Week 2: Gross ANatomy of the Stomach

Question 1

Functions of the Stomach

Answer 1

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)

Question 2

What are the major regions of the Stomach?

Answer 2

Cardiac (where oesophagus joins)
Fundus (superiorly, furthest away from exit)
Body (most of stomach)
Pyloric Artium
Pyloric Canal

Question 3

What are the important anatomical structures of the Stomach? (aka sphincters, notches, curvatures)

Answer 3

LES (aka cardiac sphincter) which leads from oesophagus to cardiac region

Cardiac notch

Angular notch

Greater Curvature

Lesser Curvature

Pyloric Sphincter

Question 4

How much pressure does LOS generate?

Answer 4

20-30mmHg

Question 5

What are the layers of muscle of the stomach, and where is each located?

Answer 5

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

Question 6

What is the usual pattern of muscle in most of the gut?

Answer 6

Usually an outer longitudinal layer and inner circular layer

Question 7

In which quadrants and regions is the stomach located?

Answer 7

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

Question 8

Surface anatomy of the LOS?

Answer 8

Just infero-lateral (left side) to the xiphoid process

Just to the left of vertebral level T11

Question 9

Surface Anatomy of Pyloric Sphincter?

Answer 9

Just to the Right of L1/L2 plane

Question 10

Describe the arrangement of the visceral and parietal peritoneum

Answer 10

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.

Question 11

What is the peritoneal cavity?

Answer 11

Potential space between the visceral and peritoneal membranes. Contains peritoneal fluid, which helps to lubricate the gut’s movement and expansion

Question 12

What is the omentum and the mesentary?

Answer 12

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)

Question 13

What are the divisions of the peritoneal cavity? How do the 2 divisions communicate?

Answer 13

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

Question 14

What is the portal triad?

Answer 14

Composed of the hepatic portal vein, the hepatic artery and common bile duct

Question 15

Structures anterior, left, right, posterior and inferior to the stomach

Answer 15

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

Question 16

What are the major arteries of the stomach, where do they arise from, and where are they located with regard to the stomach?

Answer 16

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).

Question 17

Describe the venous drainage of the stomach, in general

Answer 17

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**

Question 18

Describe the specific venous drainage of the stomach

Answer 18

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

Question 19

Describe the lymphatic drainage of the stomach

Answer 19

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)

Question 20

Describe the parasympathetic innervation of the Stomach

Answer 20

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

Question 21

Describe the Sympathetic Innervation of the Stomach

Answer 21

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

Question 22

Describe the afferent (sensory) innervation of the stomach

Answer 22

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.

Question 23

When does gastric motility occur?

Answer 23

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.

Question 24

Gastric Motility: Filling phase

Answer 24

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).

Question 25

Gastric Motility: Mixing Phase

Answer 25

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

Question 26

Gastric Motility: Emptying Phase

Answer 26

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

Question 27

Why does the stomach empty into the duodenum slowly?

Answer 27

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

Question 28

Describe the regulation of motility and secretions of the stomach

Answer 28

There are 3 phases of control: cephalic, gastric and intestinal control

These pose both neural and hormonal control

Question 29

Describe the Cephalic regulation of stomach motility and secretions

Answer 29

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

Question 30

Describe the Gastric phase of regulation of stomach motility and secretions

Answer 30

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

Question 31

Describe the Intestinal Phase of Regulation

Answer 31

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