The process of human digestion occurs in the gastrointestinal tract, which is made up of the mouth, esophagus, stomach, small intestine, large intestine and accessory organs. Digestion involves the physical and chemical breakdown of food, as well as the absorption of nutrients.
Types of Digestion
The physical breakdown of food is referred to as mechanical digestion and the chemical breakdown of food is called chemical digestion. Mechanical digestion involves chewing and grinding food, and the movement of the food through the digestive tract through a series of intestinal movements referred to as peristalsis. Chemical digestion involves the breakdown of food with digestive juices and enzymes into small pieces that are absorbed by the body.
Mouth
Digestion begins in the mouth, even before food has been consumed, according to KidsHealth.org. In anticipation of eating, the salivary glands in the mouth release saliva, which contains enzymes, electrolytes and mucus, according to "Nutrition and You," by Joan Salge Blake. The enzymes in the saliva break down the food in the mouth chemically and the mucus adds moisture so that it is possible to chew the food. Teeth contribute by grinding down the food into pieces that are small enough to be swallowed. The food mass, which is referred to as a bolus, is swallowed and travels from the mouth to the esophagus to the stomach.
Stomach
The stomach contracts and moves around to mix the food with its digestive juices. The digestive juices include hydrochloric acid, enzymes, mucus, gastrin and the protein intrinsic factor, which is necessary for proper absorption of vitamin B12. When the digestive juices mix with the bolus in the stomach, proteins are broken down into smaller structures that are easier to absorb. After several hours, the swallowed food, which is now referred to as chyme, is pushed toward the lower portion of the stomach and into the small intestine.
Small Intestine
Most of the digestion and absorption of nutrients takes place in the small intestine. The interior of the small intestine is lined with millions of finger-like projections called villi. These villi help to mix the chyme with digestive secretions from the small intestine. The villi are covered with even smaller, finger-like projections called microvilli, which increase nutrient absorption and add to the surface area of the small intestine, according to "Nutrition and You." Movements in the small intestine, called segmentation, mix the chyme with digestive juices and move the chyme into the large intestine.
Large Intestine
The large intestine is responsible for absorbing water from the food, producing certain vitamins and the formation of fecal matter. The chyme is pushed into the rectum, where it is stored until the body is ready for elimination.
Other organs
It is important to note the role of the liver, gallbladder and pancreas in human digestion. The liver is responsible for producing bile, which the gallbladder stores. Bile is required for proper fat digestion. The pancreas produces digestive enzymes and hormones, such as insulin and glucagon.
References
- Kids Health: Your Digestive System
- National Digestive Diseases Information Clearinghouse: Your Digestive System and How It Works
- "Nutrition and You"; Joan Salge Blake; 2008
