Each organ of the mammalian digestive system has specialised food–processing functions.
Using the human digestive system as a model, let′s now follow a meal through the alimentary canal, examining in more detail what happens to the food in each of the processing stations along the way.
The Oral Cavity, Pharynx, and Oesophagus
Both physical and chemical digestion of food begin in the mouth. During chewing, teeth of various shapes cut, smash, and grind food, making it easier to swallow and increasing its surface area. The presence of food in the oral cavity triggers a nervous reflex that causes the salivary glands to deliver saliva through ducts to the oral cavity. Even before food is actually in the mouth, salivation may occur in anticipation because of learned associations between eating and the time of day, cooking odors, or other stimuli. Humans secrete more than a liter of saliva each day.
With a length of more than 6 m in humans, the small intestine is the longest section of the alimentary canal (its name refers to its small diameter, compared with that of the large intestine). Most of the enzymatic hydrolysis of food macromolecules and most of the absorption of nutrients into the blood occur in the small intestine.
Enzymatic digestion is completed as peristalsis moves the mixture of chyme and digestive juices along the small intestine.
A major function of the colon is to recover water that has entered the alimentary canal as the solvent of the various digestive juices. About 7 L of fluid are secreted into the lumen of the digestive tract each day, which is much more liquid than most people drink. Most of this water is reabsorbed when nutrients are absorbed in the small intestine. The colon reclaims much of the remaining water that was not absorbed in the small intestine. Together, the small intestine and colon reabsorb about 90% of the water that enters the alimentary canal.