Anatomy Lecture Notes Section 5: The Digestive System
The Digestive System has two main components: The Gastrointestinal (G.I.) Tract and various Accessory
Structures and Organs.
The G.I. Tract is also called the alimentary (‘nourishment’) canal and consists of long muscular tube
starting from the oral cavity, where food enters the mouth, continuing through the pharynx, esophagus,
stomach, small and large intestines, to the rectum and anus, where waste is expelled as fecal matter.
The Accessory Structures and organs include: The salivary glands; mucous glands; tongue; teeth; the
liver; gallbladder and pancreas. All of these have important functions in the digestive system. Food is
propelled along the length of the G.I. tract by peristaltic movements provided by the muscular layers in
the walls of the G.I. tract. Many accessory structures assist the tract by secreting enzymes or substances
to help transform, digest, absorb or transport food as it travels along the tract.
The primary purpose of the gastrointestinal tract is to break down the large nutrients (polymers) from
the food ingested into smaller units (monomers). Once the nutrients have been broken down into their
smallest unit, they can then be absorbed across the epithelial lining into the body and these nutrients
and materials can be used in a number of ways, including providing energy for the body.
The 6 Basic Digestive Processes
1. Ingestion – taking food or drink into the mouth or oral cavity.
2. Propulsion – movement through alimentary canal. This includes movement of tongue and cheeks and
involves contraction of the muscles of swallowing, in addition to the peristaltic movement generated by
the muscular layers of the tract and the cavities created by the canal.
3. Mechanical Digestion – the Physical breakdown of food (chewing, churning), mechanical processing
and moistening of foods. This is most often required prior to chemical digestion.
4. Chemical Digestion – the Enzymatic breakdown of food (from complex to simple building blocks) by
the use of enzymes made by the body. This involves the breaking of chemical bonds.
5. Absorption – the transportation of digested products from lumen of G.I. tract across an epithelial
lining and into the blood and lymph vessels, which are considered to be inside the body.
6. Defecation – the Elimination of indigestible material and waste products from body (feces).
In summary, mechanical digestion occurs mainly in the mouth and stomach, and chemical (enzymatic)
digestion begins in the stomach (protein digestion only) and becomes significant for all nutrients in the
small intestine, where proteins, fats and carbohydrates are all chemically broken down into their basic
building blocks by a series of enzymes. Once they are broken down (catabolized) into smaller molecules,
they can then be absorbed across the epithelium of the small intestine and subsequently enter the
circulation of the body. The large intestine plays a key role in the reabsorption excess water and
electrolytes. Finally, undigested material and the secreted waste products continue along the tract and
are excreted from the body via defecation - the mass movement and elimination of feces.
, 2
The Alimentary Canal is a hollow, compartmentalized and specialized tube starting at the oral cavity and
ending at the anus.
Parotid salivary gland
Oral cavity
Sublingual Pharynx
salivary gland cavity
Submandibular Upper
salivary gland esophageal
sphincter
Esophagus Lower
esophageal
sphincter
Liver
Stomach
Pyloric sphincter
Gallbladder
Pancreas
Transverse colon
Duodenum
Jejunum
Ascending colon Descending colon
Ileum
Cecum
Appendix Sigmoid colon
Rectum
Anal canal
Anus
The Oral Cavity
The mouth or the oral cavity is the hollow space formed by the area between the lips, cheeks, tongue,
hard, soft palates and the throat. It allows food, liquid and air to enter the body. The mouth contains
many other organs, such as the teeth, tongue and the ducts of the extrinsic salivary glands. These
structures work together to aid in the ingestion and digestion of food. The mouth also plays a major role
in the production of speech through the movements of the tongue, lips and cheeks.
The lips form the anterior border of the external opening of the mouth and are very flexible and elastic
structures. They contain collagen, elastin fibers and adipose tissue covered by a thin layer of keratinized
stratified squamous epithelium. The epidermis of the lips is so thin that the color of the blood supply
immediately deep to the surface can be seen, hence the lips are usually red in color. The exterior of the
lips is continuous with the skin (covered by keratinized epithelium) while the inner surface is continuous
The Digestive System has two main components: The Gastrointestinal (G.I.) Tract and various Accessory
Structures and Organs.
The G.I. Tract is also called the alimentary (‘nourishment’) canal and consists of long muscular tube
starting from the oral cavity, where food enters the mouth, continuing through the pharynx, esophagus,
stomach, small and large intestines, to the rectum and anus, where waste is expelled as fecal matter.
The Accessory Structures and organs include: The salivary glands; mucous glands; tongue; teeth; the
liver; gallbladder and pancreas. All of these have important functions in the digestive system. Food is
propelled along the length of the G.I. tract by peristaltic movements provided by the muscular layers in
the walls of the G.I. tract. Many accessory structures assist the tract by secreting enzymes or substances
to help transform, digest, absorb or transport food as it travels along the tract.
The primary purpose of the gastrointestinal tract is to break down the large nutrients (polymers) from
the food ingested into smaller units (monomers). Once the nutrients have been broken down into their
smallest unit, they can then be absorbed across the epithelial lining into the body and these nutrients
and materials can be used in a number of ways, including providing energy for the body.
The 6 Basic Digestive Processes
1. Ingestion – taking food or drink into the mouth or oral cavity.
2. Propulsion – movement through alimentary canal. This includes movement of tongue and cheeks and
involves contraction of the muscles of swallowing, in addition to the peristaltic movement generated by
the muscular layers of the tract and the cavities created by the canal.
3. Mechanical Digestion – the Physical breakdown of food (chewing, churning), mechanical processing
and moistening of foods. This is most often required prior to chemical digestion.
4. Chemical Digestion – the Enzymatic breakdown of food (from complex to simple building blocks) by
the use of enzymes made by the body. This involves the breaking of chemical bonds.
5. Absorption – the transportation of digested products from lumen of G.I. tract across an epithelial
lining and into the blood and lymph vessels, which are considered to be inside the body.
6. Defecation – the Elimination of indigestible material and waste products from body (feces).
In summary, mechanical digestion occurs mainly in the mouth and stomach, and chemical (enzymatic)
digestion begins in the stomach (protein digestion only) and becomes significant for all nutrients in the
small intestine, where proteins, fats and carbohydrates are all chemically broken down into their basic
building blocks by a series of enzymes. Once they are broken down (catabolized) into smaller molecules,
they can then be absorbed across the epithelium of the small intestine and subsequently enter the
circulation of the body. The large intestine plays a key role in the reabsorption excess water and
electrolytes. Finally, undigested material and the secreted waste products continue along the tract and
are excreted from the body via defecation - the mass movement and elimination of feces.
, 2
The Alimentary Canal is a hollow, compartmentalized and specialized tube starting at the oral cavity and
ending at the anus.
Parotid salivary gland
Oral cavity
Sublingual Pharynx
salivary gland cavity
Submandibular Upper
salivary gland esophageal
sphincter
Esophagus Lower
esophageal
sphincter
Liver
Stomach
Pyloric sphincter
Gallbladder
Pancreas
Transverse colon
Duodenum
Jejunum
Ascending colon Descending colon
Ileum
Cecum
Appendix Sigmoid colon
Rectum
Anal canal
Anus
The Oral Cavity
The mouth or the oral cavity is the hollow space formed by the area between the lips, cheeks, tongue,
hard, soft palates and the throat. It allows food, liquid and air to enter the body. The mouth contains
many other organs, such as the teeth, tongue and the ducts of the extrinsic salivary glands. These
structures work together to aid in the ingestion and digestion of food. The mouth also plays a major role
in the production of speech through the movements of the tongue, lips and cheeks.
The lips form the anterior border of the external opening of the mouth and are very flexible and elastic
structures. They contain collagen, elastin fibers and adipose tissue covered by a thin layer of keratinized
stratified squamous epithelium. The epidermis of the lips is so thin that the color of the blood supply
immediately deep to the surface can be seen, hence the lips are usually red in color. The exterior of the
lips is continuous with the skin (covered by keratinized epithelium) while the inner surface is continuous
