Anatomy and Physiology Resource Site

13.1 - Introduction: The Alimentary Canal

The alimentary canal or gastrointestinal (GI) tract is the site for the intake, processing and absorption of nutrients. Together with several accessory organs, the alimentary canal makes up the digestive system.

Functions of the digestive system

Movement

Mixing of food; helps to break down food and coat it in fluids containing digestive enzymes
Forward propulsion of food through the alimentary canal from the oral cavity to anus

Digestion

Mechanical and chemical breakdown (hydrolysis) of ingested materials into units small enough to be absorbed
Involves the secretion of digestive substances that must be actively transported into the alimentary canal

Absorption

Nutrients move from the alimentary canal into the blood or lymph
Monomer units of nutrients, water, vitamins and electrolytes are absorbed

Types of digestion

Mechanical digestion

Physical breakdown of food into smaller pieces
The mouth and the stomach are the primary sites of mechanical digestion

Chemical digestion

Breakdown of large molecules into monomer units small enough for absorption
Chemical digestion is carried out by hydrolases (enzymes) in the mouth, stomach and small intestine

Tissue layers of the alimentary canal

There are four major tissue layers that line the length of the alimentary canal. The layers vary in thickness and function in different sections of the canal. The layers are as follows, from the innermost to ourtermost:

Mucosa

Consists of squamous epithelium, lamina propria and smooth muscle
Surrounds the hollow space inside alimentary canal, the lumen

Submucosa

Consists of soft connective tissue
Contains many of the secretory glands that produce substances for the alimentary canal

Muscularis externa

Consists of circular and longitudinal layers of smooth muscle
The circular layer of smooth muscle contracts to decrease the size of the lumen
The longitudinal layer of smooth muscle contracts to move materials laterally through the alimentary canal

Peritoneum

Serous membrane with visceral and parietal layers. The parietal peritoneum binds the organs of the alimentary canal together and to the body wall.