Food is the fuel of life and, fortunately, the source of a lot of fun and pleasure!
Along with oxygen, nutrients from food are necessary for the production of ATP. The
functions of the digestive system are related to those of the respiratory and circulatory
systems; these systems all have roles in supporting energy production in cells. The
scientific study and medical treatment of the digestive system is gastroenterology.
Digestive Processes and General Anatomy
The primary purposes of digestion are first, to break food into forms that can be
used by cells and second, to absorb nutrients so that they can be distributed to tissues.
Digestive System Functions
The specific functions of the digestive system are as follows.
, the intake of food.
, the mechanical and chemical breakdown of food into a usable form.
, the uptake of nutrients into blood and lymph.
, the absorption of water and consolidation of wastes into feces.
, the elimination of feces.
The digestive system has two anatomical subdivisions: the digestive tract and the
accessory organs. The digestive tract is a tube that runs from mouth to anus. Accessory
organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
The digestive tract has the same basic structural plan from the esophagus to the
anal canal with modifications for the functions of specific regions. The layers of the
walls from inner to outer are mucosa, submucosa, muscularis externa, and serosa.
The mucosa lines the lumen of the digestive tract. It consists of an epithelium that
overlies a lamina propria and a thin layer of smooth muscle (muscularis mucosae). In the
oral cavity, pharynx, esophagus, and anal canal the tube is lined with stratified squamous
epithelium, well-suited to resist abrasion. From the stomach through the large intestine,
the tube is lined with simple columnar epithelium.
Surrounding the mucosa is the submucosa, a layer of loose connective tissue that
contains blood and lymphatic vessels and a nerve plexus. The muscularis externa has
two layers of smooth muscle through most of the tube; an inner circular layer and an
outer longitudinal layer. The surface of the tube is serosa, a thin layer of areolar tissue
overlain by simple squamous mesothelium. Most of the esophagus, the pharynx, and
rectum are surrounded by adventitia.