Anatomy of the Digestive System

Abdominal Cavity Organs

• The abdominal cavity is bordered by abdominal walls, diaphragm, and pelvis.
• It forms the major portion of the abdominopelvic cavity, which combines and is continuous with the pelvic cavity.
• The cavity contains most of the digestive organs, spleen, kidneys, and ureters.

Outline

• The outline includes organs of the abdominal cavity, blood supply, and innervation.

Digestive Tract

• Abdominal viscera make up most of the digestive tract.
• The digestive tract includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anal canal.

Abdominal Quadrants

• The abdomen is divided into four quadrants by imaginary horizontal and vertical lines intersecting at the umbilicus.
• The quadrants are: right upper quadrant (RUQ), left upper quadrant (LUQ), right lower quadrant (RLQ), and left lower quadrant (LLQ).
• Key organs for each quadrant are:
• RUQ: Liver, gallbladder, pylorus, duodenum, hepatic flexure of colon, head of pancreas
• LUQ: Spleen, splenic flexure of colon, stomach, body and tail of pancreas.
• RLQ: Cecum, appendix, ascending colon and right ovary
• LLQ: Descending colon, sigmoid colon, and left ovary

Abdominal Regions

• The abdomen is also divided into nine regions.
• The regions are: epigastric, right hypochondriac, left hypochondriac, umbilical, right lumbar, left lumbar, hypogastric (suprapubic), right iliac (inguinal), and left iliac (inguinal).

Peritoneum

• The peritoneum is a mesothelial membrane lining the abdominopelvic cavity.
• The peritoneum has two layers:
• Parietal peritoneum – lines the inside of the abdominopelvic cavity
• Visceral peritoneum – covers the organs; continuous with the parietal peritoneum
• Forms the peritoneal cavity – a potential space between the parietal and visceral peritoneum, contains ~50ml of peritoneal fluid.
• In females, there's an opening in the peritoneal cavity allowing for the passage of oocytes from the ovaries to the fallopian tube.

Peritoneum: Mesenteries

• Mesentery – double layer of peritoneum that encloses an organ & connects it to the posterior abdominal wall. Provides pathways for nerves, vessels, and lymphatics.
• Peritoneal ligaments – double layer of peritoneum that connects one organ to another or to the abdominal wall.

Intraperitoneal vs. Retroperitoneal

• Retroperitoneal: Organs only partially covered by peritoneum, remaining external to the peritoneal cavity and posterior to the peritoneum. Examples are kidneys.
• Intraperitoneal: Organs that completely lie within the peritoneal cavity & are almost completely invested by visceral peritoneum. Examples are stomach and liver.

Intraperitoneal Organs

• Organs such as the liver, tail of pancreas, spleen, stomach, duodenum (first part), jejunum & ileum, cecum, appendix, sigmoid colon, and upper 1/3 of the rectum are within the peritoneum.

Retroperitoneal Organs

• Organs like suprarenal glands, aorta/IVC, duodenum (second, third, and fourth parts only), pancreas (except tail), ureters, colon (ascending & descending only), kidneys, esophagus, and rectum are entirely or partially retroperitoneal.

Liver

• The liver is divided into four anatomical lobes: right, left, quadrate, and caudate.
• Liver functions include detoxification, glycogen storage, hormone production, plasma protein synthesis, and bile production.

Liver: Blood Supply

• The hepatic portal vein carries 75-80% of blood to the liver.

• This blood carries nutrients absorbed by the GI tract.

• The hepatic portal vein terminates by dividing into right and left branches.

• The hepatic portal vein is formed by the union of the superior mesenteric vein and splenic vein.

• The inferior mesenteric vein typically drains into the splenic vein.

Caval Venous System

• The caval venous system drains venous blood from various structures (posterior abdominal wall, kidneys, suprarenal glands, gonads, pelvic & perineal structures, and lower limbs).
• Blood bypasses the liver and enters the right atrium of the heart.
• The inferior vena cava is formed by the union of left and right common iliac veins, located posterior to the liver.

Portal-Caval Anastomoses

• Anastomoses between caval and portal veins provide collateral circulation to compensate for obstructions in the liver or portal veins.
• Common sites of anastomoses are esophageal, superior rectal, paraumbilical, and colic veins.
• Obstruction in liver or portal veins can cause varicosities & hemorrhoids.

Stomach

• The stomach stores food, chemically and mechanically preparing it for digestion and passage into the duodenum.
• Divided into four regions: cardia (first part), fundus (dome-shaped superior portion), body (largest middle section), and pylorus (terminal end).

Stomach: Ligaments

• Greater omentum – a double-layered peritoneal sheet hanging from the greater curvature of the stomach.
• Gastrosplenic ligament – connects stomach to the spleen.
• Gastrophrenic ligament – connects stomach to the inferior surface of the diaphragm.
• Gastrocolic ligament – apron-like part of the greater omentum, connects stomach to the transverse colon.

Lesser omentum

• Double layered peritoneal fold that passes from liver to lesser curvature of the stomach & superior portion of duodenum
• Hepatogastric ligament — sheet-like part, extends between liver & lesser curvature of stomach
• Hepatoduodenal ligament — thickened free edge, extends between duodenum & liver. Conducts portal triad,

Small Intestine

• The small intestine is the primary site of nutrient absorption.
• It consists of the duodenum (short, C-shaped), jejunum (~8 feet), and ileum (~12 feet).

Small Intestine: Duodenum

• Duodenum is the first and shortest part of the small intestine.
• Parts include superior (1st) part – short, mostly horizontal, lies anterolateral to body of L1; descending (2nd) part – runs vertically along sides of L2 & L3, curves around head of pancreas; Inferior (horizontal, 3rd) part crosses anterior to IVC & aorta and posterior to superior mesenteric artery & vein at L3 level; ascending (4th) part – begins to the left of L3.

Small Intestine: Jejunum and Ileum

• Jejunum largely in the left upper quadrant.
• Ileum mostly in the right lower quadrant.
• Features of jejunum include prominent plicae circulares, fewer arterial arcades, and longer vasa recta.
• Features of ileum include fewer plicae circulares, many short loops, and short vasa recta.

Small Intestine: Mesentery

• Mesentery proper, fan-shaped fold, attaches the jejunum and ileum to the posterior abdominal wall.
• Contains blood vessels, lymphatics, nerves that supply the jejunum and ileum.

Pancreas

• Pancreas is an accessory digestive gland posterior to stomach, between duodenum and spleen.
• Functions include exocrine secretion (pancreatic juice) which enters duodenum
• Endocrine secretions (glucagon & insulin).
• The parts include head – within C-shaped curve of duodenum; uncinate process – inferior part of head, posterior to superior mesenteric artery; neck—short, overlies superior mesenteric vessels; body – to the left of superior mesenteric vessels; tail – closely related to hilum of spleen & left colic flexure.

Pancreas: Ducts

• Common bile duct crosses the posterosuperior surface of the head of the pancreas.
• Main pancreatic duct begins in tail of pancreas & merges with the common bile duct into the hepatopancreatic ampulla, then to the descending (2nd) part of duodenum at major duodenal papilla.

Large Intestine

• The large intestine absorbs water & electrolytes, produces & absorbs vitamins, propelling feces towards rectum for elimination.
• Includes cecum (with appendix), ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.

Kidneys

• Kidneys remove excess water, salts, wastes from blood, return nutrients and chemicals to blood.
• Lie retroperitoneally on posterior abdominal wall.
• Located on each side of vertebral column at level of T12-L3.
• Left Kidney slightly more superior than the right Kidney.
• Kidney consists of fibrous capsule, perinephric fat, renal fascia, & paranephric fat. Parts include cortex, medulla, renal pyramid, renal papilla, renal column, renal sinus, minor and major calyx, Renal Pelvis, & Ureter.

Blood Supply

• Foregut: Esophagus, stomach, liver, gallbladder, pancreas, upper portion of duodenum—supplied by Celiac trunk.
• Midgut: Lower duodenum, jejunum, ileum, cecum, appendix, ascending colon (proximal 2/3)—supplied by superior mesenteric artery.
• Hindgut: Distal 1/3 transverse colon, descending colon, sigmoid colon, rectum, upper anal canal—supplied by inferior mesenteric artery.

Celiac Trunk

• Celiac trunk arises from the abdominal aorta at T12.
• It branches into: Left gastric artery; Splenic artery; Common hepatic artery.

Superior Mesenteric Artery

• Superior mesenteric artery arises from abdominal aorta at L1, runs between layers of the mesentery. Branches: Ileal colic, right colic, middle colic, intestinal arteries.

Inferior Mesenteric Artery

• Inferior mesenteric artery arises from abdominal aorta at L3. Branches: Left colic, sigmoid, superior rectal arteries.

Abdominal Vasculature: Veins

• Veins drain from various abdominal organs into superior mesenteric vein and inferior mesenteric vein, ultimately to the hepatic portal vein, then to the inferior vena cava.

Innervation

• Innervation of various organs explained; sympathetic and parasympathetic systems, ganglia (prevertebral and terminal) and splanchnic nerves (Greater, Lesser, Least, and Lumbar). Vagus nerve connections are also noted.