Introduction to the Stomach (Digestive System)

As summarised on the page about transit through the alimentary canal, partly digested foodstuffs pass from the mouth down the oesophagus (by means of peristaltic action) then enter the stomach via the cardiac sphincter.

General Notes about the Stomach:

The stomach is a muscular sac that churns and mixes food. It also absorbs alcohol. Mucus and proteases are present in the stomach.

Previous pages described passage of foodstuffs through the alimentary canal as far as the oesophagus, down which each bolus passes by means of peristaltic action (peristalsis). The bolus then passes from the oesophagus into the stomach through the cardiac sphincter. This is the first of a series of pages about the stomach, beginning with a reminder that the stomach lies between the oesophagus and the small intestines.

Basic Anatomy: Parts of the Stomach

As shown (above right), the stomach is located directly below the diaphragm.
The following simple diagram of the main areas of the stomach is described below.

Above: Simple diagram of areas of the stomach
(incl. adjoining oesophagus above and duodenum below).

The upper (which may also be called the superior) part of the stomach extends, and is connected to, the oesophagus.
The lower (which may also be called the inferior) part of the stomach is connected to the duodenum, which is the first part of the small intestines.

The oesophagus and duodenum are shown labelled above to indicate the position of the stomach within the digestive system.

The parts of the stomach labelled above are the cardiac sphincter, the cardia, the fundus, the body of the stomach, the pylorus, and the pyloric sphincter.


1.	Cardiac Sphincter	The cardiac sphincter is the site at which material enters the stomach, a bolus at a time, from the oesophagus. Each bolus passes from the oesophagus into the stomach through the cardiac sphincter.
1.	Cardiac Sphincter	The area of the stomach surrounding the cardiac sphincter is called the cardia and is the first region of the stomach into which material passes (from the oesophagus).
2.	Fundus	The fundus is the area of the stomach located above the cardiac sphincter.
3.	Body (of stomach)	The largest area of the stomach is called the body.
4.	Pylorus	The lowest and narrowest part of the stomach is called the pylorus.
5.	Pylonic Sphincter	The pylonic sphincter is also called the "pyloric valve" and is the route by which the contents of the stomach is squeezed out of the stomach as chyme, passing into the first part of the small intestine - called the duodenum. The outlet (from the stomach) itself is called the pyloric outlet.

Layers of the Stomach

The walls of the stomach consist of several layers of different types of tissues. Due to their complexity, these structures are best represented by a series of diagrams illustrating the individual layers. They can also be summarised in words, as follows:

OUTSIDE the STOMACH

Layer	Sub-layers	Description
Serosa		A serous membrane forms the outermost layer of stomach. It is composed of simple squamous epithelium and areolar connective tissue. It has a dense network of blood vessels (arteries & veins) that supply the stomach. The serosa is part of the wider visceral peritoneum.
Muscularis externa (3 layers of muscle fibres lying in 3 different orientations)	Longitudinal layer of muscle	Layer of smooth muscle fibres - the longitudinal layer of muscle is continuous with the layer of smooth muscle surrounding the oesophagus.
	Circular layer of muscle	Layer of smooth muscle fibres - the middle layer of muscle is called the circular layer. It wraps around the body of the stomach and extends downwards to form the pyloric sphincter (via which the contents of the stomach exits into the small intestine).
	Oblique layer of muscle	Layer of smooth muscle fibres - the innermost layer of muscle surrounds the inner-layers of the stomach and is prominent in the fundus area.
Submucosa		The submucosa is a layer of areolar loose connective tissue that contains blood vessels, lymphatic vessels, and nerves). The submucosa connects the muscularis (layer/s of the stomach) to the mucosa (layer/s of the stomach).
Mucosa	Muscularis mucosa	Band of smooth muscle and elastic fibres.
	Lamina propria	Layer of loose connective tissue that includes blood vessels, smooth muscle fibres, lymphatic tissue and sensory nerve endings.
	Mucosal epithelium	Contains specialised cells that secrete chemicals necessary for digestive processes. These chemicals include gastric juice and the hormone gastrin - and are described on the next page.

CENTER of STOMACH

(i.e. "Lumen of Stomach")

Diagram of the outer-layers of the Stomach

Above: Diagram of the outer-layers of the stomach, including the serosa and muscularis externa.

The diagram above is intended to indicate the different orientations of the smooth muscle fibres in the three layers of the muscularis externa. It does not include detail about the inner-layers of the stomach (i.e. the submucosa and mucosa), which are described on the next page.

The next page is about the anatomy of the inner-layers of the stomach.