Digestion is defined in "Nutrition and You" by Joan Salge Blake as the process during which foods are broken down into small components that are able to be absorbed by the gastrointestinal tract. The gastrointestinal tract consists of the mouth, esophagus, stomach, small intestine, large intestine, rectum and anus. All of the components of the gastrointestinal tract play separate important rules in the digestion and absorption of food.
Types of Digestion
In order to completely understand digestion, it is important to realize that there are two types of digestion that take place in the body. The first type is called mechanical digestion. Mechanical digestion involves the chewing and breaking food in the mouth as well as the physical movement and muscle contractions of the other areas of the gastrointestinal tract. The second type of digestion is called chemical digestion. Chemical digestion is the process of using digestive juices and enzymes to chemically break down the food so that nutrients can be absorbed by the gastrointestinal tract.
Mouth and Esophagus
The process of digestion begins in the mouth. When the brain is exposed to food, either through sight, smell or taste, it triggers the glands in the mouth to release saliva, which contains an enzyme called salivary amylase. Saliva mixes with the food in the mouth to moisten it and begin the breakdown process and the salivary amylase begins to break down macronutrients. The teeth work with the saliva to physically break down food. Once the food is sufficiently chewed, it is referred to as a bolus. The bolus is propelled to the back of the mouth and swallowed, where it travels from the esophagus and into the stomach.
Stomach
When the bolus reaches the stomach, it undergoes both mechanical and chemical digestion. The stomach mechanically digests the bolus by undergoing a series of contractions and movements that further break down the food. The stomach also secretes hydrochloric acid and a number of enzymes that help to chemically digest the food. Once the food has been digested by the stomach, it is referred to as chyme.
Small Intestine
According to "Nutrition and You" by Joan Salge Blake, the small intestine is where most of the human digestive and absorption takes place. The small intestine goes through a series of physical movements called segmentation that helps to mechanically break down the food. Once the food is sufficiently broken down, small finger-like projections on the inner intestinal wall called villi absorb most of the nutrients from the food.
Large Intestine
After the chyme passes through the small intestine, it reaches the large intestine. By this time, most of the food has been digested and almost all of the nutrients have been absorbed. The large intestine plays an important role in absorbing excess water from the chyme and forming fecal matter. Once fecal matter is formed, it travels through the rest of the large intestine where it is stored in the rectum until it is eventually excreted through the anus.
Acessory Organs
Although not a direct part of the gastrointestinal tract, the liver, gallbladder and pancreas are important parts of the human digestive process. The liver produces bile, which is stored in the pancreas. Bile is critical in the proper digestion of fat. The pancreas produces important hormones that trigger the release of enzymes from the digestive tract.
References
- National Digestive Diseases Information Clearinghouse: Your Digestive System and How It Works
- "Nutrition and You"; Joan Salge Blake; 2008
- Kids Health: Your Digestive System
