Functions of the stomach
- Stores ingested food – distension
- Mechanical digestion – peristaltic contraction churns contents
- Initial chemical digestion of proteins – secretion of gastric juice including acid and pepsinogen (pepsin, a protease)
- Secretion of intrinsic factor and gastroferritin for absorption of vitamin B12 and iron in more distal gastrointestinal tract (GIT)
- Regulated movement of food into duodenum
- Endocrine function (eg, gastrin)
Layers of muscle in the stomach
3 layers : outer longitudinal, middle circular, inner oblique
- Longitudinal layer - incomplete anteriorly and posteriorly
- Oblique layer - incomplete along curvatures
Surface anatomy of LOS
Lower oesophageal sphincter – left side of vertebra T11; just inferolateral (left side) of xiphoid process, inferior to 7th costal cartilage
Surface anatomy of pyloric sphincter
to right of L1/2 vertebrae (transpyloric plane)
Peritoneum associated with the stomach
- Developmentally, stomach has dorsal and ventral mesenteries – then rotates 90 degrees
- Lesser omentum, including hepatogastric ligament (along lesser curvature) – derived from ventral mesentery
- Greater omentum (along greater curvature) and gastrosplenic ligament – derived from dorsal mesentery
How do the greater and lesser sac communicate?
Via the epiploic (omental) foramen, posterior to portal triad of liver
Where is the lesser sac located?
omental bursa – posterior tostomach and lesser omentum
Describe the surface anatomy of the stomach
The stomach is located in the upper left quadrant, and mainly in the epigastric region (as well as the left hypochondriac, umbilic and possibly left flank, depending on distention level). Although the shape of the stomach is variable, the ends are relatively fixed.
Describe the anatomical relations of the stomach
Located anteriorly to the stomach is a small part of the liver, the chest wall and lower ribs (6, 7 and 8 and associated cartilages) and anterior abdominal wall (rectus abdominal muscle etc). To the left, and posteriorly is the spleen and some ribs. To the right is the liver. Posterior to the stomach is the lesser sac of the peritoneal cavity (omental bursa) and the pancreas. Inferior to the stomach is the transverse colon and loops of the small intestine. Superior to the stomach is the heart and diaphragm.
Describe the arterial supply of the stomach
All of the arteries of the stomach are from the coeliac artery, directly or indirectly (foregut derivative). The left gastric artery arises from the celiac trunk and the right gastric artery from the hepatic artery proper. Both travel along the lesser curvature. The right gastro-omental artery arises from the gastroduodenam artery and the left gastro-omental artery arises from the splenic artery. Both of which travel along the greater curvature. The short gastric artery supply the fundus.
Describe the venous drainage of the stomach.
The general principle of the venous system of the stomach is that venous drainage of most of the GI tract drains to portal vein (hepatic portal vein), then througha second set of capillaries in the liver (liver sinusoids), and then to hepatic veins to IVC. The veins accompany the stomach arteries. The stomach veins drain to splenic, superior mesenteric or directly to hepatic portal vein – there is not a coeliac vein.
Describe the lymphatic drainage of the stomach
The stomach is drained by four groups of lymph nodes. The left gastric arterial nodal group follows the left gastric artery and drains into the coeliac nodes. These nodes drain the lesser curvature of the stomach to the left. The short gastric and left gastroepiploic vessels nodal group which drain the left side of the greater curvature of the stomach. The pylorice nodes drain the pyloric part of the stomach. All these vessels enter into the coeliac node, which passes through the intestinal lymph trunk and then enters the cisterna chyli or the abdominal confluence of lymph trunks. The cisterna chyli drains into the thoracic duct.
Describe the parasympathetic innervation of the stomach
The preganglionic parasymthatic neuron cell bodies are located in the medulla. The right vagus nerve innervates the posterior posterior portion of the stomach while the left vagus nerve supplies the anterior part of the stomach. The anterior gastric nerve (left vagus) mainly supplies the anterior portion of the body and the posterior gastric nerve (right vagus) innervates the posterior body but also innervates a small portion of the anterior body.
The vagus nerves contribute to the oesophageal plexus, and enter the abdomen associated with the oesophagus as the oesophagus passes through the diaphragm. After entering the abdomen as the anterior and posterior vagal trunks they send branches to the abdominal prevertebral plexus.
The gastric branches of the vagal trunk are located at the stomach wall, where the preganglionic fibres synapse in the intramural ganglia, and innervate smooth muscle and glands as the enteric nervous system.
Describe the sympathetic innervation of the stomach
Prenganglionic sympathetic neuron cell bodies in the lateral horn of the spinal cord segments T6-T9 have axons which travel through sympathetic chain (but do not synapse) to the greater thoracic splanchnic nerve. The preganglionic sympathetic neurons synapse in the coeliac region and the postganglionic axons follow blood vessels to smooth muscle, glands and ENS.
Describe the afferent innervation of the stomach
Some afferents travel with the vagus nerve (stretch) for vagovagal reflexes etc. and some travel with sympathetic (pain) except through the dorsal root to dorsal horn of the same spinal cord segments (T6-9). Referred pain from the stomach is through dermatomes T6-T9, especially in the epigastric region.
What is the MMC?
During fasting, stomach quiescent except for a strong contraction once every 1.5 hours – migrating motility complex (MMC), propels residual stomach contents into the duodenum.
What are the stages of gastric motility?
During eating there are three stages: filling, mixing and emptying phases.
Describe the filling phase.
During the filling phase there is stretch-relaxation of smooth muscle. This is due to the intrinsic property of stomach smooth muscle and also because of a long reflex (vasovagal), which mediates relaxation. This allows distension of the stomach, without increasing tension on wall and intraluminal pressure – up to 1-1.5L. This occurs in the orad region.
Describe the mixing phase
During the mixing phase peristaltic waves travel from the body to the pylorus (caudad region). This causes food to mix with the gastric secretions, and physical digestion occurs – the semi-liquid mixture is chyme.
Describe the emptying stage
The emptying phase occurs simultaneously with the mixing phase. The pyloric sphincter relaxes periodically. The peristaltic wave in the pyloric region squeezes a small volume of chyme into the duodenum. The stomach takes about 2-4 hours to empty. This allows neutralization of acidic chyme in duodenum, and adequate time for digestion and absorption in the small intestine.
What are the different stages of control of motility?
There are three phases of control of motility and secretion of the stomach: cephalic, gastric and intestinal which are under neural and hormonal control. Within the gastric phase there are the three phases of filling, mixing and emptying.
Describe the cephalic phase
The cephalic phase is stimulated by the sight, smell and thought of food, or the presence of food in the mouth. It is mediated by the vagus nerve which increases motility and secretion. In the cephalic phase acetylcholine or gastrin releasing peptide (GRP), released by intramural neurons, which stimulates release of gastrin and histamine. Synergistic effect of acetylcholine (Ach), gastrin, histamine on gastric (acid) secretion.
Describe the gastric phase
The gastric phase of regulation is initiated by food entering the distending stomach – mechanoreceptors detect stretch. The local and vagus nerve mediate reflexes which increase gastrin and histamine release, and acid secretion.
Describe the intestinal phase
In the intestinal phase of regulation chyme in the duodenum inhibits gastric secretion and motility. Cholecystokinin, secretin, gastric inhibitory peptide (hormones) released by the duodenum inhibit gastric activity. Additionally, local (enterogastric) and long (vagovagal) reflexes slow stomach emptying so chyme doesn’t enter too fast.