Stomach and Duodenum

Position:

• Lies predominantly in the left hypochondrium and epigastric regions, extending toward the umbilical region; the most dilated part of the gastrointestinal tract, positioned between the oesophagus and duodenum.
• Highly variable in size, shape, and position depending on its degree of fullness, body habitus, and posture.

Parts:

• Cardia: Small region surrounding the gastro-oesophageal junction, where the oesophagus enters the stomach.
• Fundus: Dome-shaped superior portion, above the level of the cardial notch (the angle between the oesophagus and the greater curvature); typically filled with swallowed gas, visible as the gastric air bubble on imaging.
• Body: The largest part, between the fundus and the pyloric part (angular incisure marking the boundary), the main site of acid/pepsinogen secretion.
• Pyloric part: Divided into the pyloric antrum (wider, proximal) and pyloric canal (narrower, distal), terminating at the pylorus (the muscular sphincter controlling gastric emptying into the duodenum).

Curvatures:

• Greater curvature: Long, convex left/inferior border, giving attachment to the gastrosplenic ligament (upper part) and the greater omentum (lower part).
• Lesser curvature: Short, concave right/superior border, giving attachment to the lesser omentum; contains the angular incisure (incisura angularis), a notch marking the junction of the body and pyloric antrum.

Anterior relations:

• Anterior abdominal wall, left costal margin, diaphragm, left lobe of the liver.

Posterior relations (forming the "stomach bed"):

• The stomach bed comprises the structures against which the posterior stomach wall rests (separated by the lesser sac): diaphragm, spleen, left kidney and suprarenal gland, pancreas (body), splenic artery, transverse mesocolon, and splenic flexure of the colon.
• Mnemonic often taught: "Diaphragm, Spleen, Left kidney/suprarenal, Pancreas, splenic Artery, Transverse mesocolon, splenic flexure of Colon."

Blood supply:

• Left gastric artery: direct branch of the coeliac trunk, runs along the upper lesser curvature, the smallest but most direct coeliac branch to the stomach; also supplies the lower oesophagus.
• Right gastric artery: branch of the hepatic artery proper (or occasionally the gastroduodenal artery), runs along the lower lesser curvature, anastomosing with the left gastric artery.
• Left gastro-omental (gastroepiploic) artery: branch of the splenic artery, runs along the upper greater curvature.
• Right gastro-omental (gastroepiploic) artery: branch of the gastroduodenal artery, runs along the lower greater curvature, anastomosing with the left gastro-omental artery.
• Short gastric arteries: multiple small branches of the splenic artery, supplying the fundus, running within the gastrosplenic ligament.
• Together these form two anastomotic arcades along the two curvatures, giving the stomach an unusually rich collateral blood supply (clinically relevant to its resilience against ischaemia compared to other GI segments).

Venous drainage:

• Largely mirrors the arterial supply, with all gastric veins draining into the portal venous system: left and right gastric veins drain directly into the portal vein; left gastro-omental vein drains into the splenic vein; right gastro-omental vein drains into the superior mesenteric vein.
• The left gastric (coronary) vein has a clinically important connection with the oesophageal venous plexus, forming a key portosystemic anastomosis: the basis for oesophageal varices in portal hypertension.

Lymphatic drainage:

• Follows the arterial supply toward coeliac lymph nodes, via several named intermediate groups (gastric, gastro-omental, pyloric nodes along the respective curvatures), all gastric lymph converges on the coeliac nodes before continuing to cisterna chyli/thoracic duct.

Innervation:

• Parasympathetic: Vagus nerve, anterior vagal trunk (predominantly left vagus-derived) supplies the anterior stomach and gives the hepatic branches; posterior vagal trunk (predominantly right vagus-derived) supplies the posterior stomach and gives the coeliac branches. Parasympathetic stimulation increases motility and secretion.
• Sympathetic: Greater splanchnic nerve (T5-T9), via the coeliac plexus/ganglion, inhibits motility/secretion, mediates pain sensation (visceral, referred to the epigastrium, given the stomach's foregut origin).

• The first and shortest part of the small intestine, C-shaped, curving around the head of the pancreas; approximately 25 cm long, divided into four parts.

First part (superior):

• Extends from the pylorus to the neck of the gallbladder, ascending and to the right.
• The first 2-3 cm (immediately adjacent to the pylorus) is intraperitoneal (mobile, the "duodenal cap/bulb," attached to the lesser omentum superiorly and greater omentum inferiorly), the remainder is retroperitoneal.
• The classic site of duodenal peptic ulceration, given its direct exposure to the acidic gastric chyme immediately after the pylorus.

Second part (descending):

• Descends along the right side of the L1-L3 vertebral bodies, curving around the head of the pancreas.
• Entirely retroperitoneal.
• Receives the major duodenal papilla (opening of the hepatopancreatic ampulla/ampulla of Vater, combining the common bile duct and main pancreatic duct) at approximately its midpoint, and the minor duodenal papilla (opening of the accessory pancreatic duct, when present) slightly proximal to the major papilla.

Third part (horizontal/inferior):

• Crosses the midline anterior to the vertebral column (L3 level), passing anterior to the IVC and aorta.
• Entirely retroperitoneal.
• Passes posterior to the superior mesenteric artery and vein (which cross anterior to this part of the duodenum), a clinically important relationship.

Fourth part (ascending):

• Ascends and curves anteriorly to the left of the aorta, at the level of L2-L3, to form the duodenojejunal flexure, where it becomes the jejunum.
• Supported by the suspensory muscle of the duodenum (ligament of Treitz), a fibromuscular band connecting the flexure to the right crus of the diaphragm, a useful surgical landmark marking the duodenojejunal junction and the conventional boundary between "upper" and "lower" gastrointestinal bleeding for clinical classification purposes.
• Predominantly retroperitoneal, becoming intraperitoneal as it transitions into the jejunum.

Intraperitoneal and retroperitoneal relationships (summary):

• Only the proximal 2-3 cm of the first part is intraperitoneal; parts 2-4 (and the remainder of part 1) are secondarily retroperitoneal.

Relations to pancreas:

• The second part of the duodenum curves closely around the head of the pancreas, sharing a common blood supply (pancreaticoduodenal arcades) and embryological origin, this intimate relationship means pancreatic head pathology (tumour, pancreatitis) frequently involves or compresses the duodenum, and duodenal resection (Whipple procedure) necessitates concurrent pancreatic head resection given their shared blood supply.

SMA relationship:

• The third part of the duodenum passes posterior to the superior mesenteric artery and vein, in the angle between the SMA (anteriorly) and the aorta (posteriorly).
• This creates a potential site of extrinsic compression: superior mesenteric artery (SMA) syndrome: when the aortomesenteric angle is abnormally narrowed (e.g., from rapid weight loss, prolonged bed rest, or an inherently narrow angle), compressing the third part of the duodenum between the SMA and the aorta, causing proximal bowel obstruction symptoms.

Blood supply:

• Proximal duodenum (first part, and to the level of the major papilla), superior pancreaticoduodenal arteries (anterior and posterior), branches of the gastroduodenal artery (itself a branch of the common hepatic artery, from the coeliac trunk), reflecting the proximal duodenum's foregut origin.
• Distal duodenum (from the major papilla onward), inferior pancreaticoduodenal arteries (anterior and posterior), branches of the superior mesenteric artery: reflecting the distal duodenum's midgut origin.
• The major duodenal papilla is therefore the approximate anatomical/embryological watershed point between foregut and midgut blood supply within the duodenum, a key organising concept.

Venous drainage:

• Mirrors the arterial supply: superior pancreaticoduodenal veins drain to the portal vein (directly or via the splenic vein); inferior pancreaticoduodenal veins drain to the superior mesenteric vein.

Lymphatics:

• Follow the pancreaticoduodenal vessels to pancreaticoduodenal nodes, then onward to coeliac nodes (proximal duodenum) or superior mesenteric nodes (distal duodenum), again reflecting the foregut/midgut watershed.

Innervation:

• Parasympathetic (vagus, via the coeliac plexus for the proximal part), sympathetic (splanchnic nerves via coeliac and superior mesenteric plexuses), pain is typically epigastric (proximal, foregut-derived part) transitioning toward periumbilical (distal, midgut-derived part).

Peptic ulcer relations:

• Gastric ulcers most commonly occur along the lesser curvature, particularly near the angular incisure, related to relatively reduced mucosal blood flow and greater exposure to acid/pepsin at this watershed-like region between the two curvature arterial arcades.
• Duodenal ulcers most commonly occur in the first part of the duodenum (duodenal cap), the segment most directly exposed to acidic gastric chyme immediately after passing the pylorus, before pancreaticobiliary alkaline secretions have had a chance to neutralise it.

Posterior duodenal ulcer bleeding:

• A posterior duodenal ulcer (first part) can erode directly into the gastroduodenal artery, which runs immediately posterior to this segment, a classic cause of severe, potentially life-threatening upper GI haemorrhage, given the artery's substantial calibre and its fixed retroperitoneal position preventing the vessel from retracting/spasming to control bleeding as a mobile vessel might.

Superior mesenteric artery compression (SMA syndrome):

• Compression of the third part of the duodenum between the SMA (anteriorly) and the aorta/vertebral column (posteriorly), from a narrowed aortomesenteric angle.
• Risk factors include rapid or severe weight loss (reduction of the mesenteric fat pad that normally cushions this angle), prolonged supine bed rest, spinal surgery/scoliosis correction, and anatomically predisposed body habitus.
• Presents with symptoms of proximal small bowel obstruction (postprandial pain, nausea, vomiting, early satiety), often improved by positional changes (e.g., left lateral or prone positioning, which increases the aortomesenteric angle).