Accessory Digestive Organs

Accessory Digestive Organs: Liver, Pancreas, and Gallbladder

Liver Anatomy and Function

• The liver is located to the right of the stomach, below the diaphragm, and above the small intestine.

• It is a large organ, weighing about 1.4 kg.

• Divided into a large right lobe and a smaller left lobe, each with eight segments and 1,000 lobules.

• Lobules connect to form the common hepatic duct.

Ligaments

• The falciform ligament separates the lobes, connecting the liver to the abdominal wall and diaphragm.

• It allows fluid passage between lobes.

Main Functions

• The liver regulates blood chemical levels and produces bile.

• It processes blood from the stomach and intestines, breaking down, balancing, and creating nutrients and metabolizing drugs.

• Over 500 identified functions.

• Includes metabolism of carbohydrates into glycogen, lipids into cholesterol, and proteins into plasma proteins.

• Detoxifies drugs and neutralizes them.

• Stores glycogen, vitamins, and minerals.

• Removes defective blood cells.

• Primary function is bile production.

Liver Microstructure

Hepatic Lobules

• Cells arranged in hexagonal lobules around blood vessels.

• Each lobule contains a central vein and portal triad.

Portal Triad

• Located at the edge of each lobule, consisting of a vein, artery, and bile duct.

Hepatocytes

• Main cell type, comprising 80% of liver volume.

• Produce bile and have secretory, metabolic, and endocrine functions.

• Grooves between cells form bile canaliculi.

Bile Canaliculi

• Small ducts accumulating bile from hepatocytes.

• Flow into bile ductules and larger bile ducts.

• All ducts unite into right and left hepatic ducts, merging to form the common hepatic duct.

Hepatic Sinusoids

• Channels between hepatocyte strands that are porous and blood-filled.

• Formed by fenestrated capillaries from hepatic portal veins and hepatic arteries.

• Hepatocytes tightly packed around endothelium for easy blood access.

• Process nutrients, toxins, and waste.

• Bilirubin is processed and excreted into bile canaliculi.

• Proteins, lipids, and carbohydrates are processed and secreted into sinusoids or stored.

Kupffer Cells

• Star-shaped cells within sinusoids.

• Phagocytose dead blood cells, bacteria, and foreign materials.

Blood Flow

• Hepatic artery delivers oxygenated blood.

• Hepatic portal vein delivers deoxygenated, nutrient-rich blood from the small intestine.

• Blood combines in sinusoids and flows to the central vein.

• The blood then flows through the hepatic vein into the inferior vena cava and back to the heart.

• Blood and bile flow in opposite directions.

Bile Production

• Lipids are hydrophobic and must be emulsified before digestion.

• Bile emulsifies lipids in the small intestine.

Bile Composition

• Bile is a yellow-brown or yellow-green alkaline solution with a pH of 7.6-8.6.

• About 1 liter produced per day.

• Contains water, bile salts, bile pigments, phospholipids, electrolytes, cholesterol, and triglycerides.

Bile Salts

• Act as emulsifying agents.

• Have hydrophobic and hydrophilic regions.

• Hydrophobic region interacts with lipids, hydrophilic with watery chyme.

• Breaks large lipid globules into smaller fragments, increasing surface area for enzyme activity.

• Most components eliminated in feces, but bile salts are reclaimed by enterohepatic circulation.

• Bile salts are absorbed in the ileum and returned to the liver via hepatic portal blood.

• Recycled to maintain function.

Bilirubin

• Main bile pigment, a waste product from the breakdown of old or damaged red blood cells.

• Transported to the liver via the splenic vein.

• Gives bile its green color and is transformed by intestinal bacteria, giving feces its color.

• Lack of bile entering intestine results in white stool with high fat content.

Regulation of Bile Production

• About 1 liter of bile produced daily by hepatocytes.

• Production increases when fatty chyme enters the duodenum.

• Between meals, bile is conserved in the gallbladder.

• Hepatopancreatic ampulla closes, diverting bile to the gallbladder to be concentrated and stored.

Bile Flow and the Biliary System

• Biliary system consists of organs and ducts involved in bile production and transportation.

Process

1. Hepatocytes produce bile.

2. Bile canaliculi collect bile.

3. Tight junctions prevent bile from escaping.

4. Bile flows into bile ducts at the periphery of lobules (portal triad).

5. Portal ducts merge into right and left hepatic ducts.

6. Hepatic ducts unite and exit the liver as the common hepatic duct.

7. The common hepatic duct joins the cystic duct from the gallbladder to form the common bile duct.

8. The common bile duct enters the duodenum.

9. 50% of bile is stored in the gallbladder.

10. Gallbladder releases stored bile through the cystic duct after food ingestion.

Flowchart Summary

• Right and left hepatic ducts → common hepatic duct

• Cystic duct (from gallbladder) + common hepatic duct → common bile duct

• Common bile duct empties into the duodenum with the pancreatic duct.

Liver Blood Supply

• Forms a portal system, enabling blood from capillary beds to drain into another set without returning to the heart.

• The hepatic portal system connects the capillaries of the gastrointestinal tract with the capillaries in the liver.

Dual Blood Supply

• Hepatic portal vein (75%) and hepatic arteries (25%).

• Hepatic portal vein carries venous blood drained from the spleen, gastrointestinal tract.

• Hepatic artery supplies oxygenated arterial blood.

• Both provide oxygen.

• The blood flow of the hepatic artery and the hepatic portal vein is in the opposite direction of the flow of bile in the common hepatic duct.

Blood Flow Process

1. Hepatic artery and portal vein branch off into each hepatic lobule

2. Blood drains into the liver sinusoids.

3. Sinusoids drain into the central vein.

4. Central veins converge and drain into the hepatic vein.

5. Hepatic vein drains into the inferior vena cava leading back to the heart.

Gallbladder

• Small, pear-shaped organ below the liver.

• Located in a fossa on the posterior surface of the liver's right lobe.

• Surrounded by peritoneum and lies anterior to the duodenum.

Anatomical Structure

• Fundus (rounded distal portion)

• Body (largest part, adjacent to transverse colon and duodenum)

• Neck (tapers to cystic duct), contains Hartmann's pouch (common site for gallstones)

Microstructure

• Inner epithelial layer of columnar cells

• Lamina propria of connective tissue

• Muscularis layer of smooth muscle (for contraction)

• Perimuscularis (fibrous connective tissue)

• Outer serosa

Connection to Digestive System

• Biliary tree: Cystic duct unites with the common hepatic duct to form the common bile duct.

• The common bile duct joins with the pancreatic duct prior to emptying into the hepatopancreatic ampulla into the duodenum.

Functions

• Stores and concentrates bile (30-70 mL capacity).

• Responds to intestinal hormones.

• Regulates bile composition.

• Controls bile flow into the duodenum.

Bile Storage

• Water is absorbed, concentrating the bile.

• Over-concentration can lead to crystallization and gallstone formation.

Control of Gallbladder Contraction and Bile Secretion

• Neural activation, hormones, and bile salts control gallbladder contraction.

Neural Control

• Hepatic branch of the vagus nerve.

• Vagus nerve stimulation causes contraction and bile release.

Hormonal Control

• Duodenum releases cholecystokinin (CCK) in response to proteins or fats in chyme.

• CCK stimulates the gallbladder to contract and release bile.

• Once lipids are broken down, CCK production stops.

• Secretin is released in response to acidic contents in chyme and stimulates bile secretion from the liver.

Bile Salts Control

• Reabsorbed bile salts stimulate additional bile secretion.

Overview of Bile

• Hepatocytes produce bile.

• Transported through the biliary duct system to the gallbladder.

• Partially excretory and mainly digestive secretion.

• Aids in lipid digestion by emulsifying triglycerides.

• Fusion of cholesterol crystals initiates gallstone formation leading to obstruction of the duct system.

Pancreas

• Located in the upper abdomen behind the stomach.

• Part of the gastrointestinal system and an endocrine organ.

• Endocrine portion secretes hormones into the body for energy, metabolism, and storage.

Key Definitions

• Exocrine Pancreas: Portion that secretes digestive enzymes into the duodenum.

• Endocrine Pancreas: Portion that secretes insulin and other hormones into the blood.

Anatomical Structure

• Elongated tapered organ (13 cm long, 2.5 cm thick) located posterior to the stomach.

• Head: Widest part, situated in the C-shaped curve of the duodenum.

• Body: Extends upward.

• Tail: Tapers off near the spleen.

• The main pancreatic duct joins the common bile duct from the liver, reviewed the flowcharts to see its positioning upon revision.

Microstructure

• Exocrine and Endocrine portions (glands).

Exocrine Portion

• The exocrine portion of the pancreas accounts for around 85%.

• Main functional unit: Acinus (berry-like cluster) and duct system, produces and transports enzymes.

• Acinar cells are highly polarized and contain granules for protein secretion.

• Lobules of acini are separated by a septa or a small lumen.

• Enzymes released into progressively larger ducts.

• Intercalated duct to intralobular duct to interlobular duct to main pancreatic duct.

Pancreatic Fluid

• Pancreatic ducts carry pancreatic fluid containing digestive enzymes (protease, lipase, amylase), water, and bicarbonate.

• Bicarbonate ions neutralize acidic gastrointestinal contents.

• Proteases are stored as inactive to prevent autodigestion.

Release of Pancreatic Fluid

• Controlled by CCK and secretin released from the duodenum.

• CCK is stimulated for release when there are lipids present within the duodenum to help digest the food.

• Also controlled by the parasympathetic nervous system.

Process

1. Chyme enters the duodenum.

2. Stimulates release of CCK and secretin.

3. CCK promotes exocytosis of enzymes from acinar cells.

4. Secretin causes duct cells to add water and bicarbonate (alkaline fluid).

5. Once combined, they drain into the main pancreatic duct and common bile duct into the duodenum.

Other Factors

• Vagal innervation stimulates secretion of enzymes.

• Seeing, smelling, or tasting food stimulates secretion.

• The presence of acidic chyme stimulates secretin release.

• Fatty acids and protein in chyme stimulate CCK release.

Endocrine Portion

• Smaller functional component.

• Composed of islets of Langerhans (1,000,000 islets).

• Highly vascularized for hormone access.

Islet Cells

• Alpha, beta, and delta cells produce hormones.

• C cells (function unknown).

• Beta cells produce insulin (regulates carbohydrate, fat, and protein metabolism).
  * Major stimulator is glucose.

• Alpha cells produce glucagon (releases glucose from the liver and fatty acid from fat tissue).

Histological Structure

• Islets are pale-staining groups of cells embedded in darker exocrine tissue.

• Endocrine portion (1%), exocrine portion (99%).