PHYSIOLOGIC ANATOMY OF THE LIVER AND THE BILIARY SYSTEM

  • The liver is a dual organ with both secretory and excretory functions.
  • It is the largest gland in the body, weighing about 1.5 kg in males.
  • Anatomical location:
    • Situated in the upper right side of the abdominal cavity, immediately beneath the diaphragm.
  • Structural features:
    • Composed of lobes called hepatic lobes.
    • Each lobe contains many lobules known as hepatic lobules, which are the structural and functional units of the liver.
    • Approximately 50,000 to 100,000 hepatic lobules are present within the liver.
    • The structure of the lobule resembles a honeycomb, composed primarily of liver cells (hepatocytes).

LIVER STRUCTURE

  • Lobes of the Liver:
    • Right lobe
    • Left lobe
    • Quadrate lobe
    • Caudate lobe
    • The gallbladder is associated with these lobes, connected through a series of bile ducts.
LIVER ANATOMY DIAGRAM
  • Diagram (Fig 1): Posterior surface of liver showing lobes and their associated ducts.

BILIARY SYSTEM

  • The biliary system comprises the gallbladder and extrahepatic bile ducts (those located outside of the liver).
  • Bile duct formation:
    • Left and right hepatic bile ducts emerge from the liver and join to form the common hepatic duct.
    • Common hepatic duct unites with the cystic duct from the gallbladder to create the common bile duct.
  • Structural Features of Bile Ducts:
    • Common bile duct connects with the pancreatic duct to form the common hepatopancreatic duct (or ampulla of Vater), which opens into the duodenum.
    • There exists a sphincter known as the sphincter of Oddi located at the lower part of the common bile duct. It is constructed from smooth muscle fibers, typically remaining closed, which allows the bile secreted from the liver to enter the gallbladder for storage. Upon stimulation, this sphincter opens, permitting the bile to flow into the intestine.
DIAGRAM OF BILIARY SYSTEM
  • Diagram (Fig 2): Illustrates the biliary system including the liver, gallbladder, ampulla of Vater, and associated ducts.

BLOOD SUPPLY TO THE LIVER

  • The liver receives a significant blood supply, approximately 1,500 mL/minute, from two primary sources:
    1. Hepatic Artery:
    • Arises directly from the aorta and supplies oxygenated blood to the liver.
    • Upon entering the liver, the hepatic artery branches out extensively, with each branch entering a portal triad composed of:
      • A branch of the hepatic artery.
      • A branch of the portal vein.
      • A tributary of a bile duct.
    1. Portal Vein:
    • Formed by the convergence of the superior mesenteric vein and the splenic vein.
    • Carries deoxygenated blood rich in nutrients from the stomach, intestines, spleen, and pancreas. Portal blood contains monosaccharides, amino acids, bile salts, bilirubin, urobilinogen, and various hormones, although it has lower oxygen content compared to arterial blood.
    • The pathway from the intestine to the liver via the portal vein is referred to as enterohepatic circulation.
MIXING OF BLOOD IN THE LIVER
  • Blood from the hepatic artery mixes with deoxygenated blood from the portal vein in hepatic sinusoids.
  • Hepatic cells utilize oxygen and nutrients from the sinusoids.
  • Waste products and carbon dioxide generated by hepatic cells are also released into the sinusoids, which collect them into the central vein of the lobule.
  • Central veins from multiple lobules converge to form larger hepatic veins (right and left), which ultimately drain into the inferior vena cava.

BILE

  • Properties of Bile:
    • Volume: 800 to 1,200 mL/day.
    • Reaction: Alkaline.
    • pH: Ranges from 8 to 8.6.
    • Specific Gravity: 1.010 to 1.011.
    • Color: Golden yellow or green.
Composition of Bile
  • Bile consists of 97.6% water and 2.4% solids.
  • Solids include both organic and inorganic substances, such as bile acids, bile pigments, cholesterol, lecithin, and fatty acids.
  • Bile is secreted by hepatocytes and is funneled into canaliculi, from where it flows through small ducts to the hepatic ducts, and then into the common hepatic duct.
  • There are two pathways for bile:
    • Direct flow into the intestine.
    • Storage in the gallbladder.
  • When bile moves through biliary ducts, sodium, bicarbonate, and water are added to it.
STORAGE OF BILE
  • The majority of bile produced in the liver is stored in the gallbladder. When needed, bile is released into the intestine.
  • Changes during bile storage in the gallbladder:
    1. Volume decreases due to absorption of substantial amounts of water and electrolytes (excluding calcium and potassium).
    2. Concentration of bile salts, pigments, cholesterol, fatty acids, and lecithin increases due to water absorption.
    3. A slight decrease in pH occurs.
    4. An increase in specific gravity is noted.
    5. Mucin is added to the bile.
Bile Salts
  • Bile salts are the sodium and potassium salts of bile acids that are conjugated with glycine or taurine.
Functions of Bile Salts
  1. Emulsification of Fats:
    • The breakdown of fat globules into tiny droplets (emulsification) forms a milky fluid called emulsion, facilitated by bile salts which reduce surface tension through detergent action.
  2. Fat Absorption:
    • Bile salts help in absorbing digested fats into the bloodstream by forming complexes known as micelles.
  3. Choleretic Action:
    • They stimulate bile secretion from the liver (choleretic action).
  4. Cholagogue Action:
    • Indirectly stimulate gallbladder contraction to release bile into the intestine through the hormone cholecystokinin.
  5. Laxative Action:
    • Stimulate peristaltic movements in the intestine, promoting defecation.
  6. Gallstone Prevention:
    • Maintain cholesterol and lecithin in solution, preventing gallstone formation.
Bile Pigments
  • Bile pigments (bilirubin and biliverdin) are excretory products derived from hemoglobin breakdown during red blood cell (RBC) destruction by reticuloendothelial cells.
  • Bilirubin is recognized as the major bile pigment in humans.
Functions of Bile
  • Most functions are attributed to bile salts, which include:
  1. Digestive functions (primarily fat digestion).
  2. Absorptive function (aiding in nutrient uptake).
  3. Excretory functions (primarily elimination of bile pigments and other substances).
  4. Laxative action (promoting bowel movements).
  5. Antiseptic action (reducing bacterial growth in the intestine).
  6. Choleretic action (increasing bile production).
  7. pH maintenance of the gastrointestinal tract (neutralizing acids from the stomach).
  8. Gallstone prevention functions (avoiding precipitation of cholesterol).
  9. Lubrication function (via mucin in bile).
  10. Cholagogue action (influencing bile release).

FUNCTIONS OF LIVER

  • The liver, as the largest gland and a vital organ, performs numerous metabolic and homeostatic functions:
  1. Metabolic Function:
    • The liver is the primary site for the metabolism of carbohydrates, proteins, fats, vitamins, and hormones.
  2. Storage Function:
    • Stores vital substances, including glycogen, amino acids, iron, folic acid, and vitamins A, B12, and D.
  3. Synthetic Function:
    • Synthesizes glucose through gluconeogenesis, produces plasma proteins, including clotting factors and hormone-binding proteins, and synthesizes steroids, somatomedin, and heparin.
  4. Secretion of Bile:
    • The liver secretes bile, essential for fat digestion, which contains bile salts, bile pigments, cholesterol, fatty acids, and lecithin. It also aids in waste product elimination through feces and urine.
  5. Excretory Function:
    • Excretes various substances such as cholesterol, bile pigments, heavy metals (lead, arsenic, bismuth), toxins, bacteria, and viruses (e.g., yellow fever).
  6. Heat Production:
    • Generates a considerable amount of heat due to metabolic activities—the liver is the major organ for heat production.
  7. Hemopoietic Function:
    • During fetal development, the liver is crucial for blood cell production, and it stores Vitamin B12 and iron essential for erythropoiesis. It also produces thrombopoietin for thrombocyte production.
  8. Hemolytic Function:
    • Dismantles aged or senile RBCs (120-day lifespan) via Kupffer cells in the liver.
  9. Inactivation of Hormones and Drugs:
    • Metabolizes and inactivates hormones (e.g., insulin, cortisol) and converts fat-soluble drugs into water-soluble substances for excretion through urine or bile.
  10. Defensive and Detoxification Functions:
    • Kupffer cells play a defensive role by phagocytizing foreign entities and producing interleukins and tumor necrosis factors that stimulate immune responses. Detoxification is carried out via two primary methods:
      • Total degradation of harmful substances through metabolic pathways.
      • Conversion of toxic materials into non-toxic derivatives via conjugation with glucuronic acid or sulfates.

REGULATION OF BILE SECRETION

  • Continuous bile secretion occurs, with fluctuations based on fasting (lower levels) and post-meal (increased levels).
  • Secretion triggers and influences include:
    1. Choleretics:
    • Substances that enhance bile production (e.g., acetylcholine, secretin, cholecystokinin, intestinal acid chyme, and bile salts).
    1. Cholagogues:
    • Agents that stimulate the release of bile (e.g., bile salts, calcium, fatty acids, amino acids, inorganic acids) by inducing gallbladder contraction through cholecystokinin secretion.
    1. Hydrocholeretic Agents:
    • Substances that contribute to bile secretion from the liver along with large volumes of water (e.g., hydrochloric acid).