- Understand the role of the gallbladder in concentrating bile and coordinating its secretion with ingestion of a meal
- Describe the molecular mechanisms whereby bile is concentrated during storage
- Discuss the mechanism and significance of gallbladder secretion
- Understand how bile remains isoosmolar during concentration
- Explain how contraction of the gallbladder is regulated
- Explain why the gallbladder is vulnerable to the formation of cholesterol gallstones
- Describe the physiologic consequences of surgical removal of the gallbladder
- Understand the role of the sphincter of Oddi in regulating bile outflow into the intestine
The gallbladder serves to store and concentrate bile coming from the liver in the period between meals. Gallbladder function therefore permits coordination of the secretion of a bolus of concentrated bile with the entry of dietary lipids into the small intestine. It is important to be aware, however, that the gallbladder is not essential to normal digestion and absorption of a meal. In the absence of a functioning gallbladder, the bile acid pool continues to cycle through the enterohepatic circulation and the majority of the bile acid pool is stored in the small intestine.
The gallbladder is a muscular sac located just below the liver and lying adjacent to the liver’s surface. Its capacity is approximately 50 mL in adult humans. It is linked to the biliary system via the cystic duct, a bidirectional conduit for bile flow. During periods of fasting, bile secreted by the liver is diverted into the gallbladder on the basis of pressure relationships in the biliary system, as will be discussed in more detail later. On the other hand, when the gallbladder receives neurohumoral cues that fats are present in the small intestine, it contracts and bile flows out of the gallbladder and into the intestine via the cystic and common bile ducts.
The gallbladder has two functional layers. The innermost of these, facing the bile, is a columnar epithelium that participates actively in bile concentration. The tight junctions that link adjacent epithelial cells are among the most well-developed anywhere in the body, making the epithelium highly resistant to the passive flux of solutes. This “tight” epithelium prevents the passive loss of bile acid molecules and thus is essential to the ability of the gallbladder to concentrate the bile. It is likely also important in limiting the potentially deleterious effects of amphipathic bile acids. To this end, the epithelium also contains abundant goblet cells that secrete mucus, which is also believed to protect the epithelium from injury.
The epithelial layers are underlaid by smooth muscle that can alter the caliber of the gallbladder lumen according to the presence of neurohumoral stimuli. The muscle cells receive input from branches of the vagus nerve, and express cholinergic receptors to respond to released acetylcholine (ACh). They also express receptors for gastrointestinal hormones, and in particular for cholecystokinin (CCK...