Anatomy and Physiology of the Stomach

Embryologic Development

The stomach arises from the embryonic endoderm and comprises a portion of the foregut along with the esophagus, the first portion of the duodenum, as well as the liver, bile ducts, and pancreas. During the fourth week of gestation, the foregut is oriented as a craniocaudal tube with the primitive stomach and first portion of the duodenum forming the caudal end. The ventral mesogastrium and the dorsal mesogastrium are attached to the stomach anteriorly and posteriorly and suspend the stomach in the peritoneal cavity. The greater and lesser curvatures of the stomach are formed because the dorsal portion of the gastric wall grows at a faster rate than the ventral portion. At approximately weeks 7 and 8, as the foregut develops, it rotates 90 degrees clockwise on its long axis so that the ventral mesogastrium is positioned to the right of the stomach and the dorsal mesogastrium is to the left ( Fig. 56.1 ). The ventral mesogastrium forms the lesser omentum comprised of the gastrohepatic and hepatoduodenal ligaments and contains the liver, which grows rapidly and pushes the stomach to the left portion of the peritoneal cavity. The dorsal mesogastrium develops into the greater omentum, comprised of the gastrophrenic, gastrosplenic, and gastrocolic ligaments and is where the spleen is located during development. This rotation also positions the left vagal nerve trunk anterior to the stomach and the right vagal nerve trunk in the posterior position. The stomach descends as cephalad structures grow and is eventually located between T10 and L3 in the adult.

Gross Anatomy and Anatomic Relationships of the Stomach

The stomach is a dilated cylindrical J -shaped organ that rests in the epigastric and left hypochondrial region of the abdomen at the level of the first lumbar vertebra ( Fig. 56.2 ). It is bordered anteriorly by the left hemidiaphragm, the left lobe of the liver and a portion of the right lobe, and the parietal portion of the anterior abdominal wall. Posteriorly, the pancreas (neck, body, and tail), left kidney, and adrenal grand border the stomach. The spleen sits posterolaterally and the transverse colon inferiorly. The two points of attachment are at the gastroesophageal junction superiorly and the retroperitoneal duodenum. Ligamentous attachments also help to further anchor the stomach to surrounding organs: gastrophrenic (diaphragm), hepatogastric or lesser omentum (liver), gastrosplenic or gastrolienal (spleen), and the gastrocolic or greater omentum (transverse colon).

The anatomic regions of the stomach can be distinguished based on surgical landmarks ( Fig. 56.3 ). Beginning superiorly from the abdominal portion of the esophagus and the gastroesophageal junction, the cardiac portion of the stomach follows just inferiorly and the fundus of the stomach is superior and to the left extending above the gastroesophageal junction, forming a sharp angle with the distal esophagus known as the cardiac notch. The corpus or body of the stomach extends and curves inferiorly as a distensible reservoir and forms a sharp medial border called the lesser curvature to the right and a lateral border called the greater curvature on the left. The gastric antrum of the stomach is not anatomically distinguishable but is estimated to be a region from the angular notch along the distal lesser curvature to a point along an inferior line to the distal greater curvature. The gastric antrum empties into the pyloric canal leading to the pyloric sphincter, a palpable thickened ring of smooth muscle that empties into the first portion of the duodenum.

The visceral peritoneum covering the stomach forms its outermost serosal layer, which is contiguous with the lesser and greater omenta anteriorly and the anterior wall of the lesser sac posteriorly. The muscularis externa of the stomach wall comprises three layers: the outermost longitudinal muscle layer, the middle circular muscle layer, and the innermost oblique muscle layer ( Fig. 56.4 ). The longitudinal muscle layer of the stomach is concentrated proximally at the gastroesophageal junction and along the greater and lesser curvatures, and subsequently spreads unevenly over the corpus until joining more densely near the pylorus. Deep to the longitudinal muscle fibers, the circular muscle layer covers the stomach completely and is contiguous with the lower esophageal sphincter muscle proximally and forms a thickened band at the pylorus distally. The innermost oblique muscle layer is blended proximally with the circular muscle layer at the collar of Helvetius and splays incompletely over the anterior and posterior gastric walls. The submucosa, which is also the strength layer of the gastric wall, is the next layer deep to the muscle layers followed by the muscularis mucosae, and finally the mucosa, which contains the lamina propria (LP) composed of connective tissue, blood vessels, and mucosal epithelium. The inner surface of the stomach can be visualized as multiple irregular folds, termed rugae , which help to increase surface area of the stomach and flatten out to allow the stomach to expand and accommodate meals.

Vascular, Lymphatic, and Neural Supply to the Stomach

The vasculature of the stomach contains a well-developed network of anastomosing vessels that stem from the celiac trunk ( Fig. 56.5 ). This rich blood supply makes ischemia of the stomach rare and can make control of gastric hemorrhage a significant challenge. The greater and lesser omenta contain the majority of the blood vessels supplying the stomach. The left gastric artery is a direct branch from the celiac trunk and courses along the lesser curvature to anastomose distally with the right gastric artery, most often a branch of the common hepatic artery. The gastroduodenal artery also branches from the common hepatic (proximal to the right hepatic artery) and it supplies the greater curvature with the right gastroepiploic (right gastro-omental) artery. The left gastroepiploic (left gastro-omental) branches from the splenic artery at the superior and proximal portion of the greater curvature before anastomosing with the right gastroepiploic artery. The short gastric arteries supply the fundus and proximal body of the stomach by branching from the splenic hilum, unlike the other vessels that course through the greater and lesser omenta.

Venous drainage of the stomach parallels the arterial blood supply with eventual drainage into the portal vein. The left gastric vein (coronary vein) and right gastric vein course along the lesser curvature and drain directly into the portal vein. The greater curvature is drained by the right gastroepiploic vein into the superior mesenteric vein and by the left gastroepiploic vein, which empties into the splenic vein. The splenic vein also drains the short gastric veins and the inferior mesenteric vein and finally joins the superior mesenteric vein to form the portal vein. In cases of portal hypertension, portal venous drainage may be redirected to lower resistance paths, especially via the left gastric vein and esophageal tributaries and also the short gastric vein, resulting in gastric varices.

Lymphatic drainage of the stomach can also vary as much as the arterial and venous supply, and gastric carcinoma may spread to multiple lymph node groups. The cardia and proximal lesser curvature of the stomach drain to superior gastric lymph nodes near the left gastric artery and gastroesophageal junction. The distal portion of the lesser curvature drains into the suprapyloric lymph node region. Pancreaticosplenic nodes near the splenic hilum drain the fundus and proximal greater curvature of the stomach, and lymph from the distal greater curvature, antrum, and pylorus drains to the subpyloric lymph nodes. Ultimately, lymph drains to the celiac axis nodal basin, which then drains to the cisterna chyli nodes and into the thoracic duct.

The stomach receives input from both the sympathetic and parasympathetic nervous systems and also originates afferent fibers of the enteric nervous system (ENS) that provide input to the sympathetic (via splanchnic nerves) and parasympathetic systems (via the vagus). The ENS is considered the third branch of the autonomic nervous system (the other two being the sympathetic and parasympathetic) and although it is relatively poorly understood, it is recognized that it contains as many neurons as the spinal cord and can function autonomously.

Presynaptic efferent parasympathetic neurons originate in the dorsal motor nucleus and travel in the left and right vagus nerves that enter the abdomen through the esophageal hiatus on the anterior and posterior surfaces of the esophagus, respectively ( Fig. 56.6 ). These fibers synapse with postsynaptic neurons located between the circular and longitudinal muscle layers—the myenteric (Auerbach) plexus—and within the submucosal (Meissner) plexus. Afferent fibers originating in the stomach travel in the vagus and synapse with cell bodies in the nucleus of the solitary tract of the brainstem.

Presynaptic efferent sympathetic fibers travel in the sympathetic chain alongside the eighth to tenth thoracic vertebrae and synapse with neurons located in the splanchnic (celiac) ganglia before terminating in the gastric neuronal plexuses. Sympathetic afferents from the stomach have cell bodies located in the dorsal root ganglia of the thoracic spinal nerves.

Microscopic Anatomy and Physiology of the Stomach

Gastric Mucosa

The mucosal lining of the stomach is characterized by a simple columnar epithelium (SCE) that uniformly lines the stomach. Surface mucous cells (SMs) make up the gastric pits (GPs), which lead to long, branched, tubular glands, giving the gastric mucosa a leafy appearance, termed the gastric foveolae . Each gland has distinct regions from the surface down: the gastric pit, isthmus, neck, and base ( Fig. 56.7 ). The stomach can be divided into three glandular regions and these glands are composed of various cell types: cardiac glands in the cardia, oxyntic glands in the fundus and body, and antral glands in the pyloric portion of the stomach.

Cardiac glands are chiefly comprised of mucous cells along with a few scattered parietal cells, undifferentiated cells in the neck, and a majority of endocrine cells at the base of the glands. The cardiac glands make up the 10- to 30-mm transition zone between the squamous epithelium of the distal esophagus and the oxyntic glands of the fundus, and have a primary function of producing mucus. Although thought to be congenital, the expression of these glands varies among ethnic populations. When the basal half of these glands expresses more parietal cells, they are termed oxyntocardiac glands .

Oxyntic glands are located in the fundus and body of the stomach and are appropriately named for their acid-producing functions, based on the Greek oxynein , meaning “acid-forming.” The main cell types are the surface epithelial cells, the mucous cells located in the GPs and in the isthmus and neck, the parietal cells that secrete hydrochloric acid (HCl) and intrinsic factor and are heavily concentrated in the neck, the basal chief (zymogenic) cells that secrete pepsinogen, and enterochromaffin-like (ECL) cells that produce histamine—a powerful stimulus for parietal cell acid production—located throughout the gland.

The antral mucosa is distinct from fundus/body mucosa in its lack of acid-producing cells and greater proportion of gastrin-secreting G cells. Gastric mucosal cells secrete an electrolyte-rich solution that aids in churning, mixing, and lubricating food. Gastric fluid also acts as a vehicle for proteolytic enzymes that are active in the fluid phase. The volume and electrolyte composition of gastric fluid depend on stimuli such as vagal/cholinergic tone and hormonal/paracrine factors (i.e., gastrin, histamine). In healthy individuals, the basal secretory rate of the stomach is more than 60 mL of fluid hourly, which, in experimental studies, can increase to more than double that when stimulated by histamine. Total average daily fluid production is more than 1.5 L. The electrolyte composition of gastric fluid is similarly dependent on external stimuli and is summarized in Table 56.1 .

TABLE 56.1

Gastric Fluid Volume and Electrolyte Composition in 200 Healthy Volunteers

From Meeroff JC, Rofrano JA, Meeroff M. Electrolytes of the gastric juice in health and gastroduodenal diseases. Am J Dig Dis. 1973;18:865.

	Basal	Histamine
Volume (mL/h)	67.9 ± 27.0	149.1 ± 18.3
H + (mEq/L)	27.1 ± 13.8	95.0 ± 20.6
Na + (mEq/L)	48.1 ± 15.7	23.4 ± 6.1
K + (mEq/L)	13.4 ± 3.1	15.2 ± 2.2
Cl − (mEq/L)	98.5 ± 20.1	139.9 ± 16.3

Gastric Cells and Physiology of Secretory Products

Knowing the various secretory products produced by gastric cells is important in understanding the grand scheme of the stomach's complex role in digestion ( Table 56.2 ).

TABLE 56.2

Important Gastric Secretory Products

Modified from Barrett KE. Gastric secretion. In: Barrett KE, et al., eds. Gastrointestinal Physiology. 2nd ed. New York: McGraw-Hill; 2014 [chapter 3].

Product	Source	Functions
HCl	Parietal cell	Hydrolysis; sterilization of meal
Intrinsic factor	Parietal cell	Vitamin B 12 absorption
Pepsinogen	Chief cell	Protein digestion
Mucus, bicarbonate	SM	Gastroprotection
Trefoil factors	SM	Gastroprotection
Histamine	ECL cells	Regulation of gastric secretion
Gastrin	G cells	Regulation of gastric secretion
Gastrin-releasing peptide	Nerves	Regulation of gastric secretion
ACh	Nerves	Regulation of gastric secretion
Somatostatin	D cells	Regulation of gastric secretion

Ach, Acetylcholine; ECL, enterochromaffin-like; HCl, hydrochloric acid; SM, surface mucous cells.

Mucous Cells

Mucous cells are located on the surface and in the neck of the gastric glands (GGs). SMs that line the stomach lumen and the GPs have a columnar shape and secrete an alkaline, highly viscous mucous substance rich in bicarbonate ions that helps to protect the stomach mucosa from abrasive food particles and erosive gastric acid. The mucous cells located deeper in the isthmus and neck of the GGs secrete a more acidic mucin substance and have a rounded nuclei with apical secretory granules. These mucous neck cells (MNs) are the anchored pluripotent stem cells that divide to replace all other cell types in the gastric gland. The SMs have a 4- to 7-day turnover rate, whereas deeper secretory cells turn over at a much slower rate.

Parietal Cells

Parietal (oxyntic) cells are located in the neck and deeper parts of the GGs and secrete HCl and intrinsic factor. This cell has a rounded or pyramidal appearance with a round nucleus and a highly eosinophilic cytoplasm due to the mitochondrial density (30% to 40% cell volume) needed to operate the cells' H ⁺ /K ⁺ pump. Water is converted to a hydrogen ion (H ⁺ ) and a hydroxide ion (OH ⁻ ) ( Fig. 56.8 ). The H ⁺ is pumped into the gastric lumen in exchange for K ⁺ , which is maintained in the cytosol above chemical equilibrium by the basolateral Na ⁺ /K ⁺ ATPase and the sodium/potassium/chloride cotransporter (NKCC1). Research in recent years has proposed that more regulatory apical membrane channels participate in the critical process of pumping K ⁺ back into the cell from the gastric lumen. An OH ⁻ combines with CO ₂ to form a bicarbonate ion ( ), which is transported across the basolateral membrane into the bloodstream. This process is catalyzed by the carbonic anhydrase II enzyme. A chloride ion is simultaneously transported across the basolateral membrane into the parietal cell lumen and across the apical membrane to combine with H ⁺ to form HCl. When the parietal cell is stimulated by acetylcholine (Ach), histamine, or gastrin to secrete gastric acid, important intracellular events take place as the cell shifts from a resting to a secretory state. In particular, intracellular canaliculi and tubulovesicles that house the H ⁺ /K ⁺ proton pumps fuse together and with the apical membrane of the cell. This amplifies the working surface area of the cell 5- to 10-fold and the concentration of proton pumps to the apical membrane increases as well as the parietal cell's power to produce HCl.

Proton pump inhibitors (PPIs) block acid secretion by directly inhibiting the H ⁺ /K ⁺ exchange ATPase at the apical membrane. Inhibitors such as omeprazole are weak bases and become protonated in the highly acidic environment immediately surrounding the parietal cell membrane and subsequently form a covalently linked mercapto complex that inactivates the enzyme. By blocking the final step in the acid-secretion pathway, PPIs are able to attenuate acid secretion stimulated by gastrin/histamine and vagal/cholinergic pathways.

Intrinsic factor is a 45-kDa glycoprotein that is secreted by parietal cells and is required for cobalamin (vitamin B ₁₂ ) uptake in the terminal ileum. Secretion of intrinsic factor is independent of acid secretion and is unaffected by PPIs and histamine receptor blockers, although a higher gastric pH may inhibit absorption of food-bound vitamin B ₁₂ . Although intrinsic factor is synthesized and secreted in the acidic environment of the stomach, it binds with cobalamin at an optimum pH of approximately 7 and is fairly resistant to breakdown by acid and proteolytic enzymes in the stomach. Vitamin B ₁₂ is initially bound by haptocorrin (R factor), after which, exposure to the higher pH and proteolytic enzymes of the duodenum dissociates the haptocorrin–B ₁₂ complex and allows for intrinsic factor binding. Upon reaching the terminal ileum, the intrinsic factor–B ₁₂ complex is endocytosed by specialized epithelial cells. Patients undergoing proximal gastric resection or total gastrectomy and those with pernicious anemia require parenteral injections of vitamin B ₁₂ .

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