The Digestive System
Alimentary canal (GI tract)
Extends from the mouth to the anus, ~9 m (29 feet) in length
Involved in the digestion, absorption and metabolism of nutrients
Includes main and accessory organs:
Main organs: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and
anal canal
Accessory organs: teeth and tongue, salivary glands, liver, gallbladder, pancreas, and vermiform
appendix
Mouth
The roof of the mouth is formed by the hard and soft palates
The uvula is a downward projection of the soft palate
The floor of the mouth is formed by the tongue and its muscles
Papillae are small elevations on the mucosa of the tongue
Taste buds are found in many papillae
The lingual frenulum is a fold of mucous membrane that helps anchor the tongue to the floor of
the mouth
Disorders of the Mouth and Teeth
Problems include infections, cancer and congenital defects
Infections and cancer may spread
Leukoplakia is a precancerous condition of mouth tissue
Snuff Dipper’s Pouch is caused by the use of chewing tobacco
Squamous cell carcinoma is the most common form of mouth cancer
Dental caries
Tooth disease resulting in a permanent defect; commonly called a cavity
Infection may spread to other adjacent tissues or to the blood
Gingivitis is gum inflammation or infection
May result from poor oral hygiene, diabetes, vitamin deficiency, or pregnancy
Periodontitis involves inflammation of periodontal membrane
Often a complication of advanced or untreated gingivitis, it is the leading cause of tooth
loss in adults
Thrush or oral candidiasis is caused by a yeast-like fungal organism
Patches of “cheesy” looking exudate form over an inflamed tongue and oral mucosa
that itches and bleeds easily
This condition is common in immunosuppressed individuals (AIDS) or after the use of
antibiotic therapy
Congenital defects of the mouth
Cleft lip and cleft palate are the most common types:
They may occur alone or together
They are caused by a failure of mouth structures to fuse during embryonic development
Salivary Glands
Three pairs of glands produce about one liter of saliva each day
Parotid glands
The largest of the salivary glands, are located in front of the ear at the angle of the jaw
Ducts open into the mouth opposite the second molars
They become inflamed in mumps
Submandibular glands: ducts open on either side of the lingual frenulum
Sublingual glands: ducts open into floor of the mouth
Saliva contains salivary amylase, which begins the digestion of carbohydrates
Esophagus
A muscular, mucous-lined tube about 25 cm (10 inches) long; it connects the pharynx with the
stomach
Muscular walls help to push food toward the stomach
Sphincters in the GI tract help to keep ingested material moving in one direction: “down the
tube”
Each end of the esophagus is “guarded” by a sphincter: the upper esophageal sphincter (UES)
and the lower esophageal sphincter (LES)
GERD: gastroesophageal reflux disease (severe/ongoing heartburn); involves the backflow of
acidic stomach contents into the esophagus
Causes include problem foods and hiatal hernia; surgical and non-surgical treatment options
are available
Nonsurgical treatments include dietary changes, weight loss, acid blocking medications, and
drugs that strengthen the LES
Stomach
The size of the stomach varies; it expands after a large meal, but is about the size of a large
sausage when empty
The pylorus is the lower part of the stomach; the pyloric sphincter muscle closes the opening of
the pylorus into the duodenum
The stomach wall is composed of many smooth muscle fibers; their contractions produce
churning movements (peristalsis)
Lined by mucous membrane, the stomach has many microscopic glands that secrete gastric
juice and hydrochloric acid
Disorders of the Stomach
Ulcers are open wounds caused by the acid in gastric juice
They often occur in the duodenum or the stomach
They are associated with infection by the bacterium Helicobacter pylori and the use of NSAIDs
Stomach cancer is associated with the consumption of alcohol or preserved food and the
chewing of tobacco
Small Intestine
The small intestine is about seven meters (20 feet) long, but only around two centimetres in
diameter; it is made up of smooth muscle fibers that contract to produce peristalsis
It is lined with mucous membrane and many microscopic glands that secrete intestinal juice;
villi (microscopic, finger-shaped projections from the surface of the mucosa) contain blood and
lymph capillaries
Disorders of the Small Intestine
Enteritis: intestinal inflammation
Gastroenteritis: inflammation of the stomach and intestines (e.g. Crohn’s Disease)
Malabsorption syndrome involves a group of symptoms resulting from failure to absorb
nutrients properly (e.g., anorexia, abdominal bloating, cramps, anemia, and fatigue)
Liver and Gallbladder
The liver is the largest gland; it fills the upper right section of the abdominal cavity and extends
over into the left side; it secretes bile
The gallbladder is located on the undersurface of the liver; it concentrates and stores bile
Disorders of the liver and gallbladder
Gallstones: calculi (stones) made of crystallized bile pigments and calcium salts
Hepatitis: liver inflammation
Characterized by liver enlargement, jaundice, anorexia, discomfort, gray–white feces, and dark
urine
Caused by a variety of factors—toxins, bacteria, viruses, and parasites
Cirrhosis—degeneration of liver tissue involving replacement of normal (but damaged) tissue
with fibrous and fatty tissue
Large Intestine
Divisions
Cecum
Colon—ascending, transverse, descending, and sigmoid
Rectum
The large intestine opens to the exterior of the body via the anus
Lines with mucous membrane, the walls of the large intestine contain smooth muscle fibers
that contract to produce churning, peristalsis, and defecation
Disorders of the large intestine often relate to abnormal motility (the rate of movement of
intestinal contents)
Diarrhea results from abnormally increased intestinal motility; it may result in dehydration or,
in severe cases, convulsions
Constipation results from decreased intestinal motility
Colitis is the general name for any inflammatory condition of the large intestine
Colorectal cancer is a common malignancy of the colon and rectum associated with colonic
polyps, advanced age, low-fiber, high-fat diets, and genetic predisposition