3.3 Digestive System

Digestive System

Digestive anatomy is 11% of General Anatomy on the NBCE Part I test plan: oral cavity and pharynx, digestive viscera, development, and histology. Peritoneal relations are the usual way viscera items are written (which organ is secondarily retroperitoneal; what bounds the omental bursa). GI physiology (motility, secretion, absorption) is a separate Physiology topic—keep this section on structure.

Oral Cavity and Pharynx

The oral vestibule lies between lips/cheeks and teeth; the oral cavity proper is inside the dental arches. The hard palate is maxilla plus palatine bone; the soft palate is muscular. Tensor veli palatini is V3 (and hooks around the pterygoid hamulus); levator veli palatini, palatoglossus, palatopharyngeus, and musculus uvulae are CN X (pharyngeal plexus). Palatine tonsils sit in the tonsillar fossa between palatoglossal and palatopharyngeal arches. Waldeyer ring is the ring of tonsillar tissue (pharyngeal, tubal, palatine, lingual).

Tongue muscles: all intrinsic muscles and genioglossus, hyoglossus, and styloglossus are CN XII. Palatoglossus is CN X. Genioglossus protrusion is the bedside XII test (the tongue deviates toward the lesion). Sensation: anterior two-thirds general sensation lingual nerve (V3), taste chorda tympani (VII); posterior third general sensation and taste CN IX; vallecula/epiglottis CN X. Tongue mucosa comes from arches 1–4; muscle comes from occipital somites with XII.

Salivary glands:

Parotid gland is crossed by CN VII, retromandibular vein, and external carotid; the auriculotemporal nerve (V3) hitchhikes postganglionic IX fibers to the gland. Teeth: 20 deciduous, 32 permanent; maxillary teeth V2 (superior alveolar), mandibular V3 (inferior alveolar).

Pharynx: nasopharynx (pharyngeal tonsil, torus tubarius of the auditory tube), oropharynx, laryngopharynx (piriform recess—internal laryngeal nerve). Constrictors (superior from the skull base, middle from the hyoid, inferior from thyroid and cricoid cartilages) are CN X. Stylopharyngeus is the one muscle of CN IX. Inferior constrictor’s cricopharyngeus portion is the upper esophageal sphincter.

Digestive Viscera and Peritoneal Relations

Intraperitoneal organs have a mesentery and are nearly surrounded by peritoneum (stomach, liver, spleen, jejunum, ileum, transverse colon, sigmoid, tail of pancreas, first part of duodenum). Secondarily retroperitoneal organs started with a mesentery that fused to the posterior wall: D2–D4, pancreas except tail, ascending colon, descending colon. SAD PUCKER is a memory hook for primarily retroperitoneal structures (suprarenal, aorta/IVC, duodenum D2–3, pancreas except tail, ureters, colon ascending/descending, kidneys, esophagus, rectum)—use it, then check the secondarily retroperitoneal list so you do not mis-call the pancreas tail or spleen.

Lesser omentum (ventral mesentery) = hepatogastric + hepatoduodenal ligaments. The portal triad (proper hepatic artery, portal vein, common bile duct) lives in the hepatoduodenal ligament. Greater omentum hangs from the greater curve (dorsal mesogastrium). The omental bursa (lesser sac) lies behind the stomach; it communicates with the greater sac through the omental (epiploic) foramen of Winslow:

Dependent peritoneal recesses: hepatorenal pouch of Morison (supine), rectouterine pouch of Douglas or rectovesical pouch (upright). Paracolic gutters conduct fluid along the colon.

Esophagus through anal canal

The esophagus is about 25 cm, begins at the cricoid (C6), and enters the abdomen at T10 with the vagal trunks (IVC at T8, aorta at T12). Constrictions: cricopharyngeus, aortic arch, left main bronchus, diaphragmatic hiatus. Cervical esophagus is supplied by inferior thyroid arteries; thoracic by bronchial and esophageal aortic branches; abdominal by left gastric. Muscle: upper third skeletal (recurrent laryngeal), middle mixed, lower third smooth (esophageal plexus of vagus). The Z line is the squamocolumnar mucosal junction. Thoracic esophagus has adventitia; serosa appears only after the hiatus.

Stomach: cardia, fundus, body, pyloric antrum and canal. Lesser curve = left and right gastric arteries; greater curve = left and right gastro-omental plus short gastrics. Venous left gastric → portal (esophageal varices). Posterior stomach bed: left crus, spleen, left kidney and suprarenal, pancreas, splenic artery, transverse mesocolon.

Duodenum (four parts):

1. Superior (cap)—mostly intraperitoneal; peptic ulcers.
2. Descending—major duodenal papilla (hepatopancreatic ampulla of Vater: CBD + main pancreatic duct, sphincter of Oddi); minor papilla (accessory duct of Santorini) is proximal to it.
3. Horizontal—crossed anteriorly by the SMA and SMV (SMA syndrome if the angle is too acute).
4. Ascending—suspensory muscle (ligament) of Treitz from the right crus marks the duodenojejunal flexure.

Jejunum (left upper): long vasa recta, few arterial arcades, tall dense plicae circulares. Ileum (right lower): short vasa recta, many arcades, Peyer patches, sparser plicae. Meckel diverticulum is a vitelline-duct remnant on the antimesenteric ileum (rule of 2s: 2% of people, 2 feet from the ileocecal valve, 2 inches long, 2 types of ectopic mucosa).

Large intestine identifiers: taeniae coli, haustra, omental appendices. Cecum and appendix are intraperitoneal (mesoappendix). McBurney point is one-third of the way from the right ASIS to the umbilicus. Transverse and sigmoid colon have mesenteries; ascending and descending colon are secondarily retroperitoneal. Marginal artery of Drummond links SMA and IMA; watershed zones include Griffith point (splenic flexure) and Sudeck point (rectosigmoid).

Rectum begins at about S3 as taeniae spread into a continuous longitudinal coat; three transverse rectal folds (Houston). Anal canal (~4 cm):

Internal anal sphincter is smooth muscle (autonomic continuation of circular muscle). External sphincter is skeletal, pudendal (S2–S4).

Liver, gallbladder, pancreas

Liver: right, left, caudate, and quadrate lobes morphologically; Couinaud segments I–VIII functionally (each with its own portal triad). Caudate (I) is independent and drains directly to the IVC. Porta hepatis: portal vein posterior, hepatic artery anterior-left, bile duct anterior-right. Blood supply ~75% portal, ~25% hepatic artery; all outflow is hepatic veins → IVC. Bare area between the coronary ligaments has no peritoneum and contacts the diaphragm. Falciform ligament splits into coronary/triangular ligaments; its free edge is ligamentum teres (obliterated umbilical vein). Ligamentum venosum (ductus venosus remnant) lies in the fissure between left lobe and caudate.

Gallbladder: fundus, body, neck (Hartmann pouch). Fundus reaches the ninth costal cartilage in the midclavicular line. Cystic duct + common hepatic duct = common bile duct, which passes behind D1 and through the pancreatic head to the major papilla. Cystohepatic (Calot) triangle (cystic duct, common hepatic duct, inferior liver) contains the cystic artery, usually from the right hepatic.

Pancreas: head and uncinate process (uncinate hooks behind SMA/SMV), neck (portal vein forms behind it), body, tail (intraperitoneal in the splenorenal ligament). Main duct of Wirsung joins the CBD; accessory Santorini is the dorsal-bud duct. Arterial supply is both celiac (gastroduodenal / pancreaticoduodenal arcades and splenic branches) and SMA (inferior pancreaticoduodenal)—a watershed that also explains why the pancreas is a foregut organ with midgut arterial contribution at the head.

Development

The gut tube is endoderm with splanchnic mesoderm walls. Buccopharyngeal membrane ruptures in week 4 to open the stomodeum. Thyroid diverticulum descends from the foramen cecum of the tongue.

Foregut (celiac): esophagus through the proximal duodenum at the major papilla, plus liver, gallbladder, pancreas, and (by mesentery) spleen. Stomach rotates 90° clockwise so the left vagus becomes anterior. Liver grows in the septum transversum from a hepatic diverticulum; extrahepatic bile ducts share that bud. Dorsal pancreatic bud forms the body, tail, and most of the head and the accessory duct. Ventral pancreatic bud (from the hepatic diverticulum) rotates posteriorly to form the uncinate process and inferior head and the main duct. A bifid ventral bud that rotates both ways produces annular pancreas around D2.

Midgut (SMA): distal duodenum through the proximal two-thirds of the transverse colon. In week 6 the midgut physiologically herniates into the umbilical cord, rotates 270° counterclockwise around the SMA, and returns in week 10. The cranial limb (jejunum/proximal ileum) returns first to the left; the caudal limb (cecum) returns last to the right and then descends. Failed rotation leaves Ladd bands and a narrow SMA pedicle prone to volvulus. Omphalocele is persistent herniation in a covering sac at the umbilicus; gastroschisis is a right paraumbilical wall defect without a sac.

Hindgut (IMA): distal third of transverse colon through the upper anal canal. The cloaca is divided by the urorectal septum into urogenital sinus and anorectal canal. The pectinate line is the endoderm–ectoderm (proctodeum) junction; failure of the anal membrane to rupture is one form of imperforate anus.

Histology of GI Layers

From lumen outward, most of the tract has four layers:

1. Mucosa — epithelium, lamina propria, muscularis mucosae
2. Submucosa — Meissner (submucosal) plexus, vessels, glands where present
3. Muscularis externa — inner circular, outer longitudinal, Auerbach (myenteric) plexus between them
4. Serosa (visceral peritoneum) or adventitia (retroperitoneal / thoracic esophagus)

Organ specializations:

Small-intestine crypts of Lieberkühn house stem cells, goblet cells, enteroendocrine cells, and Paneth cells at the base (eosinophilic granules: lysozyme, defensins). Enterocytes have a brush border of microvilli.

Liver histology is read three ways: the classic lobule (hexagon around a central vein), the portal lobule (bile-centered), and the acinus (zones 1–3 from portal triad to central vein; zone 3 is most ischemic and most sensitive to some toxins). Hepatocytes form plates with space of Disse between them and the sinusoids. Kupffer cells are sinusoidal macrophages; hepatic stellate (Ito) cells in Disse space store vitamin A and produce collagen in fibrosis. Bile flows toward the portal triad, opposite to blood.

Gallbladder: simple columnar epithelium with clear apical cytoplasm, no muscularis mucosae and no submucosa, and Rokitansky–Aschoff sinuses (mucosal invaginations into muscle). Pancreas: serous acini with apical zymogen granules; centroacinar cells begin intercalated ducts (bicarbonate, secretin-responsive). Islets of Langerhans: beta cells (insulin) central, alpha (glucagon) peripheral, delta (somatostatin).

When you can name the peritoneal boundary, the artery of the gut segment, and the histologic giveaway (Brunner, Peyer, parietal, Hassall is thymus not gut), work mixed anatomy items at /practice/nbce-part1.