The digestive tract is the body system that deals with the digestion of food. It is also called the digestive canal or the digestive tract. It runs like a tube from the mouth where food is introduced to the anus where the food remains, in the form of faeces leave the body. The digestive tract consists of organs and glands that assist it do its function. The organs and the glands have been modified to do their function. The glands produce digestive juices that aid in the digestion of food. This essay aims at exploring the structure and the function of the digestive tract. For a good discussion of the topic, essay divided into sections.

2.1 The need for the digestion of food for the purpose of both absorption and assimilation of nutrients.

The body cannot make use of the food that one eats without it being broken down to small molecules. There are various reasons why human beings consume food. Such include energy for carrying out activities and the energy needed for development and growth. Fats for deposition to provide protection is also extracted from the food particles. In addition, minerals and vitamins also have a role to carry out in the body and are also extracted from the nutrients. For the products of digestion to be useful to the body, the have to be broken down into smaller particles. Starch is absorbed in the form of glucose. Fats are absorbed in the form of fatty acids and glycerol (Hill n.d). Proteins are usually absorbed in the form of the final products of amino acids. The molecules of vitamins, minerals and water are absorbed in the smallest form. The purpose of digestion in the absorption is, therefore, to break down the food that is eaten to the simplest form that can be absorbed. The lining of the intestines is responsible for absorption. The poles through which the particles pass through during absorption are very tiny. These food particles, therefore, need to be broken down in order to pass through these poles to the bloodstream. The bloodstream is not the destination of the food molecules. They have to be assimilated into the cells of the tissues. This means that the molecules have to travel from the blood vessels to the cells. Only tiny molecules can pass through the semipermeable membrane of the cells. This shows the importance of digestion in terms of assimilation. Once inside the cells, the molecules have to be utilized by the cell for many functions. One of the commonest functions is the production of energy. This involves penetration of organelles like mitochondria which even have less tiny poles (Hill n.d). Digestion of food to produce the tiniest molecules is thus of vital importance for the purpose of assimilation and absorption.

2.2. Physical processes involved in digestion and their importance.

The first action in eating involves introducing food into the mouth through a process referred to as egestion. It is a process which makes use of the hands, and the opening of the mouth to receive the food. From the mouth to the intestines, the processes of physical and chemical digestion go simultaneously to complement one another in the digestion of food. This section looks at the physical digestion of food.

The first process after foo has been introduced into the mouth is mastication. It involves the teeth and the tongue, as well as, the walls of the mouth. The walls of the mouth are muscular, and they help in the mixing of food (ABPI 2012). Mastication involves breaking down of the ingested food into smaller particles using the teeth. It is accompanied by several other processes. Mixing of food is done by the tongue and the movements of the walls of the mouth. Chewing is another process that involves getting small particles out of the ingested food. The processes result into small balls of the ingested and masticated food that are referred to as boluses. The boluses are ready for swallowing.

Swallowing comes with another physical process of digestion referred to as peristalsis (ABPI 2012). It has also been referred to as propulsion. The walls of the gut contracts and relaxes enabling the food bolus to be propelled down the digestive canal. From the mouth to the stomach through the esophagus, it is the process of peristalsis that aids digestion.Physical digestion also takes place in the stomach. The walls of the stomach move by contracting and relaxing to aid in the churning of food and combining it with the digestive enzymes. At the intestines, the muscular movements are also present. The villi lining of the intestines also moves enabling separation of food particles easing absorption by the bloodstream. The release of digestive juices by the glands is also a physical process of digestion. An example is bile that is released by the gall bladder through the bile duct.

2.3. Sites of production of digestive juices and enzymes and their role in digestion.

Digestion cannot take place were it not for the digestive juices, hormones and enzymes that aid the process. The sites of production are located throughout the tract. In the mouth, digestive juices are produced by the salivary glands. There are three main salivary glands: the sublingual glands, the submandibular glands and the parotid gland. The salivary amylase is a digestive enzyme that aids in the digestion of starch. The enzyme also performs the function of moistening food so that swallowing would be easy.

The stomach lining is another site for the production of digestive juices. An example of an enzyme produced in the stomach is gastrin that digests protein. The walls of the stomach also produce an acidic medium that aids in the breakdown of food. The acid is bacteriostatic thus functions to kill the bacteria present in the food. Activation of protein-digesting enzymes is also achieved by the hydrochloric acid (Hill n.d). The lining also produces mucus. Mucus prevents self-digestion of the stomach wall. Intrinsic factor that is involved in the absorption of vitamin B12 into the intestines is also produced by the lining of the stomach.

Another production site for digestive enzymes is the pancreas. It produces sodium hydrogen carbonate which helps in the neutralization of hydrochloric acid that is produced in the stomach. Pancreatic amylase also produced in the stomach helps in the digestion of starch. Trypsin comes in the form of an enzyme that is produced in the pancreas that helps in the digestion of complex molecules of proteins into peptides. Pancreatic lipase is an enzyme also produced by the pancreas, and it helps in the digestion of fats into fatty acids and glycerol.

The gallbladder is also a site of production of digestive juice that is called bile. The goal bladder produces the juice and releases it through bile duct to the liver during eating. The bile performs the function of breaking down fat in the stomach.

The small intestine also counts as a site of production of digestive juices. The duodenal mucosa produces cholecystokinin, which is responsible for inhibiting the stomach’s secretory activity. It also produces secretin, a hormone that increases the production of bile when it acts in the liver. Histamine is a substance produced by the stomach wall (Santos and Danac 1999). It causes the stimulation of the release of hydrochloric acid from the inner wall of the stomach. The wall of the small intestine also produces a substance called gastric inhibitory peptide. Its role is to act in the pancreas to stimulate the release of insulin.

3.1. Main organs of the digestive system and their functions.

The mouth is the first recognizable organ of the digestive system. Its recognizable features are the tongue and the teeth. It is also endowed with salivary glands that produce saliva and digestive enzymes. The mouth starts the digestion process. Mastication, which is the most important process in physical digestion, takes place in the mouth. The digestion of starches begins in the mouth with the help of salivary amylase.

The esophagus is tube-like, and it joins the mouth to the stomach (Hill n.d). Its main role is the process of digestion to enablefood to pass down to the stomach. It does the function through the process of peristalsis. In between the mouth and the esophagus, is the pharynx. It holds the epiglottis. It functions to prevent food from entering the windpipe during swallowing. The esophagus has muscular walls that enable it to perform the function of peristalsis by relaxing and contracting.

Stomach is a muscular bag that stores food for some time. Both physical and chemical digestion happens here. Mixing and churning of the boluses into boluses is done in the stomach. For it to conduct its function better, the stomach is endowed with muscular walls and has many layers making it elastic. The muscular walls contracts and relaxes to perform the functions of churning and mixing of food. Its walls are enriched with glands that produce digestive juices. The walls also have mucus that prevents auto digestion or damage by the hydrochloric acid (Sullivan 2004).

The intestines are divided into sections. There is the large, and the small intestine. The small intestine has the duodenum and the ilium. The small intestine performs most of the absorption. The action of the enzymes is also present in the intestines to aid digestion (Santos and Danac 1999). The lining of the intestines is endowed with millions and millions of microvilli that help with the process of absorption. The microvilli make movements that enable separate food particles for easier absorption. The wall of the intestine is muscular and has many layers as an adaptation to the process of absorption. The area underlying the microvilli is full of blood vessels and lymph vessels to maximize the surface area for absorption. Fats are absorbed through the lymph vessels. The intestine is also coiled to increase the surface area for absorption. This also ensures that food travel slowly through the intestine for easier absorption. The large intestine is divided into the transverse section, the descending section, the sigmoid colon and the ascending colon. It is performs the function of absorption of water (Waugh 2010). The large intestine also performs the function of holding faeces for some time before it is released out of the body. It also includes the anal canal, caecum and the appendix. The anal canal leads to the outside of the body. It completes the digestion process through the process of egestion. The muscular wall of the large intestine helps the process of movement of faeces and the process of egestion. Bacteria that can survive with oxygen, or without, are believed to reside in the large intestine (Sullivan 2004). They are responsible for the decomposition of faeces before it is expelled out of the body.

3.2. Layers of gut wall, and relation of parts to function.

The histological structure of the gut wall is in such a manner that it favors the function that the structure should perform. Most of the organs are covered with a visceral peritoneum. The very inner layer of the canal wall is referred to as the mucosa. It performs the function of production of hormones, mucus and enzymes. It is responsible for the protection against disease, and absorption takes place in it. Submucosa is the layer below mucosa. It is rich in blood supply and lymph supply for the purpose of absorption. Muscularis externa is a layer of muscle whose function is segmentation and peristalsis (Waugh 2010). The outermost layer is referred to as serosa. Its main function is maintenance of stability and protection to the inner layers.

Every part of the digestive tract is adapted to its function in a particular way. For example, the mouth has the teeth, which perform the function of mastication. The esophagus has a strong muscular wall which performs the function of peristalsis. The muscular walls of the stomach enable it to perform the functions of churning and mixing of food. The endowment of the structures with glands enables the production of digestive juices and enzymes. The stomachs layer is fed with mucus that prevents self-digestion by the digestive enzymes. Small intestine has a number of adaptations too. It has many microvilli to maximize the area of absorption. It is also fed with many blood vessels in order to allow absorption to take place. It is also coiled allowing more area for absorption of nutrients. The large intestine is also coiled to enable it have a large surface area for absorption of water. Its muscular wall enables it to expel the end contents of digestion from the body (Waugh 2010)

REFERENCES

ABPI. 2012. Resources for schools: Diet and digestion. Retrieved online from http://www.abpischools.org.uk

Hill, M. n.d. ‘Chapter five: Digestive system and nutrition’. Retrieved online from http://highered.mcgraw-hill.com/sites/dl/free/0072347325/80938/mhb8ch05.pdf

Santos, C.N. & Danac, C.A. 1999. Biology II for high school. Quezon City: Rex Bookstore Inc.

Sullivan, R.J. 2004. Digestion and nutrition. New York: Info base Publishing.

Waugh, A. 2010. Ross and Wilson anatomy and physiology in health and illness (11^thed).London: Elsevier.

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