Introduction to the Stomach (Digestive System)
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 and begins 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.
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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.
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- 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.
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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. |
| 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.
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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: |
| |
Layer |
Sub-layers |
Description |
| |
| OUTSIDE the STOMACH |
| |
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") |
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Diagram of the outer-layers
of the Stomach |
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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. |
