Herbert, the hamburger soldier, has been assigned to traverse the gastrointestinal tract in order to deliver nutrients and other essential minerals to human being X. He has been tasked to pursue this assignment with three other soldiers, namely Frenchie, the French fries, Chocie, the chocolate shake and Apples, the apple pie. The initial approach to the task was easy because target X showed maximum cooperation by taking Herbert, Frenchie, Chocie and Apples into his mouth for strategic entry into the gastrointestinal tract.
The succeeding trek would then be more challenging because it involves smaller, yet powerful entities known as enzymes and muscles. The initial entry into the gastrointestinal tract finds the soldiers in the oral cavity, which is also called the mouth. This cavity is the venue for mechanical digestion of food using various types of teeth. The incisors are responsible for biting into food, while the canines are specialized for gnawing. The premolars and molars are responsible for chewing food into smaller pieces.
In order for the food to stay in place within the oral cavity, tongue pushes the food against the teeth. As for the vertical control of food within the oral cavity, the hard palate is positioned above, while the soft palate is positioned at the bottom, of the oral cavity. Saliva is also produced in the mouth and this contains two essential substances that are responsible for the mastication of food. Salivary amylase is an enzyme responsible for the destruction of simple sugars in the mouth, while mucin is a carbohydrate that assists in making the chewed food into a thick substance.
The chewed and masticated food, which is composed of the degraded soldiers, is then called a bolus. The bolus is then swallowed, passing through the pharynx. The epiglottis is a fleshy flap that is positioned within pharynx and directs food into the esophagus and into the stomach and not into the trachea, which is a component of the respiratory tract. In order to keep the food for regurgitating or going back into the oral cavity, a cardiac sphincter or valve is present at the anterior end of the stomach.
The soliders that comprise the bolus are aware that the stomach is a cavity that serves as a reservoir of masticated food. In this cavity, additional substances are introduced for chemical digestion, including a highly acidic gastric juice that is essentially made up of hydrochloric acid. The enzyme, gastrin, is then secreted by cells that line the stomach, facilitating in further destruction of the food. Gastrin is produced in the gastric pits of the epithelial cells of the stomach. These gastric pits are equipped with gastric glands that hold the enzyme until it is needed for digestion.
The gastric glands are generally situated along the rugae of the epithelial lining of the stomach. Another enzyme that is secreted in the stomach is pepsin, which is responsible for the degradation of proteins into amino acids. The precursor form of the protein pepsin is known as pepsinogen. The digested food is then transported into the small intestines, through the pyloric sphincter or the small constriction at the distal end of the stomach. The region connecting the stomach to the small intestines is the duodenum, which is a small passageway that is connected to a number of other ducts.
One of these ducts is the bile duct, which is the drainage for bile that is produced by the liver. The bile secreted by the liver cells is then transported for storage in the gall bladder. Bile is a greenish, bitter substance that is responsible for the emulsification of fat that is contained in the food. Another duct is the pancreatic duct, which drains enzymes that come from the pancreas. The pancreas generates pancreatic amylase, which has the capacity to further degrade carbohydrates in food. This enzyme is also mixed with bicarbonate buffer, resulting in the production of pancreatic juice.
There are also other enzymes that are introduced into the digested food, including proteases, lipases and carbohydrases, which assist in the digestion of macromolecules from food. The introduction of a number of enzymes in the small intestine thus results in a processed liquid that is now called chyme. The small intestines serve as the site where the chyme is further churned by the peristaltic movement of the muscles that lines this organ. There are also good bacteria that assist in the degradation of macromolecules in the chyme.
In addition, there are also bad bacteria that may be present in the small intestines that are responsible for toxification of the food. The chyme is then transferred to the large intestine, which is also called the colon. The large intestine comprised of three regions, based on the positioning and direction of the movement of chyme. The first section of the small intestines is the ascending colon or ileum because it is situated from the bottom right of the abdominal cavity. The appendix is also present in this region, which is a rudimentary organ that has no function in the human body.
In lower species, the appendix serves as a reservoir for enzymes that assist in the digestion of food. The cecum is that pouch that is located between the small and the large intestine. An ileocaecal valve is also positioned in this area, preventing the reflux of the digested food back into the duodenum. The ascending colon is where the micronutrients are further absorbed by the epithelial cells and are transported to the rest of the cells of the body. The second section of the large intestine is the transverse colon, which runs from the right to the left side of the abdominal cavity.
The third region of the large intestine is the descending colon, or jejunum, which is the site where water is removed from the chyme, thus resulting in a compacted and dry material mainly comprised of fiber and other excretory materials, known as the feces. The collected feces are stored in the rectum, which is a muscular organ that releases its contents when contracted. At the end of the rectum is the anus, a circular muscle similar in function to the mouth, which contracts in order to release the feces.