GI Tract Anatomy: Stomach, Small Intestine, and Large Intestine from Esophagus to Rectum

The GI tract is a continuous muscular tube from mouth to anus that processes food through mechanical and chemical digestion, absorbs nutrients and water, and eliminates waste. The stomach has four regions (cardia, fundus, body, pylorus) and produces hydrochloric acid and pepsin for protein digestion. The small intestine has three segments: the duodenum (receives bile and pancreatic enzymes, 25 cm), the jejunum (primary site of nutrient absorption, characterized by thick walls and prominent circular folds), and the ileum (absorbs vitamin B12 and bile salts, has more Peyer's patches and thinner walls). The large intestine frames the small intestine as an inverted U — cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum — and its primary function is water and electrolyte recovery. The entire GI tract drains venous blood through the portal system to the liver before returning to systemic circulation.

The stomach sits in the left upper quadrant, tucked under the diaphragm and partially behind the left costal margin. It connects to the esophagus at the cardia (the gastroesophageal junction) and exits through the pyloric sphincter into the duodenum. Between these two gates lies a muscular, J-shaped organ with three unique muscle layers (most of the GI tract has two — the stomach adds an oblique inner layer that helps churn food). Four anatomical regions from top to bottom: the cardia (the small zone around the esophageal opening), the fundus (the dome-shaped upper portion that often contains a gas bubble visible on chest X-rays), the body (the largest region, where most acid is produced), and the pylorus (the muscular funnel that controls emptying into the duodenum). The pyloric sphincter is a thick ring of smooth muscle that opens and closes to regulate gastric emptying — it prevents the duodenum from being flooded with acidic chyme and also prevents bile from refluxing back into the stomach. The gastric pits contain specialized cell types that are heavily tested. Parietal cells (in the body and fundus) secrete hydrochloric acid and intrinsic factor — intrinsic factor is essential for B12 absorption in the ileum, which is why gastrectomy or parietal cell damage causes B12 deficiency and eventually megaloblastic anemia. Chief cells secrete pepsinogen (the inactive precursor to pepsin, which digests proteins). Mucous neck cells produce the alkaline mucus that protects the stomach lining from its own acid. G cells (concentrated in the pylorus) secrete gastrin, the hormone that stimulates parietal cells to produce more acid. The blood supply comes from branches of the celiac trunk: the left gastric artery (along the lesser curvature), the right gastric artery (from the hepatic artery proper), and the left and right gastroepiploic arteries (along the greater curvature). The rich anastomotic network along both curvatures means the stomach rarely suffers ischemia — you can ligate several of these arteries and the stomach still receives adequate blood flow. AnatomyIQ has interactive stomach models that let you tap each region and see the cell types, acid production, and blood supply at that location. This content is for educational purposes only and does not constitute medical advice.

The small intestine is where 90% of nutrient absorption occurs. It is 6-7 meters long in a living person (shorter than the 9+ meters measured at autopsy because smooth muscle tone keeps it shorter during life) and is divided into three segments with distinct characteristics. The duodenum is the shortest segment (about 25 cm — roughly 12 finger-widths, which is the origin of its name). It is mostly retroperitoneal (except the first part, which is intraperitoneal and mobile) and curves around the head of the pancreas in a C-shape. The duodenum receives bile from the common bile duct and pancreatic juice from the main pancreatic duct — both enter through the major duodenal papilla (ampulla of Vater) in the second (descending) part of the duodenum. The sphincter of Oddi controls the flow of bile and pancreatic secretions. Brunner's glands in the duodenal submucosa produce alkaline mucus that neutralizes the acidic chyme arriving from the stomach — these glands are unique to the duodenum and are a classic histology identification feature. The jejunum (the middle ~40% of the small intestine) is the primary absorptive workhorse. It has thicker walls, more prominent circular folds (plicae circulares), longer villi, and a richer blood supply than the ileum — all features that maximize surface area and absorption. The mesentery of the jejunum has a single row of arterial arcades with long, straight vasa recta (vessels running to the intestinal wall). On cross-section, jejunum has fewer fat deposits in its mesentery than ileum — these visual differences help surgeons distinguish jejunum from ileum intraoperatively. The ileum (the distal ~60%) is thinner-walled, has less prominent circular folds, and has more Peyer's patches (aggregated lymphoid tissue in the submucosa — part of the gut immune system). The ileum has two unique absorptive responsibilities: it is the only site where vitamin B12 (bound to intrinsic factor) can be absorbed, and it is where bile salts are recaptured and recycled back to the liver through the enterohepatic circulation. Resection or disease of the terminal ileum (as in Crohn's disease, which preferentially affects this area) causes B12 deficiency and bile salt malabsorption — leading to fat-soluble vitamin deficiency and diarrhea.

The large intestine frames the small intestine like an inverted U, running from the cecum in the right lower quadrant up, across, and down to the rectum. It is about 1.5 meters long and has three distinctive external features: teniae coli (three longitudinal muscle bands that run the length of the colon and are shorter than the colon itself, causing it to pucker), haustra (the sacculations or pouches created by the teniae), and epiploic appendages (small fat-filled pouches hanging off the serosal surface — sometimes confused with pathology on imaging). The segments in order: the cecum (a blind pouch in the right iliac fossa where the ileum enters — the appendix hangs off the posteromedial wall of the cecum), the ascending colon (retroperitoneal, running up the right side), the hepatic flexure (the turn under the liver), the transverse colon (intraperitoneal, suspended by the transverse mesocolon — the most mobile colonic segment), the splenic flexure (the turn under the spleen — the highest point of the colon, relevant for colonic gas distribution), the descending colon (retroperitoneal, running down the left side), the sigmoid colon (intraperitoneal, an S-shaped curve in the left lower quadrant — the most common site for diverticulosis), the rectum (extraperitoneal in the pelvis), and the anal canal. The primary function of the large intestine is water and electrolyte absorption. About 1.5 liters of fluid enters the cecum daily from the ileum, and the colon absorbs all but about 100-200 mL, producing formed stool. When the colon cannot keep up (infection, inflammation, or rapid transit), the result is diarrhea. When the colon absorbs too much water (slow transit, dehydration), the result is constipation. The blood supply follows a pattern: the superior mesenteric artery (SMA) supplies everything from the duodenum through the transverse colon, and the inferior mesenteric artery (IMA) supplies the descending colon through the upper rectum. The splenic flexure — where SMA territory meets IMA territory — is a watershed zone vulnerable to ischemia when blood pressure drops. Ischemic colitis preferentially affects this zone, which is why left-sided abdominal pain in an elderly patient with cardiovascular disease raises concern for mesenteric ischemia.

All venous blood from the GI tract (stomach through upper rectum) drains into the portal vein, which carries it directly to the liver before it returns to the heart. This is the hepatic portal system — a unique venous arrangement where blood passes through two capillary beds in series (intestinal capillaries first, then hepatic sinusoids). The portal vein is formed by the junction of the superior mesenteric vein and the splenic vein behind the neck of the pancreas. The inferior mesenteric vein drains into the splenic vein (or sometimes directly into the SMV or the SMV-splenic junction). This means all the nutrients absorbed by the GI tract — glucose, amino acids, fatty acids, vitamins — pass through the liver for first-pass metabolism before reaching systemic circulation. The liver processes, stores, or detoxifies these substances, which is why oral medications have a first-pass effect that reduces their bioavailability. Portal hypertension (increased pressure in the portal system, most commonly from cirrhosis) has predictable anatomical consequences. Blood finds alternative routes around the blocked liver through portosystemic anastomoses — connections between portal venous tributaries and systemic veins. These overloaded collateral pathways produce: esophageal varices (left gastric vein → esophageal veins — these can rupture and cause life-threatening GI bleeding), caput medusae (paraumbilical veins → superficial abdominal veins — visible dilated veins radiating from the navel), and hemorrhoids (superior rectal vein → middle and inferior rectal veins — portal blood finding its way into systemic veins through the rectal plexus). These three clinical findings — varices, caput medusae, hemorrhoids — are classic exam associations with portal hypertension.