Human Digestive System

The human body is a complex unit of organs and organ systems whose functions are coordinated and related to realize an efficient, fully functional body. These systems include the circulatory system, which transports nutrients and oxygen to the rest of the body parts through blood, the nervous system and the digestive system. Others include the reproductive system and the respiratory system (Rosdhal, 2008). The digestive system involves numerous organs that function as a unit. The organs digest and absorb food and nutrients into the body. The absorbed nutrients are then transported to the rest of the body tissues and organs by blood. The process of digestion begins in the mouth and ends in the anus where the indigestible and the undigested food products are expelled from the body by a process called defecation. This essay discusses the processes that ingested food undergoes in various organs of the digestive system until its final stage where it is expelled from the body.

In the mouth, the teeth physically break down large food particles into smaller particles in a process called chewing. It is recommended by doctors and specialists that one should chew food properly (about twenty times) before swallowing (Rosdhal, 2008). This act increases the surface area of the food particles available for the enzymes to digest. The tongue aids in the process of chewing by pushing the food particles to various parts of the mouth. At the same time, the sight and smell of food alone trigger the parotid gland and salivary gland to secrete saliva (Morrison, 2001). These glands are situated on the back of the roof of the mouth and under the tongue. Saliva contains enzyme ptyalin that breaks down starch. Other functions of saliva include aiding in the formation of bolus (the circular compaction of food particles) and moistening of food. After complete physical breakdown of the food, the tongue pushes the bolus to the back of the mouth and into the pharynx. The pharynx links the oral cavity to the esophagus, a long, muscular tube that extends to the stomach and is important for swallowing. In the junction of the esophagus and the trachea, there exists a valve-like organ called the epiglottis. It closes during swallowing and prevents food particles from entering the trachea when swallowing. The bolus travels down the esophagus with the aid of gravity and rhythmic contraction and relaxation of its muscular walls in a process called peristalsis. Peristalsis ensures that one can swallow, notwithstanding the physical position one maintains (Morrison, 2001). During the movement of the bolus down the esophagus, the food continuously mixes and churns due to the close contact with the wall of the esophagus. The mucus lining the wall also makes it smooth to allow the bolus to move down easily. The bolus finally enters the stomach through a sphincter (gate) that separates the stomach from the esophagus.

Entry of the food in the stomach stimulates the release of certain juices that further aid in chemical digestion of food. In the stomach, secretion of hydrochloric acid occurs. It kills any pathogens and disease-causing organisms in the food. The hydrochloric acid could also digest the walls of the stomach due to its high acidity. The stomach wall is lined with mucus to prevent this (Rosdhal, 2008). The stomach also secretes gastric juice, and it contains various enzymes that digest different types of food. Enzyme pepsin is secreted in its inactive form, pepsinogen, so that it does not adhere to the walls of the stomach (Morrison, 2001). Pepsin digests and breaks down proteins to form smaller peptides. Rennin digests milk. Continuous mixture of the food, juices and secretions in the stomach results to formation of a semi-liquid mass of food called chyme. Chyme stays in the stomach for 2-6 hours (Morrison, 2001). It all depends on the amount of food one binged and the type of food. Heavy foods that are not easily digestible, such as meat, stay for longer in the stomach. In the same manner, when one takes a plateful of food, the food will likely remain in the stomach for longer compared to a situation where one takes only a small amount of food. After 2-6 hours, the chyme leaves the stomach and enters the small intestine. Again, there is a sphincter separating the small intestine from the stomach. It prevents regurgitation and mixing of intestinal contents with contents from the stomach.

The small intestine is divided into three parts: the duodenum, the jejunum and the ileum.  Secretions of the liver, pancreas and glands from the intestinal wall itself aid in the digestion of food. The duodenum, the first part of the small intestine, provides a site for digestion and absorption of fats. Bile juice, secreted by the liver and stored in the gall bladder, breaks down large fat molecules into smaller molecules in a process called emulsification. Emulsification ensures that surface area of the fats available for enzymatic action is increased (Morrison, 2001). The bile juice also aids in the absorption of the fats, a process that begins in the small intestine. Pancreas secretes pancreatic juice and bicarbonates. The bicarbonates neutralize the acidic chyme (due to the acidity in the stomach). Intestinal juice produced by glands on the walls of the small intestine also contributes to digestion of food. However, the main process that occurs in the small intestine is absorption. The ileum has various adaptations to ensure that absorption occurs effectively. It is long and coiled to slow down the movement of the digested food. This allows the food to remain in contact with the walls of the small intestine for longer. As a result, many nutrients are absorbed. It is being lengthy also ensures that it provides large surface area for absorption of food. In addition, the walls of the ileum possess villi and microvili, the structures responsible for the process of absorption of nutrients (Rosdhal, 2008). The indigestible and undigested food products pass on to the large intestine also separated from the small intestine by a sphincter. The sphincter prevents reentry and mixing of the contents from both organs.

The large intestine is divided into ascending and descending colon, cecum, appendix and rectum. Absorption of water, vitamins and certain minerals occurs in the colon. The cecum is small and functionless in humans. In herbivores, however, it is the site where digestion of cellulose occurs. The rest of the indigestible and undigested food products are passed on to the rectum where they are temporarily stored. When it is socially convenient, these products are expelled from the body through the anus (Rosdhall, 2008). The waste products are called feces or stool. This complex process of ingestion and digestion takes about eight hours to complete. However, the type of food ingested and its amount also determine the time the food lasts in the body. It has been suggested that people should refrain from drinking a lot of water during and after a meal. The water is said to dilute the enzymes and, therefore, neutralizes their effect (Morrison, 2001).