Digestive System & Abdominopelvic Cavity I. The Digestive System The digestive system can be functionally divided into two primary sets of organs or organ systems which occupy the largest part of the ventral body cavity, the abdominopelvic cavity. The abdominopelvic cavity is bordered by:

superiorly: diaphragm anteriorly: anterior body wall, abdominal muscles (rectus abdominus, external and internal obliques,

transversus abdominus) posteriorly: posterior body wall, lower pairs of ribs, some back muscles, vertebral column inferiorly: pelvis

1. Alimentary Canal = gastrointestinal tract (GI); this is a muscular digestive tube that runs through the abdominopelvic region of the body. It is involved in the absorption of food and excretion of waste. Organs: mouth, esophagus, stomach, small intestine, large intestine (colon), anus 2. Accessory Digestive Organs = secretions of these organs help to break down food; others act as a filter. Organs: gallbladder, liver, pancreas Both of these sets of organs/organ systems (along with the teeth and tongue of the oral cavity) take part in the digestive process either mechanically or chemically. For now, focus on those parts of the digestive system that are within the abdominopelvic cavity. II. Anatomy of the Abdominopelvic Cavity

PERITONEUM The ventral body cavity contains a large network of serous membranes, the abdominal parts of which are classified as either visceral peritoneum or parietal peritoneum. Visceral Peritoneum: covers the external surfaces of most digestive organs and is continuous

with parietal peritoneum. Parietal Peritoneum: lines the walls of the abdominopelvic cavity

In between the visceral and parietal peritoneum is the peritoneal cavity. This is a potential space that contains serous fluid, which is a lubricating substance secreted by the peritoneum. This allows organs to easily slide against each other during digestion.

MESENTERIES A mesentery is a double layer of peritoneum that extends from the body wall to and organ in the abdominopelvic cavity. Mesenteries hold some organs in place, suspending them from the body wall. They also tend to transmit important vessels and/or nerves. Most mesenteries are dorsal, extending from organs to the posterior abdominal body wall. However the stomach and the liver also receive ventral mesenteries from the anterior abdominal body wall. Organs that have mesenteries are called intraperitoneal or peritoneal. (The spleen is an exception in that its mesenteries connect to other organs but not to the body wall.) Organs that have no mesenteries are called retroperitoneal (the kidneys are an example of this).

Peritoneal organs of the abdominopelvic cavity and their mesenteries: stomach: greater omentum (dorsal) & lesser omentum (ventral) liver: falciform ligament & lesser omentum (ventral) small intestine (ileum & jejunum): mesenteries (dorsal) transverse colon: transverse mesocolon (dorsal) sigmoid colon: sigmoid mesocolon (dorsal)

There are also mesenteries that connect organs to each other, or superiorly to the diaphragm. Examples: stomach diaphragm =gastrophrenic ligament stomach spleen =gastrosplenic ligament (gastrolienal) stomach transverse colon =gastrocolic ligament spleen kidneys =splenorenal ligament (lienorenal)

Organs that do not have a mesentery are called retroperitoneal. Some of these organs have lost their mesenteries (secondarily retroperitoneal) while others originated there (primarily retroperitoneal). They are outside of the peritoneal cavity. Retroperitoneal organs of the abdominopelvic cavity (no mesenteries): Secondarily retroperitoneal: most of the duodenum, ascending colon, descending colon, rectum, & pancreas. Primarily retroperitoneal: kidneys, ureters, bladder, prostate, uterus and ovaries. III. Gross Anatomy of the Digestive System

STOMACH Esophagus travels through opening in the diaphragm and connects to the stomach. The stomach is j shaped, muscular organ of the alimentary canal. Fundus, body, rugae, & pyloris

Arterial supply: right and left gastric arteries from celiac trunk (lesser curvature) Right (from gastroduodenal a.) and left gastro-epiploic (from splenic a.) arteries (greater curvature) Venous drainage: gastric veins drain into portal, splenic and superior mesenteric veins

LIVER Falciform ligament: a ventral mesentery that binds liver to anterior abdominal wall, attaches to

anterior aspect of hepatic fissure Round ligament (Ligamentum teres): extends from umbilicus to liver (remnant of umbilical

vein) Ligamentum venosum: cord-like remnant of ductus venosus; attaches to posterior aspect of liver

fissure Liver Anatomy: Divided into 4 lobes: 1. right lobe (on diaphragmatic surface) 2. left lobe (on diaphragmatic surface) 3. quadrate lobe (on visceral surfaceinferior) 4. caudate lobe (on visceral surfacesuperior) Porta hepatis: (doorway to the liver) hepatic portal vein (into liver) right & left hepatic arteries (from celiac trunk into liverblood for liver tissue) right & left hepatic ducts (carry bile away from the liver, become common hepatic duct) Bile is

important for fat digestion Also on the visceral side of liver:

gall bladder (bile storage vessel) inferior vena cava

Liver Innervation: Parasympathetic (Vagus) and Sympathetic fibers from the celiac plexus

**Be aware of the livers part in the portal system and that it acts as a filter.

GALLBLADDER The cystic duct emerges from gallbladder to join the common hepatic duct from the liver and forms the common bile duct, which empties into duodenum. Bile from the liver (which is constantly made) is stored in the gallbladder for future use.

PANCREAS Exocrine (pancreatic juice) & endocrine (insulin & glucagon) gland of importance in digestion and

producing hormones that control sugar levels in the body. It is a secondarily retroperitoneal organ Main pancreatic duct (which opens into the duodenum) travels through the length of the

pancreas Pancreas Innervation & Blood Supply: Sympathetic from thoracic splanchnic nerves, parasympathetic from vagus (all via the celiac

plexus) blood supply from hepatic, splenic and superior mesenteric vessels

SPLEEN

A lymphatic organ, very vascularhas white and red pulpfilters out old RBCs blood supply from splenic artery, drained via portal system

SMALL INTESTINE

The small intestine has 3 divisions: 1. Duodenum (5%) portion that connects to pyloric part of stomach, also receives bile from liver and gallbladder via the bile duct. Receives secretions from the pancreas (pancreatic juice) via the pancreatic duct. The bile duct and the pancreatic ducts normally unite to form the hepatopancreatic duct which emerges in the duodenum at the duodenal papilla. 2. Jejunum (35%) most lies in the umbilical region of the abdominal cavity 3. Ileum (60%) lies in the suprapubic and right inguinal regions of the abdominal cavity Primarily, the small intestine receives blood from the superior mesenteric artery. But the

duodenum also receives blood from the celiac trunk via gastroduodenal arteries. Likewise, the superior mesenteric vein drains most of the small intestine, except for a small part of the duodenum drained by the portal vein.

The duodenum receives sympathetic (sympathetic trunk) and parasympathetic (vagus) innervation from nerve plexuses around the pancreaticoduodenal arteries (branches of the gastroduodenal arteries)

The rest of the small intestine receives sympathetic innervation from greater splanchnic nerves via the celiac and superior mesenteric plexuses, and parasympathetic innervation from the vagus.

LARGE INTESTINE (colon)

The large intestine has distinctive outer features such as teniae coli (muscular strips that run longitudinally around caecum and colon), haustra (puckers in the colon caused by teniae coli), epiploic appendices (fat-filled pouches of visceral peritoneum that hang from the colon) Gross anatomy: Ascending Colon: ascends along right side of posterior abdominal wall to level of right kidney, makes turn called right colic flexure (a.k.a. hepatic flexure) caecum: blind sac at start of ascending colon, unites with ileum at ileocaecal junction appendix: vermiform blind tube that opens into wall of caecum; has lymphoid tissue; easily

infected. Transverse Colon: extends across peritoneal cavity, bends at left colic flexure (a.k.a. splenic

flexure) Descending Colon: descends on left side of posterior abdominal wall, approaches pelvis and

becomes Sigmoid (s-shaped) Colon. Rectum: terminal end of sigmoid colon, no teniae coli, well-developed muscular wall, three transverse folds (rectal valves) that aid in defecation Anal Canal: last part of large intestine, has longitudinal folds of mucosa (anal columns), that form anal valves; interceding spaces are anal sinuses which release mucus for lubrication. Pectinate line separates visceral and external part of anus (superior to line, no pain because of visceral sensory innervation; inferior to this line, pain due to somatosensory innervation) (Clinical Note: Hemorrhoids are varicose veins of the anal canal. They are commonly caused by irritation or build up of venous blood pressure. They can also be caused by a condition known as portal hypertension which can result from cirrhosis of the liver. In a cirrhotic liver, blood flow is restricted due to fibrous scar tissue that forms during repair of hepatocytes (liver cells) that have been damaged by toxins or infection. This causes a build up of pressure in the portal vein and its tributaries, leading to engorgement and distention.) Anal Sphincters: internal sphincter is smooth muscle (involuntary); external sphincter is skeletal muscle (conscious control). Innervation and Blood Supply of Large Intestine: - First of large intestine receives sympathetic innervation from superior mesenteric plexus and celiac plexus; parasympathetic innervation is from vagus - blood supply is from superior mesenteric vessels - Second receives sympathetic innervation from hypogastric and inferior mesenteric plexuses; parasympathetic innervation is from pelvic splanchnic nerves (sacral outflow) - blood supply is from inferior mesenteric vessels; anal canal and rectum are supplied by rectal branches of internal iliac vessels