Digestive System and How It Works - Essay Example

The Digestive System Introduction: The digestive system in the human body has the functions of ingesting food, digesting it and absorbing the required nutrients for the body. For this purpose it is made up of a long hollow tube that extends from the lips of the mouth to the anus between the two lower limbs, with other organs whose secretions assist in the digestive process. The Mouth: Ingested food enters the digestive system at the mouth or buccal cavity, which is lined by a mucous membrane, with the lips or labia protecting the external opening. The cheeks make up the lateral walls of the mouth with upper and lower palates completing the outer aspects of the oral cavity. Attached to the floor of the mouth is the muscular tongue with taste buds that provide us with our sense of taste. Within the mouth are found hard structures called teeth. On the posterior side is the opening into the esophagus and also a small structure called the epiglottis. Food entering the mouth is masticated by the teeth. There are three pairs of ductless salivary glands parotid, submandibular, and sublingual that secretes saliva into the mouth. The food that is chewed is mixed with saliva with the tongue. Saliva contains the enzyme ptyalin that initiates the digestive process by breaking down sugar and starch into simpler molecules. The ph of saliva is alkaline and ptyalin functions in this alkaline environment. When the mastication process is complete food is pushed into the esophagus in a voluntary motion called swallowing that is assisted by the tongue and the soft palate. The epiglottis covers the trachea during swallowing to prevent food from going down the respiratory tract via the trachea, as the food passes of the trachea in swallowing (1). Swallowing is the only voluntary action in the movement of food in the digestive tract. The rest of the journey food in the digestive tract is through a motion called peristalsis that is unique to the digestive tract, where the digestive tract behind the food contracts to push the food forward in the digestive tract. (2). The Esophagus: The esophagus is the in essence the first part of the digestive tract and extends for about ten inches connecting the mouth to the stomach. At the upper end and the lower end of the esophagus are two sphincter muscles that control the entry and exit of food from the esophagus. On swallowing the sphincter at the upper end of the esophagus relaxes to allow food to into the esophagus from where it travels downwards and as food nears the lower end the sphincter muscle the lower end opens allowing food from the esophagus to enter the stomach. During the journey of food in the esophagus the digestive action of the enzyme ptyalin on food continues. (2). The Stomach: The stomach is a C shaped muscular bag like structure found on the left side of the abdomen. The Cardio-esophageal sphincter guards the entry at the esophagus end, while the pyloric sphincter guards the exit into the small intestine at the lower end. The stomach is made up of the fundus, which is the first part of the C, the main body of the stomach and the lower end called the pylorus. The stomach secretes mucous and acid that aid in the protecting it. The acid has also a role to play in the processes in the digestive processed that occur in the stomach. The stomach acts as a temporary storehouse for the food that is consumed and during this period it is churned by the muscular layers of the stomach to form a creamy substance called chyme. Once the food has been ground into chyme the pyloric sphincter muscle relaxes allowing food to enter the duodenum of the small intestine. (1). The digestive process in the stomach besides breaking the food into finer particles for ease in digestion revolves around the secretions of the stomach wall called gastric juices. Gastric juices consist of hydrochloric acid and the enzymes pepsin and rennin. The digestive activity of saliva stops in the acidic medium of the stomach. The dilute hydrochloric acid in the gastric juices makes the medium of the chime acidic for the action of the gastric enzymes of rennin and pepsin to act. Renin breaks down milk proteins for further digestive action in the small intestine, while pepsin breaks down the other proteins found in food into peptides (3). This semi digested food then enters the duodenum of the small intestine. Duodenum: The duodenum is the first part of the small intestine. It is C shaped ten inch long tube that curves around the head of the pancreas and the entry of the common bile duct from the gall bladder. It is the location, where most of the final digestive process of converting food into an absorbable form takes place through its own enzyme secretion as well as the exocrine secretions of the pancreas called pancreatic juice and bile secreted by the liver. Lipase secreted by the duodenum breaks down fatty acids. (1). The Pancreas: The pancreas is the most important digestive gland in the human body responsible for the secretion of many enzymes that convert food into an absorbable form. Trypsin and chymotrypsin are two peptidases similar to pepsin in the stomach and continue the digestive action of converting proteins into peptides. Carboxypolypeptidase secreted by the pancreas then acts on the peptide fragments splitting them further into amino acids. The pancreas also secretes several nucleases that degrade nucleic acids. Amylase secreted by the pancreas degrades starch into glycogen. The pancreas in addition secretes four enzymes that act on the disaccharides converting them into monosaccharide. Sucrase is responsible for breaking down sucrose into glucose and fructose, while maltase is responsible for breaking down maltose into glucose, and isomaltase is responsible for the break down of maltose and isomaltose into glucose, while lactase breaks down lactose into glucose and galactose. (3). The Liver and the Gall Bladder: The liver is located on the right side of the lower abdomen and secretes bile, which is stored in the gall bladder. Bile is forced into the duodenum, when food enters it and the gall bladder contracts forcing bile through the common bile duct into the duodenum. Bile contains bile salts, bile pigments, cholesterol and phospholipids. Bile salts and phospholipids emulsifies fat present in the food for the action of lipase secreted by the duodenum. (1). The Small Intestine: The rest of the small intestine is made up of the jejunum, which is eight feet long and the ileum, which is twelve feet long. The surface area of the jejunum and ileum is increased through circular folds called plica circulares, as the jejunum and the ileum is the location from where the digested food is absorbed. The circular folds contain and the cells of the villi called microvilli through which the nutrients from the ingested food us carried out by the rich venous and capillary drainage system present (1). The Large Intestine: The small intestine leads into the large intestine, which is made up of three parts, namely the ascending colon, the transverse colon, and the descending colon. The large intestine is responsible for reabsorbing the water in the digested residues and converting this residue into the solid mass called feces for excretion. There are no villi in the large intestine, but there are many goblet cells, which secrete mucus o lubricate the feces for easy passage and excretion. (1). The Rectum and the Anus: The rectum is the end of the large intestine in which the feces can be found prior to excretion. The external opening of the rectum is guarded by a sphincter muscle called anus. It has both voluntary and involuntary capabilities and the ability to distinguish between gas and solid material. Gas is passed in an involuntary opening of the anus, while in normal conditions the opening of the anus for excretion of solid matter is voluntary. (1). Works Cited 1. Johnson, D.R. “Introductory Anatomy: Digestive System”. Faculty of Biological Sciences, University of Leeds. 11 May 2008. . 2. “Your Digestive System and How It Works”. National Digestive Diseases Information Clearing House (NDDIC). 11 May 2008. . 3. “Digestive enzymes”. Vitamins & health supplements guide. 11 May 2008. . Read More