Digestive System Overview

• The objectives list covers the overview of the digestive system, peritoneum and mesenteries, organs of the GI tract, accessory digestive organs, and the blood supply and innervation of the digestive system.

Digestive System Components

• The Gastrointestinal (GI) Tract is a continuous tube from the mouth to the anus.
• Accessory organs are not part of the GI tract but assist the tract in digestion.
• Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Digestive System Functions

• Ingestion involves solid and liquid materials entering the oral cavity.
• Digestion is the breakdown of materials into smaller structures and molecules.
• Propulsion involves the materials being swallowed and moving through the GI tract.
• Secretion is the release of fluids to aid in digestion and protection of the GI tract.
• Absorption involves digestion products, water, vitamins, and electrolytes moving from the GI tract into the blood or lymph.
• Elimination of wastes completes the digestive process.

Abdominal Cavity

• There are 4 quadrants of the abdominal cavity.
• These are commonly used to locate organs when discussing them.
• Transverse and midsagittal planes intersecting at the umbilicus form these quadrants.
• They are the Left Upper, Right Upper, Left Lower, and Right Lower quadrants.

GI Tract Wall Layers

• The wall of the GI tract, from the esophagus to the large intestine, has four concentric layers called tunics.
• Mucosa is the deepest layer, in contact with the lumen.
• Submucosa is the layer above the mucosa.
• Muscularis is above the submucosa.
• Adventitia or Serosa is the superficial, outer layer.
• Serosa is covered by the visceral peritoneum on intraperitoneal organs.
• Adventitia is on retroperitoneal organs.

Peritoneum

• The peritoneum is the serous membrane that lines the abdominopelvic wall and many organs.
• The peritoneal cavity is the space between the parietal peritoneum (which lines the cavity) and the visceral peritoneum (which lines the organs).
• It allows for movement and expansion of organs and is filled with serous fluid.

Intraperitoneal Organs

• Intraperitoneal organs push into the abdominal cavity and are suspended via peritoneal reflections.
• This arrangement allows movement and passage for blood vessels.
• They are completely surrounded by visceral peritoneum.
• Examples include the stomach, part of the duodenum, jejunum, ileum, cecum, appendix, and transverse colon.

Retroperitoneal Organs

• Retroperitoneal organs are located between the parietal peritoneum and the posterior wall.
• They have no visceral peritoneum.
• Examples include most of the duodenum, the pancreas, ascending and descending colon, and the rectum.

Mesentery

• Mesentery is a double-layer reflection of peritoneum that forms as an organ pushes into the abdominal cavity, wrapping itself in visceral peritoneum.
• It allows for movement and serves as a conduit for blood vessels and nerves from the posterior wall to the organ.
• If an organ is "suspended" from another organ/structure due to embryonic rotation, a double-layer of peritoneum is called a ligament or omentum.

Mesenteries

• Mesentery proper runs from the abdominal wall to the small intestine.
• Transverse mesocolon runs from the abdominal wall to the transverse colon. Sigmoid mesocolon is associated with the sigmoid colon.

Ligaments

• The coronary ligament runs from the superior surface of the liver to the diaphragm and is located at the margins of the bare area.
• The falciform ligament runs from the liver to the anterior abdominal wall.

Omentums

• Greater omentum connects the greater curvature of the stomach to the transverse colon and hangs down like an apron anterior to much of the digestive tract.
• Lesser omentum connects the lesser curvature of the stomach/duodenum to the liver.

Pharynx

• The pharynx (throat) is a common space used by both respiratory and digestive systems.
• There are three segments:
  • The nasopharynx is posterior to the nasal cavity and conducts air.
  • The oropharynx is posterior to the oral cavity and conducts air, food and drink.
  • The laryngopharynx conducts air and provides passage for food and drink to the esophagus.

Esophagus

• The esophagus transports swallowed materials from the pharynx to the stomach.
• Approximately 25 cm long it courses anterior to the vertebral bodies and posterior to the trachea.
• It passes through the esophageal hiatus in the diaphragm.
• The superior esophageal sphincter is a thickened ring of circular skeletal muscle where the pharynx and esophagus meet.
• The inferior esophageal sphincter is a thin band of circular smooth muscle where the esophagus and stomach meet, and is weak allowing materials can reflux back into the esophagus from the stomach.

Stomach Location and Function

• Located inferior to diaphragm in left upper quadrant of abdomen and is an intraperitoneal organ.
• It performs mechanical and chemical digestion, creating chyme.

Stomach Regions

• Composed of the cardia where the esophagus enters the stomach, the dome-shaped superior fundus, the large, central body, and the funnel-shaped pylorus connected to the duodenum.
• There is has a superior concave lesser curvature and an inferior convex greater curvature.

Stomach Structures

• The internal surface has gastric folds or rugae, allowing the stomach to expand when full and return to normal shape when empty.
• The pyloric sphincter is a thick ring of smooth muscle regulating the entrance of chyme into the duodenum.
• There are 3 layers of smooth muscle in the Muscularis.
• Longitudinal, circular, and oblique layers churn/blend the stomach's contents.
• It is supplied by the left gastric artery from the celiac trunk/artery.

Small Intestine Characteristics

• It extends from the pylorus of the stomach to the cecum of the large intestine.
• It fills most of the abdominal cavity.
• Its function is to finish chemical digestion and absorb 90% of nutrients and water.
• Primary blood supply comes from branches of the superior mesenteric artery.
• It has three segments:
  • The duodenum is the first 10 inches (25 cm).
  • The jejunum is the middle section 7.5 feet (2.5 m).
  • The ileum is the final 10.8 feet (3.6 m).

Duodenum

• Mostly retroperitoneal the duodenum originates at the pyloric sphincter.
• Arches into C-shape around the head of the pancreas.
• Bile from the liver and pancreatic juices enter through the major duodenal papilla.

Jejunum

• This is an intraperitoneal segment that is the primary region for chemical digestion and nutrient absorption

Ileum

• This is an intraperitoneal segment whose distal end terminates at the ileocecal valve.
• The ileocecal valve controls the entry of materials into the cecum of the large intestine.

Large Intestine

• It extends from the ileocecal junction to the anus, surrounding the centrally located small intestine.
• It absorbs water and ions, and forms and stores feces.
• It includes tenia coli (distinct longitudinal bundles of smooth muscle fibers), haustra (sacs formed as a result of tenia coli causing large intestine to bunch up), and omental appendices (lobules of fat hanging off of haustra).

Large Intestine Segments and Blood Supply

• The segments include: the cecum (intraperitoneal), the ascending colon (retroperitoneal), the hepatic flexure, the transverse colon (intraperitoneal), the splenic flexure, the descending colon (retroperitoneal), the sigmoid colon (intraperitoneal), the rectum (retroperitoneal), and the anal canal.
• Its blood supply comes from the superior and inferior mesenteric arteries: cecum, ascending colon, and proximal 2/3 of transverse colon. Distal 1/3 transverse, descending, sigmoid & rectum are also supplied.

Cecum

• The cecum is located in the right lower abdomen.
• It contains the ileocecal valve, where the ileum enters into the large intestine.
• The appendix is a thin, hollow, fingerlike sac lined by lymphatic nodules that open into the cecum.

Anal Canal

• The anal canal is the last few centimeters of the large intestine terminating at the anus.
• It passes through an opening in the levator ani muscles of the pelvic floor.
• Anal sphincters close off the opening to the anal canal, used to relax (open) during defecation.
• Internal anal sphincter is involuntary smooth muscle.
• External anal sphincter is voluntary skeletal muscle.

Accessory Organs

• These are organs that are not part of the GI tract but support it.

Liver Location and Functions

• It is located in the right upper quadrant immediately inferior to the diaphragm.
• It functions to produce and emulsify bile in chemical digestion, detoxify drugs, metabolize and processes from the blood, store excess nutrients and vitamins, synthesize blood plasma proteins, and remove debris from blood and recycles worn out blood cells.
• The liver's blood supply comes from the common hepatic artery via a branch of celiac trunk/artery.

Liver Structure

• The liver is made up of four lobes: a right lobe, a left lobe, a quadrate lobe, and a caudate lobe.
• Ligaments include the falciform ligament (separates right and left lobes, attaches liver to abdominal wall), the ligamentum teres hepatis (remnant of fetal umbilical vein), and the coronary ligament (attaches superior liver to diaphragm, surrounds bare area).

Gallbladder

• The gallbladder is located on the visceral/inferior surface of the liver.
• It stores and concentrates bile produced by the liver until needed for digestion.
• The cystic duct allows bile to enter and exit the gallbladder. It contains the neck, and is attached to the liver by the body.

Biliary Tree

• This is a network of thin ducts that transport bile from the liver and gallbladder to the duodenum.
• The left and right hepatic ducts drain bile from R&L lobes of the liver and merge to form the common hepatic duct.
• The cystic duct and common hepatic duct merge to form the bile duct.
• The pancreatic duct merges with the bile duct. Bile and pancreatic juices enter the duodenum through the major duodenal papilla.

Pancreas

• It is located posterior to the stomach and is retroperitoneal.
• The regions are the head (fits into C-shaped duodenum), neck (anterior to SMA and SMV), body, and tail (associated with the spleen).
• It secretes pancreatic juices into the duodenum to break down fats and proteins.
• It also serves as endocrine gland makes glucagon and insulin.
• The main pancreatic duct joins the bile duct and drains digestive enzymes from the pancreas into the duodenum.
• If present, the accessory pancreatic duct drains into the duodenum, superior/proximal to the main pancreatic duct. the location indicates that the pancreas was two "buds" during embryonic development

Gut Development & Blood Supply

• The foregut includes the pharynx, esophagus, stomach, proximal duodenum, liver, gallbladder, biliary tree, and pancreas.
• The midgut includes the distal duodenum, jejunum, ileum, cecum, appendix, ascending colon, and proximal 2/3 of the transverse colon.
• The hindgut encompasses the distal 1/3 transverse colon, descending colon, sigmoid colon, rectum, and superior anal canal.
• The foregut's blood Unpaired Branch of Aorta is the Celiac Trunk/Artery.
• The midgut's blood supply is from the Superior Mesenteric Artery (SMA).
• The hindgut gets their blood from Inferior Mesenteric Artery (IMA)-

Innervation

• Three autonomic plexuses (celiac, superior mesenteric, and inferior mesenteric) follow the distribution of the unpaired arteries.
• Recall that sympathetic innervation inhibits digestion.
• Recall that parasympathetic innervation promotes digestion.
• Celiac plexus = T5-T9
• Superior Mesenteric Plexus = T8-T12
• Inferior Mesenteric Plexus = L1-L2
• The celiac plexus and superior mesenteric plexus = vagus nerve (CN X).
• The inferior mesenteric plexus = pelvic splanchnic nerves (S2-S4).