Simply taking in nutrients and passing them through the digestive system is not nearly enough. Absorption must first occur so that fuel can enter the cell. The time frame for absorption varies, depending upon where nutrients are being absorbed. For instance, the vast majority of absorption takes place in the small intestine, but matter remains in the small intestine only a very short time. On the other hand, matter remains in the large intestine for hours, but very little absorption takes place in the large intestine.
Digestion begins in the mouth when food is mixed with saliva, containing digestive enzymes. Although there is a lot of chemical and mechanical activity during chewing, there is very limited nutrient absorption in the mouth. Some glucose absorption takes place in the mouth in the time the food is held for chewing -- less than a minute.
Although the process of digestion begins immediately when the first bite of food enters the mouth, absorption does not begin until the food reaches the stomach. Food may remain in the stomach undergoing digestion and limited absorption for approximately 3.5 hours. Although the food is in the stomach for quite some time, the only absorption that occurs in the stomach consists of some fatty acids and alcohol.
The liquefied digested material passes from the stomach to the small intestine, where more than 90 percent of nutrient absorption takes place. However, this material travels through the small intestine in only a few minutes. The lining of the intestine is covered with small protrusions, called villi, that are covered with epithelial cells for absorption. When nutrients come into contact with the epithelial cells, they are absorbed and transported to the bloodstream, where they are then delivered to the tissues or other organs.
Almost all absorption has taken place by the time the waste products reach the large intestine. The large intestine's main function is waste storage. However, some absorption does take place in the large intestine. Water and sodium are absorbed, as well as a small amount of vitamin K. Large intestine absorption occurs over several hours.
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