The human digestive system has the job of processing ingested food in order to extract nutrients, which are then taken up into the bloodstream. Consisting of the gastrointestinal tract—which stretches from mouth to anus and includes the stomach and intestines—and several accessory organs responsible for supplying chemical digestive aids, the digestive system combines mechanical and chemical digestion with nutrient absorption in order to provide for the nutritional needs of all body cells.
Mechanical Digestion
The process of digestion begins in the mouth, where the combined efforts of teeth, tongue and saliva break up chunks of food. In his book, “Anatomy and Physiology,” Gary Thibodeau, Ph.D. notes that the chewing process is important to later digestive processes because digestion and absorption are most efficient when surface area is maximized. Once a ball of food, called a bolus, has been thoroughly chewed, it’s moved to the back of the throat by the tongue and swallowed. Mechanical digestion continues in the stomach, as the muscles in the stomach wall churn and blend food with stomach secretions, further breaking up food pieces and producing a soupy mixture with maximized surface area that lends itself well to the next digestive processes.
Chemical Digestion
While chemical digestion begins in the mouth to a limited extent with some breakdown of starch, the bulk of chemical digestion starts in the stomach. Here, stomach acid and digestive enzymes begin to break the chemical bonds that hold large nutrient molecules together, releasing smaller nutrient molecules that are easily absorbed. After food has spent some time in the stomach, it’s released in small amounts into the first part of the small intestine, or duodenum. Here, pancreatic and bile secretions mix with food, neutralizing stomach acid and further breaking down nutrient molecules. Once food leaves the duodenum to pass into the central portion of the small intestine, it is essentially nothing more than a collection of raw nutrient molecules—sugars from starch, amino acids from protein and triglycerides from fat.
Absorption
The small intestine is the part of the digestive system responsible for most nutrient absorption, notes Dr. Lauralee Sherwood in her text, “Human Physiology.” Here, specialized cells forming the lining of the intestinal tract, called the digestive epithelium, bind to nutrient molecules and transport them into the bloodstream. Vitamins and minerals are absorbed all along the intestine, even by the large intestine, which is the last section of gastrointestinal tract through which food passes before elimination via the anus. The large intestine also absorbs water, helping the body to conserve much of this precious resource.
References
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
