By: Kristy Haugen
The body is a complex web of systems. Most are not fully aware of the complexity of the digestive system. However, most know of its opposite, indigestion. Most do not realize how amazing the body can be, especially the digestive tract. When the process of digestion is described as fascinating, passing gas isnâ€™t going to be the major focus (flatulence), but more so the process of how gas is produced.
The digestive systemâ€™s primary role is to convert the food into substances that are capable of being absorbed. The digestive system is comprised of the following structures: the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and the anus. The liver, gallbladder, pancreas, and salivary glands also play a role in digestion but are not considered part of the alimentary canal (primary digestive organs).
Digestion begins in the mouth when food is ingested. This is a mechanical process. Through mastication, (the biting and chewing action of the teeth) the breakdown of food from larger particles into smaller particles takes place. This process doe not chemically alter the food, but increases the total surface area of the food. This in turn increases the speed and efficiency of enzyme activity. An enzyme is a protein that catalyzes, or speeds up, a chemical reaction. Enzymes are essential to sustain life because most chemical reactions in the body would occur too slowly, or would lead to different products without the assistance of enzymes.
For more information on enzymes and how enzymes work, read Liquid Vitamins Contain Enzymes Because?.
Saliva also plays an important part with digestion in the mouth. Saliva is secreted by the salivary glands which lubricates the food to facilitate swallowing. The salivary glands begin to produce saliva in response to food; whether stimulated by smell or taste. Some may experience a mouth watering sensation in response to a big juicy steak. Also, saliva initiates the digestion of carbohydrates. Amylase is the digestive enzyme found in saliva that helps with carbohydrate digestion.
Once the food has been sufficiently chewed, the tongue rolls it into a ball (bolus) and pushes it into the pharynx (the cavity that leads from the mouth to the esophagus). Swallowing (deglutition) propels the bolus downward into the upper esophagus using a peristaltic contraction (wavelike motion). At this time, the epiglottis blocks the trachea (airway) to prevent food from entering the lungs and interfering with breathing. Peristaltic contractions continue to move the bolus (food) downwards to the lower esophageal sphincter. This is the ring of smooth muscle fibers at the junction of the esophagus and stomach; it is also referred to as the cardiac sphincter. When food approaches, the sphincter relaxes to allow food into the stomach. After the food has passed through the sphincter, the muscle fibers contract to keep the food and digestive juices from re-entering the esophagus. Heartburn results when the cardiac sphincter relaxes and allows the digestive juices to re-enter the esophagus. When this happens too often, the smooth muscle of the esophagus is eroded, which can cause bleeding and persistent heartburn referred to as GERD (gastro-esophageal reflux disease). This can become a serious condition.
The stomach is a large muscular organ; the walls are lined by a thick gastric mucosa. The stomach is also lined by two types of glands: gastric and pyloric glands. These glands contain mucous cells which secrete mucus that protects the stomach lining from the harsh stomach acid (pH of 2). Chief cells located in the gastric glands secrete pepsinogen, which is a zymogen. A zymogen is an inactive form of an enzyme. The gastric glands also contain parietal cells which secrete hydrochloric acid. This aides in the conversion of pepsinogen to the active enzyme pepsin, and secrete intrinsic factor which helps to absorb vitamin B12. Hydrochloric acid is essential to kill bacteria in the food, and to help breakdown the food into an absorbable form. The pyloric glands contain peptic cells which also secrete the zymogen pepsinogen. Gastrin cells are located in the pyloric glands. These cells secrete the hormone gastrin for hydrochloric acid production in the parietal cells; and stimulate the churning of the stomach to help produce the acidic, semi-fluid, partially digested mixture referred to as chyme. Protein digestion is initiated in the stomach.
The chyme then empties into the small intestine by way of the pyloric sphincter. The pyloric sphincter is the ring of smooth muscle fibers located at the joining of the stomach and small intestine. The small intestine consists of three regions: the duodenum, jejunum, and ileum. The bulk of digestion will be done in the duodenum. The jejunum and ileum have a primary function of absorption.
The small intestine has the perfect anatomy for absorption. The extended length, highly coiled structure, along with surface villi (small finger like projections), and epithelial cells with a brush border microvilli allow for increased surface area for absorption. Nutrients are absorbed across the epithelium villi and are carried to the bloodstream through capillaries (small blood vessels) or lacteals (small lymph vessels that serve as extensions of the lymphatic vessel in the villi). Goblet cells located in the small intestine secrete mucus on the surface epithelium of the villi for protection from the digestive juices.
The pancreas releases a pancreatic juice in response to the hormone secretin that is secreted by the duodenum. This hormone cholecystokinin (CCK) is secreted in response to the acidity of the chyme in the small intestine. The pancreatic juice that is secreted has an alkaline pH to neutralize the acidity of the chyme. The pancreatic juice contains many enzymes (inactive & active) that digest carbohydrates, proteins, and lipids (fats).
The liver plays the role of secreting and synthesizing bile. Bile is a non-enzymatic digestive fluid that is used to breakdown (emulsify) fats. The gallbladder simply stores and concentrates the bile. Bile is made up of bile salts, bile pigments, and cholesterol.
Meals high in fat tend to spend a longer amount of time in the stomach since it takes more time to digest. The hormone enterogastrone is released by the duodenum. This hormone inhibits the peristalsis in the stomach, slowing the release of chyme into the small intestine. This also gives more time for the bile to properly digest the fats.
The remaining food passes from the small intestine to the large intestine. The large intestine consists of three parts: cecum, colon, and rectum. The large intestine plays a smaller role of digestion, mainly to absorb any electrolytes and water that has not already been absorbed. This process is done in the colon. Many normally harmless bacteria colonize the large intestine, such as E. coli. E. coli is important because this type of bacteria produces vitamin K as a byproduct. This is a good source of vitamin K. Also, the amount of time spent in the large intestine determines the consistency of the stool. If too little time is spent in the colon, diarrhea and dehydration result. If too much time is spent in the colon, constipation will result.
Lastly, the stool passes into the rectum. The rectum stores the feces (stool), which consist of unabsorbed digestive secretions (enzymes), water, undigested food (cellulose and fiber, etc.). The anus is the opening through which wastes are eliminated. The anus is separated from the rectum by two sphincters that regulate elimination.
The digestive process is just one extremely complex process that occurs without question. It is often taken for granted. The complexity isnâ€™t easy to understand. The next time you eat a juicy steak, understand the journey the meal is sent on is much farther than the trip to the restaurant.
- 1. McKinley, Michael P. and Oâ€™Loughlin, Valerie Dean. Human Anatomy. 1st ed. New York, NY. McGraw-Hill. 2005.
- Champe PhD, Pamela C; Harvey PhD, Richard A; and Ferrier, PhD, Denise R. Lippincottâ€™s Illustrated Reviews: Biochemistry. 3rd ed. Lippincott Williams &Wilkins. Philadelphia, Pennsylvania. 2005.
- Research & Education Association, The Best Test Preparation and Review Course For MCAT, Piscataway, New Jersey, 2002
- Kaplan, MCAT Comprehensive Review, 5th ed., Kaplan Publishing, New York, New York, 2000