Digestion is the breakdown of foods into absorbable nutrients in the gastrointestinal tract. The gastrointestinal tract, also referred to as the digestive tract, includes the mouth, esophagus, stomach, small intestine, large intestine and anus. Each part of the gastrointestinal tract plays an important role in proper food digestion.
Types of Digestion
Digestion is divided into separate categories: chemical digestion and mechanical digestion. Chemical digestion describes the breakdown of food that occurs as a result of digestive juices and enzymes. During chemical digestion, food is broken down into nutrients that can be absorbed by the body. Mechanical digestion involves the physical part of digestion, such as chewing, grinding and the physical breakdown of food. The movement of the gastrointestinal tract during digestion is also considered mechanical digestion.
Mouth and Esophagus
The process of digestion begins when salivary glands in your mouth release saliva, which moistens the food and contains enzymes that help break the food down. You also use your teeth to mechanically break down food by chewing. Once the food has been chewed well enough, it is swallowed and travels into the esophagus. Once the food reaches the esophagus, it is referred to as a bolus. The bolus travels through a valve called the lower esophageal sphincter and into the stomach.
Stomach
When the bolus reaches the stomach, the stomach contracts to mix the bolus with stomach acid and digestive juices. These juices chemically break down the bolus into a substance called chyme, which "Nutrition and You," by Joan Salge Blake, defines as a semi-liquid substance that contains digestive secretions and food particles. The stomach contractions then push the chyme toward the lower part of the stomach, where it travels through a valve called the pyloric sphincter and into the small intestine.
Small Intestine
Most digestion and absorption occurs in the small intestine. Once the chyme enters the small intestine, it is further broken down by digestive secretions. Small hair-like projections, called villi, on the inner surface of the small intestine come into contact with the chyme and absorb nutrients from it. The nutrients then travel through the intestinal walls and into the bloodstream. The small intestine also undergoes movements called segmentation that push the chyme along the small intestine and into the large intestine.
Large Intestine
Once the chyme reaches the large intestine, it has been almost fully digested, and most of the nutrients have already been absorbed. Bacteria in the large intestine break down the remaining nutrients in the chyme. Water is also added to the chyme in the large intestine. Once the digested material reaches the colon, it is turned into fecal matter, which is passed through the anus within 12 to 70 hours, according to "Nutrition and You."
Considerations
They're not considered a part of the gastrointestinal tract, but it is important to note the role that the pancreas, liver and gallbladder play in proper digestion. These organs produce and release digestive enzymes and hormones that help break down food and regulate the digestive process.
References
- "Nutrition and You"; Joan Salge Blake; 2008
- Kids Health: Your Digestive System
- National Digestive Diseases Information Clearinghouse: Your Digestive System and How it Works
