Monogastric Stomach - Anatomy & Physiology

Introduction

Stomach Anatomy 1 - Copyright Nottingham 2008

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.

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

The gut tube is formed from the folding of splanchnopleure (mesoderm and endoderm)

The mesoderm forms the skeletal muscle (oesophagus and anus), smooth muscle (lateral plate mesoderm) and connective tissue layers around the inner endoderm.

The endoderm is the inner layer forming the epithelia and glands.

1. Region enlarges. Swelling indicates where the stomach will form.

2. Dorsal surface becomes convex to form the greater curvature

3. Ventral surface becomes concave to form the lesser curvature

4. Two rotations of 90 degrees occur along the longitudinal axis and then then the dorso-ventral axis

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.

6. The ventral mesogastrium becomes the lesser omentum. It is in between the stomach and the liver. The rest of the ventral mesentry degenerates.

Stomach Anatomy 2 - Copyright Nottingham 2008

Structure and Function

Stomach split into regions: cardia, fundic, body and pyloric parts

Entire stomach is motile

pH 0.9 to 1.5

The larger part of the stomach lies to the left of the midline, under cover from the ribcage and in contact with the liver and diaphragm. The oesophagus opens into it at the cardiac sphincter.

The smaller part of the stomach has thicker walls and passes to the right of the midline into the duodenum at the pyloric sphincter.

The angular point between the two parts of the stomach is called the angular notch (incisura)

Stomach Anatomy 3 - Copyright Nottingham 2008

The fundus is a blind dome rising above the cardia

The body extends from the cardia ventrally

The pyloric part is on the right divided into a more muscular and a less muscular half

The serosa (external peritoneum) covers the entire organ

Contractions start near the cardia and spread distally, accelerating and becoming more vigorous as they reach the pylorus region.

The pyloric sphincter is open for 1/3 of the time during contractions

The empty stomach lies completely within the rib cage and does not contact the abdominal floor. Little secretion is produced and only small peristaltic contractions occur.

Once food is offered or anticipated, secretions begin

Supported by 4 folds of peritoneum
- Gastrophrenic ligament- from the greater curvature of the stomach to the crura of the diaphragm
- Lesser omentum- connecting the lesser curvature of the stomach and the initial segment of duodenum to the liver in the region of the hepatic porta
- Gastrosplenic ligament- connecting the greater curvature of the stomach to the spleen by a double fold of peritoneum
- Greater omentum- connceting the greater curvature of the stomach to the duodenum and dorsal body wall

Histology

Vasculature

Coeliac artery (which is a branch of the dorsal aorta)
- Coeliac artery splits into the hepatic artery supplying the liver, pancreas and stomach (right gastric and right gastro-epiploic arteries)
- Coeliac artery also splits into the splenic artery which supplies the spleen and the stomach (left gastro-epiploic artery)
- Coeliac artery also splits into the left gastric artery supplying the stomach

The gastro-epiploic arteries supply the greater curvature of the stomach

The gastric arteries supply the lesser curvature of the stomach

The numerous veins join the portal vein

Innervation

Sympathetic fibres run with the arteries

Parasympathetic from the vagus nerve (CN X) are within the two vagal trunks
- In the proximal region of the stomach, vagal stimulation suppresses muscular contraction (VIP)
- In the distal region, vagal stimulation increases muscular activity (ACh)

Digestive Enzymes

Proteases
- Inactive zymogen
- Activated by hydrochloric acid
- Active pepsin produced
- Completed near brush border to generate small peptides and individual amino acids
- Starts in the stomach, continues into the small intestine

Carbohydrases
- E.g. amylase
- Salivary and pancreatic to produce disaccharides
- Disaccharides converted to monosaccharides near brush border
- Cellulases from symbiotic micro-organisms (ruminant stomachs)

Lipases
- Assisted by bile salts which neutralise stomach acids and emulsifies fats
- Generates free fatty acids, monoglycerides and diglycerides

Innervation

Control of gastric secretions under hormonal (gastrin), paracrine (histamine) and neural (ACh) mediators in the cephalic and gastric phases

Gastric secretions inhibited during the intestinal phase by CCK and secretin

See control of secretions

Species Differences

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.

Equine Stomch with Margo Plicatus - Copyright RVC 2008

Equine

A region called the margo plicatus is present which separates the glandular and non-glandular parts of the equine stomach. The non-glandular area is lined with squamous epithelium (not columnar).

The stomach is relatively small (10% GIT) with a strong cardiac sphincter which prevents the animal from vomiting. The equine stomach is rarely empty and retention time is short as expulsion into the duodenum stops when feeding stops.