(16) Abdominal Viscera I Lecture Notes
Anatomy and Functional Organization of the Liver
Lobes of the Liver
In terms of gross anatomical appearance, the liver is divided into four distinct lobes. This division is facilitated by various fissures and anatomical structures:
Fissures of the ligamentum venosum and ligamentum teres hepatis: These structures divide the liver into a smaller left lobe and a larger right lobe.
The Porta Hepatis (Portal Fissure), Fossae for the Gall Bladder, and Groove for the Inferior Vena Cava: These markers further subdivide the right lobe into the caudate lobe and the quadrate lobe.
'H'-Shape Pattern: Collectively, these divisions create an 'H'-shape pattern on the visceral (posteroinferior) surface of the liver. This pattern is most visible when the inferior border of the liver is reflected superiorly.
Despite these anatomical divisions, the liver is functionally divided into only two lobes of roughly equal size. These right and left functional parts are independent and are defined by the distribution of the following structures:
Right and left branches of the hepatic artery.
Right and left branches of the hepatic portal vein.
Right and left branches of the common hepatic duct.
Peritoneal Ligaments (Folds of Peritoneum)
Lesser Omentum: This attaches the liver to the stomach and duodenum. It is composed of two parts: - Hepatogastric part: Attaches to the stomach. - Hepatoduodenal part: Attaches to the duodenum. - The free right margin of the lesser omentum contains the portal triad, consisting of the hepatic artery, bile duct, and portal vein. These structures enter the liver at the porta hepatis.
Falciform Ligament: A peritoneal fold that attaches the liver to the diaphragm and the anterior abdominal wall.
Ligamentum Teres (Hepatis): This is the remnant of the fetal umbilical vein. It is located in the inferior free margin of the falciform ligament. In the fetus, the umbilical vein brings oxygenated and nutrient-rich blood from the placenta to the liver.
Ligamentum Venosum: This is the remnant of the fetal ductus venosus. In the fetus, the ductus venosus shunts blood from the umbilical vein directly to the inferior vena cava (IVC), bypassing the fetal liver.
Surface Anatomy and Function
Normal Position: The liver is primarily located under the thoracic cage, situated just below the dome of the diaphragm on the right side. It extends a variable distance to the left, passing the median plane.
Palpability: Under normal conditions, the liver is not palpable below the right costal margin.
Primary Functions: - Synthesis of proteins and clotting factors. - Storage of iron, copper, vitamins, and glycogen. - Secretion of bile. - Fetal function: It serves as an important blood-forming organ in the fetus. - Chemical Modification: The liver modifies macromolecules, toxins, and drugs. Importantly, every substance absorbed from the gastrointestinal (GI) tract—with the exception of lipids—is first received and processed by the liver.
Blood Supply of the Liver
Proper Hepatic Artery: This is a branch of the common hepatic artery (derived from the celiac trunk). It brings oxygenated blood to the liver. Near the porta hepatis, it divides into right and left hepatic arteries. This source provides approximately of the liver's blood flow.
(Hepatic) Portal Vein: This vein collects blood from the capillary beds of the entire GI tract. It brings venous blood rich in products of digestion to the liver, providing of the blood flow. - Portal System Characteristics: Unlike the systemic venous system, the portal system lacks significant valves. It begins as a capillary plexus in the GI organs, gall bladder, pancreas, and spleen, and ends in the hepatic sinusoids. It receives blood from the superior mesenteric vein, inferior mesenteric vein, and the splenic vein.
Portal-Systemic (Portacaval) Anastomoses: These are locations where the portal venous system communicates with the systemic venous system: - Rectal: Superior rectal vein $\Leftrightarrow$ middle and inferior rectal veins (to internal iliac veins). - Esophageal: Left gastric vein $\Leftrightarrow$ esophageal veins (to azygos vein). - Paraumbilical: Paraumbilical veins $\Leftrightarrow$ epigastric veins (to external iliac veins).
Hepatic Veins: These veins exit the posterior surface of the liver to open directly into the inferior vena cava just below the diaphragm.
Clinical Considerations of the Liver and Bile System
Nerve Supply
The nerve supply to the liver is derived from sympathetic and parasympathetic nerves via the autonomic plexus surrounding the celiac trunk.
Clinical Conditions
Portal Hypertension: Results from the obstruction of normal blood flow through the hepatic circulation, frequently due to cirrhosis. Because portal vessels lack valves, venous blood from the GI tract bypasses the liver through portacaval anastomoses. Major sites of clinical significance include esophageal, umbilical, and rectal venous plexuses.
Cirrhosis: Characterized by the atrophy or destruction of liver cells and the hypertrophy of connective tissue. This tissue surrounds intrahepatic vessels and biliary ducts, impeding blood flow.
Metastasis: Due to its high vascularity, the liver is a common site for cancer metastasis from other body regions.
Jaundice: A yellowing of the skin caused by the accumulation of bile pigment. This often results from obstructions in the intrahepatic or extrahepatic bile duct system.
Anatomy and Physiology of the Gall Bladder
Structure and Location
Location: The gall bladder is located on the visceral surface of the liver, lying in the gallbladder fossa along the right edge of the quadrate lobe.
Parts: It is composed of the fundus, body, and neck.
Surface Anatomy: It is located at the intersection of the linea semilunaris (the lateral margin of the rectus abdominis muscle) and the right costal margin.
Biliary Duct System
Origin: The liver develops as a bud from the duodenum and maintains this connection via the bile duct.
Pathway: Bile is secreted into intrahepatic ducts $\rightarrow$ right and left hepatic ducts $\rightarrow$ common hepatic duct.
Bile Duct Formation: The cystic duct from the gall bladder merges with the common hepatic duct to form the bile duct (formerly referred to as the common bile duct).
Termination: The bile duct runs in the free edge of the hepatoduodenal ligament. Before entering the second part of the duodenum, it joins the main pancreatic duct to form the hepatopancreatic ampulla.
Sphincter of the Ampulla: Also known as the hepatopancreatic sphincter, this smooth muscle thickening regulates the flow. The ampulla opens into the duodenum at the apex of the greater duodenal papilla.
Functions and Regulation
Function: The gall bladder stores, concentrates, and acidifies bile produced by the liver.
Mechanism: When the hepatopancreatic sphincter is constricted, bile is forced back through the cystic duct into the gall bladder. During meals, gastrointestinal hormones and the vagus nerves cause the gall bladder to contract and the sphincter to relax.
Bile Action: Bile is amphiphilic, meaning it makes ingested fats more water-soluble so that digestive enzymes can function effectively.
Blood Supply and Innervation
Blood Supply: The cystic artery (usually from the right hepatic artery) provides blood. The cystic vein drains directly into the portal vein.
Innervation: Nerves travel along the cystic artery from the celiac plexus (sympathetic), vagus nerve (parasympathetic), and right phrenic nerve (sensory).
Clinical Considerations
Gallstones: Obstruction of the ducts leads to pain and bile backup.
Cholecystitis: Inflammation of the gall bladder, which causes pain. If bile cannot exit the gall bladder, it may enter the bloodstream and cause jaundice.
Anatomy and Physiology of the Pancreas
Nature and Structure
Type: The pancreas is both an exocrine and endocrine (Islets of Langerhans) gland.
Parts: Consists of a head, neck, body, and tail.
Position: It is a retroperitoneal structure located deep to the stomach. The head sits in the "C" curve of the duodenum, and the tail reaches the spleen.
Functional Overview
Exocrine Function: Secretes an alkaline mixture of enzymes that digest proteins, fats, and carbohydrates through the pancreatic duct and the major duodenal papilla.
Endocrine Function (Islets of Langerhans): Regulates blood glucose levels by secreting hormones directly into the blood: - Insulin (Beta cells): Lowers blood sugar. - Glucagon (Alpha cells): Raises blood sugar. - Other Hormones: Includes somatostatin, ghrelin, and pancreatic polypeptide.
Regulation: Pancreatic secretion is controlled by the vagus nerves and various hormones.
Blood Supply and Innervation
Blood Supply: Branches from the splenic artery, gastroduodenal artery (specifically the superior pancreaticoduodenal artery), and the superior mesenteric artery (specifically the inferior pancreaticoduodenal artery).
Venous Drainage: Drainage occurs via pancreatic veins into the portal, splenic, and superior mesenteric veins.
Innervation: Sympathetic and parasympathetic fibers from the celiac and superior mesenteric plexuses travel along the arteries to the gland.
Clinical Conditions
Pancreatitis: Serious inflammation of the exocrine pancreas, often caused by duct blockage or bile reflux from the hepatopancreatic ampulla.
Pancreatic Cancer: Usually involves the head of the pancreas and is a leading cause of extrahepatic biliary obstruction. Its retroperitoneal position makes early diagnosis difficult.
Cystic Fibrosis: A genetic disease that produces thick mucus in the lungs and pancreas, leading to impaired nutrient digestion and frequent lung infections.
The Spleen
Characteristics and Functions
System: Part of the lymphatic system, consisting of lymphatic tissue and blood vessels.
Functions: - Lymphocyte and monocyte production. - Immune response to antigens. - Breakdown and removal of old red blood cells (RBCs).
Bilirubin Production: The heme from hemoglobin is converted to bilirubin, sent to the liver via the portal vein, and incorporated into bile. High blood bilirubin levels indicate liver or gall bladder disease (jaundice).
RBC Storage: Minimal in humans.
Anatomy and Clinical Context
Position: Located on the left side at the level of ribs , behind the midaxillary line (MAL). It is typically the size of a cupped hand.
Blood Supply: The splenic artery (from the celiac trunk) and the splenic vein (which joins the superior mesenteric vein to form the portal vein).
Splenomegaly: Abnormal enlargement associated with mononucleosis, malaria, leukemia, and hemolytic anemia. The notched border of the spleen is used for palpation as the organ moves below the left costal margin during deep breaths.
Injury: The spleen is the most frequently injured abdominal organ; rupture leads to severe intraperitoneal hemorrhage and shock.
Blood Supply of the Abdomen
The Abdominal Aorta
The abdominal aorta spans from the vertebra to the vertebra, where it divides into the right and left common iliac arteries.
Parietal and Visceral Branches
Parietal Branches: - Lumbar arteries: Small segmental branches. - Common iliac arteries.
Visceral Branches: - Suprarenal, Renal, and Gonadal (testicular or ovarian) arteries. - Celiac Trunk: - Left gastric artery: Supplies the lesser curvature of the stomach and abdominal esophagus. - Splenic artery: Supplies the spleen, pancreas body/tail, and greater curvature of the stomach (via left gastroomental/gastroepiploic branches). - Common hepatic artery: Supplies the liver (proper, right, and left hepatic), lesser curvature (right gastric), duodenum (gastroduodenal), pancreas head (superior pancreaticoduodenal), gall bladder (cystic), and greater curvature (right gastroomental). - Superior Mesenteric Artery (SMA): Supplies the pancreas head (inferior pancreaticoduodenal), distal duodenum, small intestine (via intestinal arteries), cecum (via ileocolic artery), ascending colon (right colic artery), and right half of the transverse colon (middle colic artery). - Inferior Mesenteric Artery (IMA): Supplies the left half of the transverse colon, descending colon, sigmoid colon, and rectum. Branches include the left colic, sigmoid, and superior rectal arteries. - Marginal Artery: An anastomotic channel near the colon that connects branches of the SMA and IMA.
Iliac Arteries
Internal Iliac Arteries: The primary supply for pelvic viscera, the perineum, medial thigh, and gluteal region.
External Iliac Arteries: They become the femoral arteries after passing deep to the inguinal ligament to supply the lower limb.