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Monogastric Stomach - Anatomy & Physiology (view source)

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==Introduction==
 
==Introduction==
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The enlarged swelling of the gastrointestinal tract between the oesophagus and duodenum is called the stomach. It is a simple structure in carnivores and a compound structure in ruminants.
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The stomach functions as a reservoir of food where digestion occurs through chemical and mechanical processes. This allows food to be broken down further and absorbed.
    
==Development==
 
==Development==
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*The gut tube is formed from the folding of splanchnopleure (mesoderm and endoderm)
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*The mesoderm forms the skeletal muscle ([[Oesophagus - Anatomy & Physiology|oesophagus]] and anus), smooth muscle (lateral plate mesoderm) and connective tissue layers around the inner endoderm.
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*The endoderm is the inner layer forming the epithelia and glands.
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1. Region enlarges. Swelling indicates where the stomach will form.
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2. Dorsal surface becomes convex to form the greater curvature
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3. Ventral surface becomes concave to form the lesser curvature
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4. 2 rotations of 90 degrees occur along the longitudinal axis and then then the dorso-ventral axis
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5. The dorsal mesogastrium becomes elongated (with the spleen) and expands into a large fold along the ventral abdominal wall. This becomes the greater omentum which covers all the abdominal organs. It is a superficial structure which is free to move.
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6. The ventral mesogastrium becomes the lesser omentum. It is inbetween the stomach and the liver. The rest of the ventral mesentry degenerates.
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==Functional Anatomy==
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*Stomach split into regions: cardia, fundus, body and pylorus
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*Entire stomach motile
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*pH 0.9 to 1.5
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==Histology==
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*Columnar epithelium
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*Folded mucosa
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*Invaginations called gastric pits which are continuous with gastric glands
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*Chief (zygomatic) cells secreting pepsinogen
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    -Secreted in response to vagus and gastrin
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*Goblet cells secreting mucous
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    -Protects against autodigestion
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*Parietal (oxyntic) cells secreting hydrochloric acid in gastric pits
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    -Aids digestion
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    -Activates gastric enzymes, e.g. pepsinogen
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    -Kills microorganisms that enter with food
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    -Secreted in response to vagus or pepsin
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*3 layers of lamina muscularis: the outer longitudinal, middle circular layers are inner oblique.
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*Glands are short, coiled, branched tubular. Need to be replaced due to wear and tear. Only in mucosal layer.
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*Cardia is a narrow muscle strip
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*Pyloric sphincter is thickened tunica muscularis from the middle circular smooth muscle layer
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==Blood Supply==
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*Coeliac artery (which is a branch of the dorsal aorta)
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*Coeliac artery splits into the hepatic artery supplying the liver, pancreas and stomach (right gastric and left gastro-epiploic arteries)
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*Coeliac artery also splits into the splenic artery which supplies the spleen and the stomach (left gastro-epiploic artery)
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*Coeliac artery also splits into the left gastric artery supplying the stomach
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*The gastro-epiploic arteries supply the greater curvature of the stomach
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*The gastric arteries supply the lesser curvature of the stomach
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==Digestive Enzymes==
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*Proteases
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    -Inactive zymogen
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    -Activated by hydrochloric acid
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    -Active pepsin produced
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    -Completed near brush border to generate small peptides and individual amino acids
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*Carbohydrases
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    -e.g. amylase
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    -Salivary and pancreatic to produce disaccharides
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    -disaccharides converted to monosaccharides near brush border
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    -Celluloses from symbiotic micro-organisms (ruminant stomachs)
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*Lipases
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    -Assisted by bile salts which neutralise stomach acids and emulsifies fats
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    -Generates free fatty acids, monoglycerides and diglycerides
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==Innervation==
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*Control of gastric secretions under hormonal (gastrin), paracrine (histamine) and neural (ACh) mediators in the cephalic and gastric phases
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*Gastric secretions inhibited during the intestinal phase by CCK and secretin
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==Species Differences==
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*Horses have a region called the margo plicatus which separates the glandular and non-gloandular parts of the equine stomach
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*The equine stomach is relatively small with a strong cardiac sphincter which prevents the animal from vomiting
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*The size of the non-glandular region in the simple stomach varies between species. It is largest in the horse, pig and then smallest in the dog.
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*The canid stomach is of variable size. A full stomach can touch the bladder.
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*The cardia in the pig is thickened, taking up nearly half the area of the stomach
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*Carnivores have a subglandular layer of fibroblasts and collagen fibres for protection, e.g. from consuming bones. It is between the glands and the lamina muscularis.
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