Digestion is the process by which food is broken down into absorbable compounds called nutrients. Digestion is a multi-stage process that involves several structures that make up the digestive, or gastrointestinal, tract. During digestion, food passes through the various structures of the gastrointestinal tract including the mouth, esophagus, stomach, small intestine and large intestine.
Ingestion
The beginning stage of human digestion is called ingestion. Ingestion is the process during which you actually take food or drink into your mouth. Prior to ingestion, you may see or smell food, which triggers your salivary glands to release saliva, getting your body ready for digestion.
Digestion
The digestion stage involves both physical and chemical digestion. Physical digestion involves the physical breakdown of food. Your teeth and tongue break apart and chew food. Peristalsis, which is rhythmic muscle contractions in your digestive tract, moves food along the digestive tract. Chemical digestion involves various enzymes and digestive fluid, like saliva and stomach acid, that break the food down chemically into nutrients that can pass through your small intestine into your blood.
Absorption
The third stage of digestion, called absorption, begins in the small intestine. The small intestine consists of three segments: the duodenum, jejunum and ileum. Most absorption occurs in the middle segment of the small intestine, the jejunum. During this stage, small, finger-like projections called villi and microvilli absorb the nutrients through vessels called capillaries and lacteal. The nutrients, which include carbohydrates, fat, protein, vitamins and minerals, move from these vessels into your blood. Some absorption also takes place in the large intestine. When digested material moves through the large intestine, water and electrolytes are absorbed.
Egestion
Egestion is the final stage of digestion. During egestion, sometimes referred to as elimination, fecal matter is expelled from the body. Fecal matter remains in the rectum, the final portion of the large intestine, until it is removed from the body through the relaxation of the anal sphincter. Fecal matter consists mainly of cellulose, a type of fiber found in the cell walls of plants that cannot be digested by humans. Fecal matter also contains any potentially toxic substances that the body does not use.
Considerations
In addition to the structures of the gastrointestinal tract, the pancreas, liver and gallbladder are also involved in the digestive process. These organs produce or store digestive fluids that help chemically break down food.
References
- National Digestive Diseases Information Clearinghouse; Your Digestive System and How it Works; April 2008
- St. Charles College: The Digestive System;
- "Nutrition and You"; Joan Salge Blake; 2008
