Inside every human being finely meshed and almost unbelievably complex series of mechanical and chemical processes takes place. With little conscious control, the body's delicate organs first digest food to release its nutrients and then use products to build tissue and generate the energy of life.

The main parts—the Exposition.

The human body requires a constant supply off food in order to carry on its numerous activities. All foods have to be made soluble and some of them must undergo certain chemical changes before they can be stored and assimilated in the tissues of the body. These important transformations take place in the digestive tract or alimentary canal. Its main parts are the mouth, pharynx, esophagus, small intestine, large intestine, and rectum. The movement of the food is controlled by the muscles that line the walls of the digestive tract. Tiny glands that line the muscle wall secrete mucus, a moist substance that enables the food to slip along the tract.

Start of the Journey—the Rising Action.

Digestion begins as soon as food is taken into the mouth. The food is masticated or chewed by the teeth and moistened with the saliva which contains a substance called ptyalin. While food is still in the mouth, ptyalin acts upon complex molecules of starches, breaking them into simple sugars. The effect of ptyalin may continue inside a ball of food for some time after the food is swallowed. Then, the food reaches the stomach. It passes through the pharynx and down the esophagus. Peristalsis, the wavelike movements of the esophagus muscles, force food into the stomach.

Into the Stomach—the Climax.

The main part of the digestive process takes place in the stomach. Powerful muscles churn the food. Gastric juices mix the food. In the gastric juices are hydrochloric acid and the enzymes pepsin which together split the proteins. Two other enzymes in the gastric juices are rennin, which curdles milk; and lipase, which helps split fats. At this point, food is turned into a thick fluid mass chyme. Continuing stomach contractions push the chyme into the small intestine.

The Absorption—the Falling Action.

Final digestion and absorption take place in the upper part of the small intestine— the duodenum. Food passes through this area slowly. As a result, various enzymes in the digestive juices composed of bile from the liver, pancreatic juice from the pancreas, and intestinal juice from the intestine itself serve to neutralize the acid. All these secretions are alkaline. In bile are salts that helps break down fats into fatty acids. Pancreatic juice and intestinal juice contain powerful enzymes that convert protein molecules into amino acids; fat molecules into fatty acids and glycerin; and sucrose, maltose, lactose into simple sugars.

After all these chemical changes, simple sugars, fatty acids and glycerin, and amino acids readily absorbed into blood stream through the lining of the small intestine by millions of tiny fingerlike projections called villi.

By the time the chyme reaches the ileum, the lower part of the small intestine, absorption is practically complete. Chyme is passed through the valve into the large intestine.

The Excretion—the Denouement.

When the chyme reaches the large intestine, it contains cellular debris, bacteria, indigestible food residue, fats, some minerals, and water. A great deal of the water is absorbed as these materials pass through the large intestine and ultimately become semi-solid feces that are excreted from the body through the rectum.

—end of the journey.

Thank you for reading.

Lead photo source: https://digestive-guide.weebly.com/at-a-glance.html