The Liver and Gallbladder
The digestive function of the liver is to produce bile, which is then delivered to the duodenum to emulsify fats. Emulsification is the breaking up of fat globules into smaller fat droplets, increasing the surface area upon which fat-digesting enzymes (lipases) can operate. Because bile does not chemically change anything, it is not an enzyme. Bile is also alkaline, serving to help neutralize the HCl in the chyme. Bile consists of bile salts, bile pigments, phospholipids (including lecithin), cholesterol, and various ions. The primary bile pigment, bilirubin, is an end product of the breakdown of hemoglobin from expended red blood cells. Although some of the bile is lost in the feces (bilirubin gives feces their brown color), much of the bile is reabsorbed by the small intestine and returned to the liver via the hepatic portal vein. The liver performs numerous metabolic functions. Some of the most important follow.
- Bile is produced.
- Blood glucose is regulated. When blood glucose is high, the liver converts glucose to glycogen (glycogenesis) and stores the glycogen. When blood glucose is low, glycogen is broken down (glycogenolysis), and glucose is released into the blood.
- Proteins (including plasma proteins) and certain amino acids are synthesized.
- Ammonia (which is toxic) is converted to urea (less toxic) for elimination by the kidneys.
- Bacteria and expended red and white blood cells are broken down. From the red blood cells, Fe and globin are recycled, and bilirubin is secreted in the bile.
- Vitamins (A, D, and B12) and minerals (including Fe from expended red blood cells) are stored.
- Toxic substances (drugs, poisons) and hormones are broken down.
The liver is composed of numerous functions units called lobules. Within each lobule, epithelial cells called hepatocytes are arranged in layers that radiate out from a central vein. Hepatic sinusoids are spaces that lie between groups of layers, while smaller channels called bile canaliculi separate other layers. Each of (usually) six corners of the lobule are occupied by three vessels: one bile duct and two blood vessels (a portal triad). The blood vessels are branches from the hepatic artery (carrying oxygenated blood) and from the hepatic portal vein (carrying deoxygenated but nutrient-rich blood from the small intestine). Blood enters the liver through the hepatic artery and hepatic portal vein and is distributed to lobules. Blood flows into each lobule by passing through the hepatic sinusoid and collecting in the central vein. The central veins of all the lobules merge and exit the liver through the hepatic vein (not the hepatic portal vein). Within the sinusoids, phagocytes called Kupffer cells (stellate reticuloendothelial cells) destroy bacteria and break down expended red and white blood cells and other debris. Hepatocytes that border the sinusoids also screen the incoming blood. They remove various substances from the blood, including oxygen, nutrients, toxins, and waste materials. From these substances, they produce bile, which they secrete into the bile canaliculi, which empty into bile ducts. Bile ducts from the various lobules merge and exit the liver as a single common hepatic duct. The common hepatic duct merges with the cystic duct from the gallbladder to form the common bile duct, which, in turn, merges with the pancreatic duct to form the hepatopancreatic ampulla. This last duct delivers the bile to the duodenum. The gallbladder stores excess bile. When food is in the duodenum, bile flows readily from the liver and gallbladder into the duodenum. When the duodenum is empty, a sphincter muscle (sphincter of Oddi) closes the hepatopancreatic ampulla, and bile backs up and fills the gallbladder.
The secretions of the pancreas, called pancreatic juice, include various enzymes, including pancreatic amylase (digestion of starch), trypsin, carboxypepiydase, and chymotrypsin (proteases), as well as pancreatic lipase (digestion of fats). Sodium bicarbonate is also produced, making the pancreatic juice alkaline. An alkaline solution neutralizes the HCl in the chyme and provides an optimal environment for the action of these enzymes. Pancreatic juice is produced in clusters of exocrine cells called acini. The remaining cells in the pancreas (about 1 percent of the total) also form clusters (islets of Langerhans). These are the endocrine cells that produce the hormones insulin, glucagon, somatostatin, and pancreatic polypeptide. Pancreatic juice collects in small ducts that merge to form two large ducts. The main pancreatic duct (duct of Wirsung) exits the pancreas and merges with the common bile duct from the liver and gallbladder. This combined duct, called the hepatopancreatic ampulla (ampulla of Vater), then enters the duodenum. A smaller, second duct that exits the pancreas, the accessory pancreatic duct (duct of Santorini), joins the duodenum directly.