The Stomach: A Bag of Acid That Does Not Digest Itself
Drop a nail into a beaker of hydrochloric acid at pH 1.5 and come back in a few days: the nail is gone. That is the pH your stomach holds, several times a day, for a lifetime — and the wall containing it is made of the same soft protein the acid is designed to dissolve. Nothing about the stomach is more remarkable than the fact that it survives its own contents. Every part of its anatomy, from the three layers of muscle in its wall to the thin film of alkaline mucus on its surface, is either a device for attacking food or a device for defending the attacker from itself. Learn the stomach as that defended compromise and every clinical problem it produces — the ulcer, the bleed, the perforation, the projectile vomiting of a six-week-old baby — falls into place.
Two men are admitted the same night with the same story: months of gnawing pain in the pit of the stomach, relieved by milk, worse when hungry. By morning they are in two different theatres for two entirely different operations. The first collapsed with a board-hard abdomen and free gas under his diaphragm on an erect chest film — his ulcer sat on the ANTERIOR wall, and when it finally ate through, it opened into the free peritoneal cavity and spilled acid across the peritoneum. The second vomited what looked like a bucket of dark blood and lost his blood pressure in the ambulance — his ulcer sat on the POSTERIOR wall, and behind that wall, pressed against it across a thin curtain of peritoneum, ran the splenic artery. Same disease, same layer of mucosa, same organism in the biopsy. The only variable was which way the ulcer happened to face. That is the whole argument for learning the relations of the stomach properly: the front of the stomach opens into a space, and the back of it lies on a bed of great vessels.
Four parts, read from the top down
The stomach is not a simple sac; it is four regions with four different jobs. The CARDIA is the short ring of stomach immediately around the gastro-oesophageal junction, where the oesophagus enters after passing through the T10 hiatus in the diaphragm; its mucosa contains mainly mucous glands, and it is the watershed at which squamous oesophageal lining meets columnar gastric lining. Above and to the left of the cardia, and higher than the entrance itself, arches the FUNDUS — the dome of the stomach, which in life is almost always full of swallowed air. That trapped bubble sits under the left dome of the diaphragm and appears on every erect abdominal or chest film as a smooth, dark, half-moon shadow: the gastric bubble, one of the most reliable landmarks in plain radiology and a clue to which side the stomach is on in a patient with situs inversus. Below the fundus is the BODY, the largest part, whose glands do most of the secretory work. The body narrows into the PYLORIC PART, itself divided into a wide PYLORIC ANTRUM and a narrow PYLORIC CANAL, which ends at the PYLORIC SPHINCTER — and that sphincter is not a physiological idea but a true anatomical structure, a palpable thickening of the circular muscle coat that a surgeon can feel between finger and thumb, and that the stomach uses to hold food back until it has been ground fine enough to pass into the duodenum described in the duodenum and pancreas.
Two curvatures, and what hangs from them
Run a finger along the right border of the stomach and you follow the LESSER CURVATURE, short and concave, continuous above with the right border of the oesophagus. Near its lower end it turns abruptly at a notch called the ANGULAR INCISURE, and this notch is worth remembering by name, because the region around it is the commonest site of a gastric ulcer — a fact endoscopists rely on when they steer the scope. From the whole length of the lesser curvature the LESSER OMENTUM ascends to the liver as a double sheet of peritoneum carrying the portal triad in its free edge. The LEFT border is the GREATER CURVATURE, long and convex, four to five times the length of the lesser. From it the GREATER OMENTUM hangs down like an apron over the intestines, and from its upper part the GASTROSPLENIC LIGAMENT runs left to reach the organ described in the spleen, carrying the short gastric and left gastro-omental vessels between its layers. These folds are not decoration: they are the routes by which every vessel, nerve and lymphatic reaches the stomach, and they are mapped in detail in the mesenteries, omenta and peritoneal ligaments. A surgeon who wants to mobilise the stomach must divide them, and to divide them safely he must know exactly what runs inside each one.
Three layers of muscle, not two
Everywhere else in the gut tube the muscle coat has exactly two layers. The stomach breaks the rule. The rest of the alimentary canal is built for TRANSPORT: an outer longitudinal layer that shortens the tube and an inner circular layer that narrows it, and between them the two produce the travelling wave of peristalsis. The stomach is built for something harder — it must not merely move a bolus along, it must GRIND it — and so it adds a third, INNERMOST OBLIQUE layer, whose fibres loop over the fundus and down across the anterior and posterior walls at an angle to the other two. A tube with two layers can only squeeze and push. A bag with three layers pulling in three different directions can knead, fold and shear its contents against themselves, which is exactly what the stomach does: waves begin in the body and race towards the closed pylorus, and when they arrive, most of the contents are squirted violently BACKWARDS into the body — a process called retropulsion — so that solid food is milled between opposing streams until the particles are small enough, about two millimetres across, for the sphincter to let them through. The oblique layer is the anatomical reason a meal leaves the stomach as a smooth cream and not as the lumps you swallowed.
Think of a concrete mixer rather than a pipe. A pipe moves material from one end to the other and does nothing to it on the way; a mixer holds its load, turns it against angled blades, and does not release it until the consistency is right. The rugae are the corrugations that let the drum expand as you load it — an empty stomach holds barely fifty millilitres and its lining is thrown into deep longitudinal ridges, and as a meal arrives those ridges flatten out one by one until the same organ comfortably holds a litre and a half without the pressure inside it rising much at all. That is why you can eat a very large meal and feel full rather than feel your abdomen become tense: the stomach is not stretching so much as UNFOLDING. And it is why the surgeon who staples off a narrow tube of stomach in a sleeve gastrectomy changes a patient's eating behaviour so profoundly — he has not removed the appetite, he has removed the folds that let the bag unfold.
The lining, and the truce it keeps with acid
Look at the mucosa under magnification and the surface is pitted with millions of GASTRIC PITS, each the mouth of several tubular GASTRIC GLANDS. Four cell types matter. PARIETAL (oxyntic) cells, most numerous in the body and fundus, pump hydrogen ions into the lumen through the H+/K+-ATPase — the proton pump — and also secrete INTRINSIC FACTOR, the only substance the stomach makes that is genuinely irreplaceable, since without it vitamin B12 cannot be absorbed in the terminal ileum and the patient develops pernicious anaemia and, eventually, a subacute degeneration of the spinal cord. CHIEF (peptic) cells, packed at the base of the glands, secrete PEPSINOGEN, an inert precursor that acid itself converts into pepsin. MUCOUS NECK CELLS produce the mucus. And in the ANTRUM, scattered among the glands, sit the G CELLS, which release GASTRIN into the blood to drive the parietal cells — the reason a gastrin-secreting tumour causes ulcers everywhere. Now the defence. The surface epithelium secretes a thick adherent gel of mucus, and beneath that gel it pumps out BICARBONATE, so a steep gradient exists across a layer half a millimetre thick: pH 2 at the top, pH 7 at the cell membrane. The gel is maintained by a rich mucosal blood flow that carries away any acid that leaks back, and both the mucus and that blood flow are driven by locally produced PROSTAGLANDINS. Which explains the single commonest drug injury in medicine: the drugs discussed in NSAIDs and their big risks block prostaglandin synthesis, the mucus thins, the blood flow falls, and the acid that was always there finally reaches a wall that can no longer defend itself.
The stomach does not resist acid — it out-runs it. The entire surface epithelium is replaced every three to five days, faster than almost any tissue in the body, so a cell that is damaged is simply discarded before the damage matters. Add the mucus gel, the bicarbonate underneath it, the tight junctions between the cells and the blood flow washing the base of the barrier, and you have four independent lines of defence, of which any one alone would fail. This is why ulcers are so rarely caused by too much acid and so often by too little defence: in most patients the acid output is entirely normal, and what has changed is that Helicobacter pylori has colonised the mucus layer, or a prostaglandin-blocking drug has thinned it. Treating an ulcer is therefore two separate acts — suppress the attack with the drugs described in proton pump inhibitors, and remove whatever has been undermining the defence.
The stomach bed: what lies behind
In front there is a cavity. Behind there is a shelf of organs — and it is loaded with arteries. ANTERIORLY the stomach is related to the left lobe of the liver, to the diaphragm, and — over a triangular area in the left hypochondrium and epigastrium — directly to the ANTERIOR ABDOMINAL WALL. Nothing of consequence lies between: an anterior ulcer that erodes through the full thickness of the wall opens straight into the greater sac of the peritoneum, and its contents flood a cavity lined by somatic-innervated parietal peritoneum. Hence the textbook picture of perforation: sudden, agonising, exactly localised pain, board-like rigidity, and free gas under the diaphragm on an erect film. POSTERIORLY the picture is completely different. The posterior wall forms the anterior boundary of the LESSER SAC (omental bursa), a flat closed space of peritoneum, and across that space lies the STOMACH BED: the body and tail of the pancreas, the left kidney and left suprarenal gland, the spleen, the transverse mesocolon, and the left crus and dome of the diaphragm — and, running along the upper border of the pancreas in a famously tortuous course, the SPLENIC ARTERY. A posterior ulcer therefore does not perforate into a cavity; it burrows into a structure. It may adhere to the pancreas and produce constant boring pain radiating to the back, or it may reach the splenic artery, in which case the patient does not develop peritonitis — he exsanguinates.
Every artery from one trunk
The stomach is pure foregut, and the foregut has exactly one artery: the coeliac trunk at T12. The COELIAC TRUNK trifurcates into the left gastric, the splenic and the common hepatic arteries, and every vessel reaching the stomach is a branch or a grandchild of one of those three. Along the LESSER CURVATURE run two vessels that meet and anastomose: the LEFT GASTRIC ARTERY, which comes directly off the coeliac trunk, arches up to the cardia (giving oesophageal branches) and then turns down along the curvature; and the RIGHT GASTRIC ARTERY, a much smaller vessel arising from the common hepatic — or from its continuation, the proper hepatic — and running up to meet it. Along the GREATER CURVATURE the same pattern repeats: the LEFT GASTRO-OMENTAL (gastroepiploic) ARTERY from the SPLENIC artery, and the RIGHT GASTRO-OMENTAL ARTERY from the GASTRODUODENAL, anastomosing between the layers of the greater omentum. And to the fundus, which neither curvature reaches, run the SHORT GASTRIC ARTERIES, five or six small vessels from the splenic artery crossing in the gastrosplenic ligament. One vessel deserves separate memory: the GASTRODUODENAL ARTERY, a branch of the common hepatic, descends BEHIND the first part of the duodenum before dividing — which is why the classic catastrophic upper gastrointestinal bleed is not from the stomach at all but from a posterior DUODENAL ulcer eroding this artery. Front perforates, back bleeds: the rule holds for both organs.
The veins accompany the arteries and share their names, but they drain somewhere quite different: not to the inferior vena cava but into the PORTAL system, and therefore through the liver first. The right and left gastro-omental veins drain into the superior mesenteric and splenic veins respectively; the right gastric vein joins the portal vein directly; the short gastric veins join the splenic. The vessel to remember is the LEFT GASTRIC (CORONARY) VEIN, which also drains into the portal vein — because its oesophageal tributaries anastomose, in the wall of the lower oesophagus, with tributaries of the azygos system draining to the superior vena cava. In health this junction is trivial. When portal pressure rises in cirrhosis, it becomes a high-volume escape route, the submucosal veins at the gastro-oesophageal junction distend into OESOPHAGEAL VARICES, and a thin-walled vein sitting under acid-exposed mucosa eventually tears: the mechanism traced through the portal system and inferior vena cava and treated as GI bleeding. LYMPHATICS follow the arteries backwards — along the left and right gastric, gastro-omental and short gastric vessels — to reach the COELIAC NODES, and efferents from there travel to the cisterna chyli and the thoracic duct. Because the thoracic duct ends at the junction of the left internal jugular and left subclavian veins, malignant cells can travel retrogradely to the node that sits there, and an enlarged, hard, painless left supraclavicular node — VIRCHOW'S NODE, and the sign called Troisier's — may be the first thing a patient with gastric cancer ever notices.
Two vagi, one sympathetic pain line
PARASYMPATHETIC supply is vagal, and the rotation of the stomach during development scrambles the sides in a way worth fixing in memory: the LEFT vagus becomes the ANTERIOR vagal trunk and the RIGHT vagus becomes the POSTERIOR trunk as they descend through the oesophageal hiatus. They increase gastric secretion and motility and relax the pylorus. Each trunk gives gastric branches along the lesser curvature — the NERVES OF LATARJET — whose terminal fibres fan out onto the antrum and pylorus in a pattern the old surgeons called the crow's foot; in the era of highly selective vagotomy the whole operation consisted of dividing the branches to the acid-secreting body and fundus while carefully SPARING the terminal nerves of Latarjet, so that the stomach would still empty. The anterior trunk also supplies the liver and gallbladder, the posterior trunk the coeliac plexus and hence most of the midgut. SYMPATHETIC fibres come from spinal segments T6 to T9, travel in the greater splanchnic nerve to the COELIAC PLEXUS, and reach the stomach along the arteries. They are largely vasomotor — but they also carry the AFFERENT PAIN fibres, and those fibres re-enter the cord at T6–T9. The brain therefore refers gastric pain to the dermatomes of those segments, which lie in the EPIGASTRIUM. That single fact is the clinical signature of the whole foregut, as explained in foregut, midgut and hindgut: stomach, duodenum, liver, gallbladder and pancreas all hurt in the same place, in the midline just below the xiphisternum, no matter which of them is inflamed.
When the compromise breaks
PEPTIC ULCER remains the commonest serious disease of the organ, and in the great majority of cases the cause is Helicobacter pylori, a spiral organism that survives in the mucus layer by splitting urea into ammonia and clouding itself in alkali. Eradicate it and the ulcer usually never returns — one of the most complete reversals of a chronic disease in all of medicine. GASTRIC CANCER is the opposite story: the stomach is capacious, so a tumour can grow for a long time without obstructing anything, and the presenting features — vague dyspepsia, early satiety, weight loss, iron-deficiency anaemia, sometimes only a Virchow's node — appear late, which is why survival remains poor outside countries that screen. INFANTILE HYPERTROPHIC PYLORIC STENOSIS is the anatomy of the sphincter made visible: in a baby of three to six weeks, usually a firstborn male, the circular muscle of the pylorus hypertrophies until the canal is a slit, milk cannot leave, and the feed is expelled PROJECTILE and non-bilious minutes after every feed; the child is ravenously hungry immediately afterwards, a peristaltic wave may be seen crossing the epigastrium, and an olive-shaped mass can be palpated to the right of the midline. HIATUS HERNIA lets the cardia and part of the stomach climb through the diaphragm into the chest, abolishing the pinchcock and producing reflux. And BARIATRIC surgery is applied gastric anatomy: a sleeve gastrectomy staples away most of the greater curvature — necessarily dividing the short gastric and left gastro-omental vessels while leaving the lesser-curvature arcade to feed what remains — while a Roux-en-Y bypass leaves a small proximal pouch of the cardia and fundus joined directly to jejunum.
The vomiting that changed the blood: a woman with a scarred, stenosed pylorus after years of untreated ulcer disease vomits repeatedly for three days. She is losing hydrochloric acid — hydrogen ions AND chloride — and nothing else, because the obstruction is above the level at which alkaline pancreatic and biliary secretions enter. Her blood becomes a hypochloraemic, hypokalaemic METABOLIC ALKALOSIS, and paradoxically her urine turns acidic as the dehydrated kidney trades hydrogen ions to hold on to sodium. The biochemistry is pure anatomy: the level of the obstruction dictates the electrolyte picture. The baby with the olive: a six-week-old boy is fed, vomits the entire feed across the room, and immediately roots hungrily for more. Ultrasound measures a pyloric muscle four millimetres thick and a canal seventeen millimetres long. He is not rushed to theatre — he is rehydrated and his alkalosis corrected first, because anaesthetising an alkalotic infant is dangerous; the pyloromyotomy that follows simply splits the hypertrophied circular muscle down to the mucosa and leaves it to gape. The bleed behind the wall: a man on long-term anti-inflammatory tablets for arthritis presents with melaena and a systolic pressure of eighty. At endoscopy a vessel is seen pumping in the base of an ulcer on the posterior wall of the antrum. The clip goes on the vessel, not the ulcer — the ulcer is only the hole through which the artery became visible.
- Four parts: CARDIA around the gastro-oesophageal junction; FUNDUS, the dome above and left of it holding the gas bubble seen under the left diaphragm on an erect film; BODY, the largest and most secretory; and the PYLORIC PART — antrum, canal and the pyloric sphincter, a true thickening of circular muscle.
- The LESSER curvature bears the angular incisure — the commonest site of gastric ulcer — and gives attachment to the lesser omentum; the GREATER curvature gives attachment to the greater omentum and, above, to the gastrosplenic ligament.
- The muscle coat is UNIQUE in having THREE layers — longitudinal, circular and an INNERMOST OBLIQUE — which is what lets the stomach churn and retropulse rather than merely propel.
- Rugae are longitudinal mucosal folds that flatten as the stomach fills, allowing it to expand from about 50 mL empty to roughly 1.5 L without a large rise in pressure.
- Gastric glands: PARIETAL cells (acid + intrinsic factor), CHIEF cells (pepsinogen), mucous neck cells (mucus), and G cells in the ANTRUM (gastrin). Loss of intrinsic factor causes pernicious anaemia.
- Defence is a mucus–bicarbonate barrier plus prostaglandin-driven mucosal blood flow and rapid epithelial turnover — precisely what NSAIDs undermine.
- ANTERIOR relations: left lobe of liver, diaphragm and anterior abdominal wall — so an anterior ulcer PERFORATES into the peritoneal cavity. POSTERIOR relations across the lesser sac form the STOMACH BED: pancreas, left kidney and suprarenal, spleen, splenic artery, transverse mesocolon and diaphragm — so a posterior ulcer ERODES and BLEEDS.
- All arteries come from the COELIAC TRUNK: left gastric (from the trunk) and right gastric (from the hepatic) on the lesser curvature; left gastro-omental (from the splenic) and right gastro-omental (from the gastroduodenal) on the greater; and short gastric arteries from the splenic to the fundus.
- The GASTRODUODENAL artery lies behind the first part of the duodenum — the vessel eroded by a posterior DUODENAL ulcer and the classic cause of torrential upper GI haemorrhage.
- Venous drainage is entirely PORTAL; the left gastric (coronary) vein feeds the porto-systemic anastomosis at the lower oesophagus that becomes oesophageal varices in portal hypertension.
- Lymph follows the arteries to the COELIAC NODES, then via the cisterna chyli and thoracic duct — hence the hard left supraclavicular VIRCHOW'S NODE in advanced gastric cancer.
- Nerves: ANTERIOR vagal trunk = LEFT vagus, POSTERIOR = RIGHT (secretion and motility, with the nerves of Latarjet); sympathetic T6–T9 via the greater splanchnic nerve and coeliac plexus carry pain — referred to the EPIGASTRIUM, the foregut signature.
- Assuming the anterior wall is the dangerous one because it is nearest the surface. It is the POSTERIOR wall that is dangerous: in front lies an empty peritoneal cavity, behind lies the stomach bed with the splenic artery on it. Anterior ulcers perforate; posterior ulcers bleed.
- Mixing up the vagal trunks. Because the stomach ROTATES in development, the LEFT vagus ends up ANTERIOR and the RIGHT vagus POSTERIOR — not the other way round.
- Calling the pyloric sphincter a purely functional or physiological sphincter, as at the gastro-oesophageal junction. The pylorus is a TRUE anatomical sphincter — a discrete thickening of circular muscle you can feel — which is exactly why it can hypertrophy into infantile pyloric stenosis.
A 58-year-old man with a long history of dyspepsia vomits a large volume of fresh blood and becomes shocked. At endoscopy a chronic ulcer is found on the POSTERIOR wall of the stomach, with a visible vessel in its base. Which artery has most likely been eroded?
- Four parts (cardia, fundus, body, pyloric part with antrum, canal and a TRUE muscular pyloric sphincter), two curvatures (lesser with the ulcer-prone angular incisure and the lesser omentum; greater with the greater omentum and gastrosplenic ligament), and a UNIQUE three-layered muscle coat whose innermost oblique layer allows churning.
- The mucosa expands from 50 mL to 1.5 L as its rugae flatten, and defends itself with a mucus–bicarbonate barrier, prostaglandin-driven blood flow and three-to-five-day epithelial turnover; parietal cells make acid and intrinsic factor, chief cells pepsinogen, and antral G cells gastrin.
- Anteriorly lie liver, diaphragm and abdominal wall — so anterior ulcers perforate; posteriorly, across the lesser sac, lies the stomach bed (pancreas, left kidney and suprarenal, spleen, splenic artery, transverse mesocolon, diaphragm) — so posterior ulcers bleed.
- Blood comes entirely from the coeliac trunk (left and right gastric; left and right gastro-omental; short gastric), drains to the PORTAL system (left gastric vein → oesophageal varices) and lymph runs to coeliac nodes and on to Virchow's node; the vagi (anterior = left) drive secretion while sympathetic T6–T9 afferents refer pain to the EPIGASTRIUM.
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- Netter FH. Atlas of Human Anatomy — Stomach in situ; arteries of the stomach, liver and spleen; mucosa of the stomach.
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- Snell RS. Clinical Anatomy by Regions — Peptic ulceration, gastric carcinoma and congenital pyloric stenosis.
- TeachMeAnatomy — The Stomach; The Coeliac Trunk.
