Chapter 18: Digestive System III: Liver & Pancreas

The liver, the body's largest internal organ, performs extensive metabolic duties, including synthesizing the majority of circulating plasma proteins like albumins and very low-density lipoproteins (VLDLs), maintaining iron and copper homeostasis, processing vitamins (A, D, E, K), and detoxifying xenobiotics via Phase I oxidation and Phase II conjugation. Structurally, the liver receives a unique dual blood supply with about 75% of the flow coming from the venous hepatic portal vein and the remaining 25% from the arterial hepatic artery. Its parenchyma is organized into interconnecting plates of large, polygonal hepatocytes, separated by discontinuous sinusoidal capillaries that converge toward the terminal hepatic venule, or central vein, within the organization of the classic lobule. Specialized cells integral to the sinusoidal lining include phagocytic Kupffer cells, which manage immune defense and recycle iron from aged red blood cells, and hepatic stellate cells (Ito cells), which primarily store Vitamin A but differentiate into myofibroblasts that produce collagen during chronic liver pathology. The liver’s exocrine secretion, bile, is collected by bile canaliculi formed by adjacent hepatocytes, flows centrifugally, and reaches the biliary tree, which is lined by cholangiocytes; the canals of Hering, partially lined by both hepatocytes and cholangiocytes, are also implicated as a reservoir for hepatic stem cells. Bile is stored and concentrated up to ten-fold in the gallbladder, an action achieved by the simple columnar epithelial cells (cholangiocytes) actively transporting electrolytes out of the bile, followed by the passive, aquaporin-mediated movement of water; the gallbladder wall notably lacks a muscularis mucosa and submucosa, but may exhibit deep Rokitansky-Aschoff sinuses. Finally, the pancreas acts as both an exocrine serous gland, synthesizing potent digestive enzyme precursors (zymogens) essential for intestinal digestion, and an endocrine organ. A unique histological feature of the exocrine pancreas is the centroacinar cells, which represent the initial lining of the intercalated ducts starting inside the acinus. These ducts secrete a large volume of bicarbonate-rich fluid, regulated by the hormone secretin, to neutralize acidic chyme. The endocrine function is concentrated in the highly vascularized Islets of Langerhans, containing B cells that secrete insulin (to decrease blood glucose), A cells that secrete glucagon (to increase blood glucose), and D cells that secrete somatostatin.