process.

Digestive System Organs 

The human digestive system is formed by the gastrointestinal tract, which is composed of the mouth, pharynx, esophagus, stomach, small intestine and large intestine. Associated with these organs, we have the accessory glands, also called associated glands, which are the salivary glands, the liver and the pancreas.

Parts & Description 

• Upper digestive tube: Mouth, pharynx and esophagus. 

• Middle digestive tract: the duodenum, jejunum, and ileum of the small intestine. 

• Lower digestive tube: Large intestine (cecum, ascending, transverse, descending colon, the sigmoid curve and the rectum). 

• Attached Organs: Salivary glands, teeth, tongue, pancreas, liver and gallbladder.

Mouth 

The digestion process starts in the mouth. In this cavity, the food will undergo the action of the teeth, which act to ensure that the food is cut, crushed and crushed. This stage of digestion is called mechanical digestion, as it does not involve chemical substances that act on the food. In the mechanical digestion carried out by the teeth, the food will only become smaller, ensuring a better action of the enzymes and also aiding in swallowing.

One of the auxiliary glands of this crucial system, the salivary glands, also function in the mouth. These glands are responsible for the secretion of saliva, which acts in chemical digestion. Saliva contains salivary amylase or ptyalin, an enzyme that breaks down carbohydrates. The food, crushed and mixed with saliva, forms an agglomerate called food bolus. 

The tongue is also an important structure present in the mouth, being responsible for helping food to mix with saliva and also for moving the food bolus to the bottom of the oral cavity for swallowing. After leaving the oral cavity, the food goes to the pharynx.

Pharynx 

The pharynx is a membranous muscular tube that communicates with the mouth, through the isthmus of the throat and at the other end with the esophagus. 

To reach the esophagus, food, after being chewed, travels through the entire pharynx, which is a common channel for the digestive system and the respiratory system. 

In the process of swallowing, the soft palate is retracted upwards and the tongue pushes the food into the pharynx, which contracts voluntarily and takes the food to the esophagus. 

The penetration of food into the airways is prevented by the action of the epiglottis, which closes the orifice of communication with the larynx.

Esophagus 

The autonomic nervous system regulates the esophagus, which is a muscular  conduit.

It is through waves of contractions, known as peristalsis or peristaltic movements, the muscular conduit squeezes the food and takes it towards the stomach.

Stomach 

The stomach is a large bag located in the abdomen, responsible for the digestion of proteins. 

The entrance of the organ receives the name of cardia, because it is very close to the heart, separated from it only by the diaphragm. 

It has a small upper curvature and a large lower curvature. The most dilated part is called the "fundic region", while the final part, a narrow region, is called the "pylorus". 

The simple movement of chewing food already activates the production of hydrochloric acid in the stomach. However, it is only with the presence of food, of a protein nature, that the production of gastric juice begins. This juice is an aqueous solution, composed of water, salts, enzymes and hydrochloric acid.

The gastric mucosa is covered by a layer of mucus that protects it from gastric juice aggression, since it is quite corrosive. Therefore, when there is an imbalance in protection, the result is inflammation of the mucosa (gastritis) or the appearance of wounds (gastric ulcer). 

Pepsin is the most potent enzyme in gastric juice and is regulated by the action of a hormone, gastrin. 

When meal protein molecules come into touch with the organ wall in the stomach, gastrin is created. Thus, pepsin breaks down large protein molecules and transforms them into smaller molecules. These are proteoses and peptones. 

Finally, gastric digestion lasts, on average, from two to four hours. In this process, the stomach suffers contractions that force the food against the pylorus, which opens and closes, allowing, in small portions, the chyme (white and foamy mass), to reach the small intestine.

Small Intestine 

The small intestine is a long compartment that can be over six meters long. This is where most of the digestion process takes place. The duodenum, jejunum, and ileum are the three sections that make up the small intestine. 

The duodenum, the first portion of the small intestine, measures about 25 cm and is where the chyme joins with secretions from the pancreas, liver and small intestine itself. These secretions are:

• Pancreatic juice: The pancreas is an attached gland and is responsible for producing an alkaline solution rich in bicarbonate and also in enzymes, such as trypsin and chymotrypsin, which impact nucleic acids, pancreatic nucleases that impact proteins, and pancreatic lipase that impacts lipids. 

• Bile: Produced by the liver, bile is unique from other gastrointestinal tract secretions in that it is devoid of enzymes. This secretion contains salts that serve as emulsifiers or detergents. Bile is stored in the gallbladder despite being produced in the liver. 

• Intestinal or enteric juice: The lining of the small intestine is also responsible for secreting substances. Among the enzymes found in intestinal or enteric juice, maltase, which acts on maltose, sucrase, which acts on sucrose, and lactase, which acts on lactose, stand out.

The villi present in the intestine ensure greater absorption. 

In the duodenum most of the digestion takes place, with the other parts of the small intestine concerned mainly with the absorption of nutrients. After leaving the duodenum, the product of the digestive process goes to the jejunum, which is about 2.5 meters long and goes to the ileum, which is about 3.5 meters long. 

As mentioned, the small intestine also has the function of absorbing nutrients. In this organ, the presence of folds called villi is observed. In the cells of the villi, several microscopic folds, called microvilli, are also observed. These folds ensure an increase in the contact surface, thus providing an increase in the absorption rate.

Large Intestine

The large intestine measures about 1.5 m in length and 6 cm in diameter. It is the site of water absorption (both ingested and digestive secretions), storage and elimination of digestive waste. 

It is divided into three parts: the cecum, the colon (which is subdivided into the ascending, transverse, descending, and sigmoid curve) and rectum. 

In the cecum, the first portion of the large intestine, food residues, already constituting the “fecal cake”, pass to the ascending colon, then to the transverse and then to the descending colon. In this portion, the fecal bolus remains stagnant for many hours, filling the portions of the sigmoid curve and the rectum. 

The rectum is the final part of the large intestine, which ends with the anal canal and the anus, through which feces are eliminated. 

To facilitate the passage of the fecal cake, the glands of the mucosa of the large intestine secrete mucus in order to lubricate the fecal cake, facilitating its transit and elimination. 

Note that plant fibers are neither digested nor absorbed by the digestive system, pass through the entire digestive tract and form a significant percentage of fecal mass. Therefore, it is important to include fiber in the diet to help the formation of feces.

Summary of the Digestive System  

The digestive system is responsible for ensuring the breakdown of food into smaller particles and the absorption of nutrients that are needed by the body. 

The mouth, pharynx, esophagus, stomach, small intestine, and large intestine make up the gastrointestinal tract, which is also where the pancreas, liver, and salivary glands are located. 

In the mouth, food is torn and crushed by the teeth and, with the help of the tongue, is mixed with saliva. 

The food bolus goes from the mouth to the pharynx and from the pharynx to the esophagus, being taken through peristaltic movements to the stomach. 

In the stomach, the food bolus undergoes the action of gastric juice and is called chyme.

From the stomach, the chyme goes to the small intestine, where it will suffer the action of the pancreatic juice, the bile and the secretions produced by the small intestine itself. 

Along with a large portion of the digesting process, nutrient absorption also takes place in the small intestine. 

Feces are created in the large intestine and sent out through the anus.