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Digestion – The Stomach
1- Introduction: The stomach absorbs water, ions and drugs, it helps to mechanically
break down the bolus and mixes the bolus with gastric secretions
2- The stomach is a J-shaped enlargement of the GI tract in the abdomen directly
inferior to the diaphragm.
a- The stomach connects the esophagus with the duodenum (the first portion
of the small intestine)
b- The stomach serves as a holding area (reservoir) for distribution the bolus.
Only a small quantity of the bolus can be released into the small intestine
at any one time.
c- The position and size of the stomach varies continuously. This is due in
large part to the movement of the diaphragm.
d- In the stomach digestion of starch continues and the digestion of fats and
proteins begins
e- In the stomach the semi-solid bolus is converted into a liquid called
“chyme”
3- The Anatomy of the Stomach
a- The stomach has four main regions: the cardia, fundus, body and pyloris
b- When the stomach is empty the mucosa lies in fold called rugae, that can
be seen with the unaided eye.
c- The concave medial border of the stomach is the lesser curvature and the
convex lateral border is the greater curvature
d- Diagram
4- Histology of the Stomach
a- Epithelial cells in the stomach lining form columns of secretory cells
called gastric glands. These cells line the many narrow channels called
gastric pits. Secretions flow from the gastric glands into the pits and then
into the lumen of the stomach
b- The gastric glands contain three types of cells. These are all exocrine
glands (Secrete their products directly into the lumen and not through a
duct)
1- Mucus neck cells -
2- Parietal cells –
3- Chief cells –
c- Gastric juice
5- Mechanical and Chemical Digestion in the Stomach – Upon entering the stomach
the presence of the bolus causing the stomach to start producing gentle mixing
waves every 15-25 seconds. These waves macerate the food, mix it with gastric
juice and reduce it to a soupy liquid called chyme. Everything that leaves the
stomach is in liquid form.
a- The mixing waves become more significant as you go down the stomach.
These waves occur every 15-20 seconds.
b- During each mixing wave a small amount of chyme (3ml) is pushed
through the pyloric sphincter into duodenum. This is called gastric
emptying
c- Most of the chyme is pushed back into the stomach where churning
continues
d- The bolus may remain in the upper part of the stomach (the fundus) for up
to one hour. While it is hare it is not mixed with gastric juices and
digestion of CHO by salivary amylase continues. However, once the bolus
begins to churn the salivary amylase is inactivated and lingual lipase that
digests fats is activated
e- During mixing parietal cells secrete H+ ions and Cl- ions separately into
the lumen of the stomach, thus producing HCL.
1- Diagram
f- The site, smell, taste and even thought of food causes the release of
acetylcholine by neurons in the stomach this stimulates parietal cells to
increase the production of H+ and Cl-. These senses also increase the
production of gastrin by the G-cells increasing the mobility of the
stomach.
g- The HCL denatures and unfolds proteins in food. These proteins are then
digested which is secreted by chief cells. Pepsin destroys the peptide
bonds between the amino acids. Pepsin is effective at very low Ph (2)
1- What keeps pepsin from eating the proteins in the stomach muscle? –
Pepsin is stored and secreted as the inactive form pepsinogen. It only
becomes active when it comes into contact with the stomach acids. In
addition the epithelial lining of the stomach is protected from the
gastic juice by a 1-3mm thick layer of mucus produced by the mucus
neck cells.
h- Very few things are absorbed in the stomach. Alcohol and aspirin are
absorbed here, some fatty acids and a little bit of water
i- Generally the stomach has been completely emptied 2-4 hours after eating.
Foods rich in CHO spend little time in the stomach, high protein foods
stay a little longer and fatty foods stay the longest.
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