9.4 Liver, gallbladder, and pancreas

The liver, gallbladder, and pancreas are key players in digestion. They work together to break down food, absorb nutrients, and regulate blood sugar. These organs produce and store essential substances like bile and enzymes that help us digest fats, proteins, and carbs.

Understanding how these organs function is crucial for grasping the digestive system as a whole. From producing bile to releasing hormones, the liver, gallbladder, and pancreas have unique roles that keep our bodies running smoothly and help us get the most out of our food.

Liver, Gallbladder, and Pancreas Structure and Function

Liver Structure and Hepatocyte Function

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• The liver is the largest internal organ located in the upper right quadrant of the abdominal cavity
• It has four lobes (right, left, caudate, and quadrate) and is covered by a fibrous capsule
• The liver performs over 500 functions including nutrient metabolism, detoxification, plasma protein synthesis, and bile production
• Hepatocytes are the main functional cells of the liver arranged in lobules surrounding a central vein
• Blood enters the liver through the hepatic artery and portal vein and exits through the hepatic veins

Gallbladder and Pancreas Structure and Function

• The gallbladder is a small, pear-shaped organ located beneath the liver that stores and concentrates bile produced by the liver
• It contracts to release bile into the duodenum when stimulated by cholecystokinin (CCK)
• The pancreas is an elongated gland located behind the stomach that has both endocrine and exocrine functions
• The exocrine portion secretes digestive enzymes into the duodenum, while the endocrine portion secretes hormones (insulin, glucagon) into the bloodstream

Bile's Role in Lipid Digestion

Bile Composition and Function

• Bile is a greenish-yellow fluid produced by the liver and stored in the gallbladder
• It is released into the duodenum to aid in lipid digestion and absorption
• Bile contains bile salts, which are amphipathic molecules that act as emulsifiers to break large fat globules into smaller droplets, increasing their surface area for enzymatic action
• Bile also contains bilirubin, a pigment derived from the breakdown of red blood cells, which gives feces its characteristic brown color

Micelle Formation and Lipid Absorption

• Bile salts form micelles, which are small spherical structures that encapsulate the products of lipid digestion (fatty acids and monoglycerides)
• Micelles transport the lipid digestion products to the intestinal mucosa for absorption
• The close proximity of the micelles to the intestinal wall facilitates the diffusion of fatty acids and monoglycerides across the cell membrane and into the enterocytes
• Inside the enterocytes, the fatty acids and monoglycerides are reassembled into triglycerides and packaged into chylomicrons for transport via the lymphatic system

Endocrine and Exocrine Functions of the Pancreas

Endocrine Function and Hormones

• The endocrine portion of the pancreas consists of the islets of Langerhans, which contain alpha, beta, and delta cells that secrete hormones directly into the bloodstream
• Alpha cells secrete glucagon, which raises blood glucose levels by promoting glycogenolysis and gluconeogenesis in the liver
• Beta cells secrete insulin, which lowers blood glucose levels by promoting glucose uptake and storage in cells (muscle, liver, adipose tissue)
• Delta cells secrete somatostatin, which inhibits the release of other pancreatic hormones (insulin, glucagon) and digestive enzymes

Exocrine Function and Digestive Enzymes

• The exocrine portion of the pancreas consists of acinar cells that secrete digestive enzymes into the pancreatic ducts, which merge to form the main pancreatic duct that joins the common bile duct to enter the duodenum at the hepatopancreatic ampulla
• The pancreas secretes a variety of enzymes including proteases (trypsin, chymotrypsin), lipases, and amylases, which digest proteins, fats, and carbohydrates, respectively
• Pancreatic secretions are alkaline due to the presence of bicarbonate ions ($HCO_3^-$), which help neutralize the acidic chyme from the stomach
• Pancreatic enzymes are secreted as inactive zymogens (trypsinogen, chymotrypsinogen) to prevent autodigestion of the pancreas and are activated in the duodenum by enterokinase

Common Disorders of the Liver, Gallbladder, and Pancreas

Liver Disorders

• Hepatitis is inflammation of the liver caused by viral infections (hepatitis A, B, C), alcohol abuse, or autoimmune disorders
• It can lead to liver damage, cirrhosis, and impaired liver function, affecting nutrient metabolism and detoxification
• Cirrhosis is a chronic liver disease characterized by the replacement of healthy liver tissue with scar tissue, leading to impaired liver function and portal hypertension
• Cirrhosis can be caused by chronic hepatitis, alcohol abuse, or non-alcoholic fatty liver disease (NAFLD)

Gallbladder and Pancreatic Disorders

• Cholelithiasis (gallstones) is the presence of solid stones in the gallbladder or bile ducts, which can cause obstruction and inflammation
• Gallstones can lead to cholecystitis (gallbladder inflammation), biliary colic, and pancreatitis
• Pancreatitis is inflammation of the pancreas, which can be acute or chronic
• Acute pancreatitis is often caused by gallstones or alcohol abuse, while chronic pancreatitis is usually due to long-term alcohol abuse or genetic factors
• Pancreatitis can lead to impaired digestion and absorption of nutrients, as well as endocrine dysfunction (diabetes mellitus)
• Pancreatic cancer is a malignant tumor of the pancreas, often affecting the exocrine portion
• It can cause obstructive jaundice, abdominal pain, and weight loss, and is associated with a poor prognosis due to late detection