Week 2: Gross ANatomy of the Stomach Flashcards
Functions of the Stomach
Stores in ingested food - distends to accommodate
Mechanical digestion via peristalsis
Initial digestion of proteins via secretion of gastric juice, including acid and pepsinogen (converts to pepsin)
Secretes Intrinsic factor - binds B12 for absorption
Gastroferratin - binds iron
Regulated movement of food into the duodenum: releases small amounts of chyme at a time to allow duodenum to neutralise the stomach acid, allowing time also for duodenal digestion and absorption to occur (metered release so as not to overwhelm the duodenum)
What are the major regions of the Stomach?
Cardiac (where oesophagus joins) Fundus (superiorly, furthest away from exit) Body (most of stomach) Pyloric Artium Pyloric Canal
What are the important anatomical structures of the Stomach? (aka sphincters, notches, curvatures)
LES (aka cardiac sphincter) which leads from oesophagus to cardiac region
Cardiac notch
Angular notch
Greater Curvature
Lesser Curvature
Pyloric Sphincter
How much pressure does LOS generate?
20-30mmHg
What are the layers of muscle of the stomach, and where is each located?
Outer Longitudinal Layer: Prominent along the greater and lesser curvatures, and absent anteriorly/posteriorly
Circular Layer: Present throughout
Innermost Oblique Layer: Mainly present anteriorly and posteriorly, and incomplete along the curvatures
What is the usual pattern of muscle in most of the gut?
Usually an outer longitudinal layer and inner circular layer
In which quadrants and regions is the stomach located?
Mainly upper left quadrant, and a little in the upper right quadrant
Mainly Epigastric and Left Hypochondriac Regions, a little in the umbilical, and perhaps some in the left flank, depending on distension
Surface anatomy of the LOS?
Just infero-lateral (left side) to the xiphoid process
Just to the left of vertebral level T11
Surface Anatomy of Pyloric Sphincter?
Just to the Right of L1/L2 plane
Describe the arrangement of the visceral and parietal peritoneum
The parietal Peritoneum surrounds the abdominal cavity. It is continuous with the visceral peritoneum which surrounds the organs (largely, the intraperitoneal organs) of the abdominal cavity.
What is the peritoneal cavity?
Potential space between the visceral and peritoneal membranes. Contains peritoneal fluid, which helps to lubricate the gut’s movement and expansion
What is the omentum and the mesentary?
The omentum refers to the double-layer of visceral peritoneum that stretches from the liver to the stomach (the lesser omentum), and that which stretches from the stomach, loops down inferiorly, and then returns superiorly to the transverse colon (the greater omentum).
The lesser omentum includes the hepatogastric ligament, and stretches from the lesser curvature of the stomach to the liver. Derived from ventral mesentary
The greater omentum includes the gastrosplenic ligament (section between stomach and spleen), and is derived from dorsal mesentary
The mesentary refers to double layers of visceral peritoneum which suspend some of the intraperitoneal organs from the abdominal wall: e.g. the transverse colon to the posterior abdominal wall, and from the small intestine to the posterior abdominal wall)
What are the divisions of the peritoneal cavity? How do the 2 divisions communicate?
Greater sac: most of the peritoneal cavity
Lesser sac (aka the omental bursa): Posterior to the stomach and lesser omentum
There is communication via the omental (epiploic) foramen, which is posterior to the portal triad
What is the portal triad?
Composed of the hepatic portal vein, the hepatic artery and common bile duct
Structures anterior, left, right, posterior and inferior to the stomach
Anterior: Some of the liver, the chest wall: lower ribs 6, 7, & 8 and their costal cartilages, the diaphragm (also superiorly), rectus abdominalis
Right: Liver
Left: Chest wall
Posteriorly: Pancreas, Adrenal gland, bit of kidney (L), transverse colon
Inferiorly: Transverse colon, small intestine
What are the major arteries of the stomach, where do they arise from, and where are they located with regard to the stomach?
Along the lesser curvature: Left gastric artery, arising from the coeliac trunk, and the Right gastric artery, arising from the Hepatic artery proper (arises from coeliac trunk)
Along Greater Curvature: Left Gastroomental artery, arising from the splenic artery which arises from the coeliac trunk. And the Right gastroomental artery which arises from the Gastroduodenal artery, which arises from the Hepatic artery proper (arising from coeliac trunk).
Fundus area supplied by the Short Gastric Arteries, which arise from the Splenic artery (which arises from the coeliac trunk).
Describe the venous drainage of the stomach, in general
General Principle: Most of the venous drainage from GIT drains to the Hepatic Portal Vein, then through the Liver Sinusoidal Capillary networks, then to Hepatic Veins, which drain into the IVC.
Thus, veins from the GIT perfuse the liver before returning to the IVC.
Stomach veins drain to the Splenic, Superior Mesenteric veins, or directly into the Hepatic Portal Vein.
*There is no coeliac vein**
Describe the specific venous drainage of the stomach
Short Gastric Arteries and the Left Gastroomental Artery drain into the Splenic Vein
The Right Gastroomental vein drains into the Superior Mesenteric vein.
The Superior Mesenteric and Splenic Veins then combine to form the Hepatic Portal Vein
The Left and Right Gastric Veins drain directly into the Hepatic Portal Vein
The Hepatic Portal Vein drains into Liver Sinusoids (capillary networks) where its blood mixes with blood from the Hepatic Artery. This blood then drains into the Hepatic Vein, which drains into the IVC
Describe the lymphatic drainage of the stomach
Superior Gastric Nodes - run along lesser curvature
Pericardial Nodes - at cardiac region of stomach
Pancreaticolineal Nodes - between stomach and spleen
Inferior Gastric Nodes - run along greater curvature
Subpyloric Nodes - Pyloric region
Nodes of the stomach drain to the Coeliac Nodes, then into the Intestinal Lymph Trunk, which drains into the Cisterna Chyli, which drains into the Thoracic duct, which rejoins the circulation at the union of the L Subclavian and L Internal Jugular vv (L Brachiocephalic)
Describe the parasympathetic innervation of the Stomach
Preganglionic parasympathetic neuron cell bodies lie in the medulla.
Their preganglionic axons run with the L & R Vagus Nerves (CNX).
These nerve fibers contribute to the oesophageal plexus, then rearrange into the Anterior and Posterior Vagal Trunks
These enter the abdomen with the oesophagus via the oesophageal hiatus
Gastric Branches of the vagal trunks then enter Intramural Ganglia, where they sinapse.
Some post-ganglionics then directly innervate stomach smooth muscle or glands, or synapse with the enteric nervous system
Describe the Sympathetic Innervation of the Stomach
Preganglionic Sympathetic Neuron cell bodies lie in the lateral horn of the spinal cord, in segments T6-T9
Their axons travel through ventral root, to the spinal nerve, then through white rami communicanes to the Sympathetic Chain.
The preganglionic nerves do not synapse in the Sympathetic chain - they pass through the sympathetic chain and form part of the Greater Thoracic Splanchnic Nerve.
They then synapse in the coeliac ganglion. Postganglionic axons follow blood vessels to stomach and synapse directly with smooth muscle, glands, or synapse with the Enteric Nervous System
Describe the afferent (sensory) innervation of the stomach
Stretch reflex afferents travel with the vagus nerve
Pain fibres travel with sympathetics, except run through the dorsal root to the dorsal horn of spinal cord segments T6-T9 (because afferent fibres run through dorsal root to dorsal horn, whereas efferent fibres run from lateral horn through the ventral root)
Pain fibres running with sympathetics to dorsal horns of T6-T9 explains referred pain for stomach - correlates to T6-T9 dermatomes, especially in epigastric region.
When does gastric motility occur?
During eating: Filling, mixing and emptying phases
During fasting: stomach quiescent except for a strong contraction once every 1.5 hours, when migrating motility complex (MMC) causes a peristaltic contraction of the stomach to propel any residual stomach contents into the duodenum. The MMC is regulated by motilin.
Gastric Motility: Filling phase
During filling, the stomach undergoes stretch-relaxation - an intrinsic property of the stomach muscle
Filling of the stomach triggers a reflex-mediated relaxation which is vasovagal
This property allows distention of the stomach, without increasing wall tension and intraluminal pressure up to a capacity of 1-1.5L.
This occurs mainly in the orad region (more proxinal portion).
Gastric Motility: Mixing Phase
Peristaltic waves move from body to the pylorus (caudal region)
(Less peristaltic pressure occurs in the fundus region, and the strongest peristaltic motions occur in pylorus region - strength increases as move from fundus to pyloric region)
This movement mechanically breaks down food, and also mixes it with gastric secretion.
The semi-liquid product = Chyme
Gastric Motility: Emptying Phase
The emptying of chyme into duodenum occurs simultaneously with the mixing phase
Periodically, the pyloric sphincter relaxes, and the peristaltic wave in the pyloric region squeezes a small volume of the chyme through to the duodenum
Stomach takes around 2-4 hours to empty
Why does the stomach empty into the duodenum slowly?
Generally takes 2-4 hours for the stomach to empty
Slow emptying allows neutralisation of acidic chyme in duodenum
Also allows adequate time for digestion and absorption in the small intestine
Describe the regulation of motility and secretions of the stomach
There are 3 phases of control: cephalic, gastric and intestinal control
These pose both neural and hormonal control
Describe the Cephalic regulation of stomach motility and secretions
Triggered by the sight, smell and/or thought of food, or presence of food in the mouth
It is mediated by PSNS - vagus nerve. Result = increase in motility and secretions
ACh or gastrin-releasing peptide )GRP) is released by intramural PSNS neurons, and stimulates the release of gastrin and histamine into the stomach
There is then a synergistic effect between ACh, gastrin and histamine on gastric acid secretions
Describe the Gastric phase of regulation of stomach motility and secretions
Triggered by food entering and distending the stomach
Mechanoreceptors detect the stretch, afferents are carried with vagus nerves.
Both local and vagus-mediated reflexes ensue, resulting in increased gastrin and histamine release, and more acid secretion
The motility reflexes involve relaxation and stretch (vasovagal) during filling, and then peristalsis later on
Describe the Intestinal Phase of Regulation
The intestinal phase of regulation also occurs a bit simultaneously with the mixing phase of the stomach
The presence of Chyme in the stomach feeds back, and inhibits gastric secretion and motility
To do this, the duodenum releases Cholecystokinin, secretin, and gastric inhibitory peptide (hormones), which then inhibit gastric activity - inhibit peristalsis and lowers HCl secretions
Local (enterogastric) reflexes are also involved
This feedback mechanisms slows stomach emptying so chyme doesn’t enter the duodenum too quickly
