Describe the four layers of the GI tract.

The GI tract is composed of four layers or also know as Tunics. Each layer has different tissues and functions. From the inside out they are called: mucosa, submucosa, muscularis, and serosa.

Mucosa: The mucosa is the absorptive and secretory layer. It is composed of simple epithelium cells and a thin connective tissue. There are specialized goblet cells that secrete mucus throughout the GI tract located within the mucosa. On the mucosa layer there are Villi and Micro Villi.

Submucosa: The submucosa is relatively thick, highly vascular, and serves the mucosa. The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa. The submucosa also has glands and nerve plexuses.

Muscularis: The muscularis is responsible for segmental contractions and peristaltic movement in the GI tract. The muscularis is composed of two layers of muscle: an inner circular and outer longitudinal layer of smooth muscle. These muscles cause food to move and churn with digestive enzymes down the GI tract.

Serosa: The last layer is a protective layer. It is composed of avascular connective tissue and simple squamous epithelium. It secretes lubricating serous fluid. This is the visible layer on the outside of the organs.
Source:
http://en.wikibooks.org/wiki/Human_Physi...

1.  Mucosa is the moist innermost layer; it lines the cavity of the organ.

2.  The submucosa is found beneath the mucosa layer. It contains blood vessels, nerve endings, lymph nodules, and lymphatic vessels.

3.  The muscular externis is the next layer, which is typically made up of a circular and a longitudinal layer of smooth muscle.

4.  The outermost layer is the serosa, which consists of two single layers of cells. The innermost serosa is the visceral peritoneum while the outermost layer is the parietal peritoneum. Between these layers is serous fluid.