Diagram showing the relationship of hepatic, pancreatic, and gallbladder ducts. The gallbladder is a blind pouch joined to a single cystic duct in which numerous mucosal folds form the spiral valve (of Heister). The cystic duct joins with the common hepatic duct, and together they form the common bile duct that leads into the duodenum. At the entry to the duodenum, the common bile duct is joined by the main pancreatic duct to form the hepatopancreatic ampulla (of Vater), and together they enter the second part of the duodenum. Sphincters can be found at the distal part of these ducts. The sphincters of the common bile duct (of Boyden), the main pancreatic duct, and the hepatopancreatic ampulla (of Oddi) control the flow of bile and pancreatic secretion into the duodenum. When the common bile duct sphincter contracts, bile cannot enter the duodenum; it backs up and flows into the gallbladder, where it is concentrated and stored.
common hepatic duct spiral valve (of Heister) -jj cystic duct gallbladder bile common duct sphincter of common bile duct (of Boyden)
major duodenal papilla main pancreatic duct x (of Wirsung)
sphincter of main pancreatic duct hepatopancreatic ampulla (of Vater)
The adult human liver secretes, on average, about 1 L of bile a day
The composition of bile and the functions of most of its components are listed in Table 17.1. As noted in the table, many components of the bile are recycled via the portal circulation.
• About 90% of the bile salts, a component of bile, is reabsorbed by the gut and transported back to the liver in the portal blood. The bile salts are then reabsorbed and resecreted by hepatocytes. Hepatocytes also synthesize new bile salts to replace those that are lost.
• Cholesterol and phospholipid lecithin, as well as most of the electrolytes and water delivered to the gut with the bile, are also reabsorbed and recycled.
Bilirubin glucuronide, the detoxified end product of hemoglobin breakdown, is not recycled. It is ultimately excreted with the feces and gives them their color. Failure to absorb bilirubin or failure to conjugate it or secrete glucuronide can produce jaundice.
Bile flow from the liver is regulated by hormonal and neural control. The rate of blood flow to the liver and the concentration of bile salts in the blood exert regulatory effects on the bile flow. Bile flow is increased when hormones such as cholecystokinin (CCK), gastrin, and motilin are released by enteroendocrine cells during digestion. Steroid hormones (i.e., estrogen during pregnancy) decrease bile secretion from the liver. In addition, parasympathetic stimulation increases bile flow by prompting contraction of the gallbladder and relaxation of the sphincter of Oddi. Bile that leaves the liver via the common hepatic duct flows through the cystic duct to the gallbladder. The gallbladder stores and can increase the concentration of bile up to 10-fold. Following stimulation, the gallbladder contracts and delivers the bile to the duodenum via the common bile duct.
The liver has both sympathetic and parasympathetic innervation
The liver (and gallbladder) receives nerves from both sympathetic and parasympathetic divisions of the autonomic nervous system. The nerves enter the liver at the porta hepatis and ramify through the liver in the portal canals along with the members of the portal triad. Sympathetic fibers are believed to innervate blood vessels; parasympathetic fibers are assumed to innervate the large ducts (those that contain smooth muscle in their walls) and possibly blood vessels. The cell bodies of parasympathetic neurons are often present near the porta hepatis.
The gallbladder is a pear-shaped, distensible sac with a volume of about 50 mL in humans (see Fig. 17.15). It is attached to the visceral surface of the liver. The gallbladder is a secondary derivative of the embryonic foregut, arising as an evagination of the primitive bile duct that connects the embryonic liver to the developing intestine.
The gallbladder is a blind pouch that leads, via a neck, to the cystic duct. Through this duct it receives dilute bile from the hepatic duct. Hormones secreted by the enteroendocrine cells of the small intestine in response to the presence of fat in the proximal duodenum, stimulate contractions of the smooth muscle of the gallbladder. As a result of these contractions, concentrated bile is discharged into the common bile duct, which carries the bile to the duodenum.
Mucosa of the gallbladder has several characteristic features
The empty or partially filled gallbladder has numerous deep mucosal folds (Fig. 17.16). The mucosal surface consists of simple columnar epithelium. The tall epithelial cells exhibit the following features:
• Numerous well-developed apical microvilli
• Apical junctional complexes that join adjacent cells and form a barrier between the lumen and the intercellular compartment
• Localized concentrations of mitochondria in the apical and basal cytoplasm
• Complex lateral plications (Fig. 17.17)
These cells closely resemble the absorptive cells of the intestine.
Both cells share the above characteristics, as well as localization of Na 7K+-activated ATPase on their lateral plasma membranes and secretory vesicles filled with glycoproteins in their apical cytoplasm.
The lamina propria of the mucosa is particularly rich in fenestrated capillaries and small venules, but there are no lymphatic vessels in this layer. The lamina propria is also very cellular, containing large numbers of lymphocytes and plasma cells. The characteristics of the lamina propria resemble those of the colon, another organ specialized for the absorption of electrolytes and water.
Mucin-secreting glands are sometimes present in the lamina propria in the normal human gallbladder, especially near the neck of the organ, but they are more commonly found in inflamed gallbladders. Cells that appear identical to enteroendocrine cells of the intestine are also found in these glands.
The wall of the gallbladder lacks a muscularis mucosae and submucosa
External to the lamina propria is a muscularis externa that has numerous collagen and elastic fibers among the
Photomicrograph of the wall of the gallbladder. The mucosa of the gallbladder consists of a lining of simple columnar epithelial cells and a lamina propria of loose connective tissue, which typically exhibits numerous deep folds in the mucosa. Beneath this layer is a relatively thick layer, the muscularis externa. There is no muscularis mucosae or submucosa. The smooth muscle bundles of the muscularis externa are randomly oriented. External to the muscle is an adventitia containing adipose tissue and blood vessels. The portion of the gallbladder not attached to the liver displays a typical serosa instead of an adventitia. X175.
bundles of smooth muscle cells. Despite its origin from a foregut-derived tube, the gallbladder does not have a muscularis mucosae or submucosa. The smooth muscle bundles are somewhat randomly oriented, unlike the layered organization of the intestine. Contraction of the smooth muscle reduces the volume of the bladder, forcing its contents out through the cystic duct.
External to the muscularis externa is a thick layer of dense connective tissue (see Fig. 17.16). This layer contains large blood vessels, an extensive lymphatic network, and the autonomic nerves that innervate the muscularis externa and the blood vessels (cell bodies of parasympathetic neurons are found in the wall of the cystic duct). The connective tissue is also rich in elastic fibers and adipose tissue. Where the gallbladder attaches to the liver surface, this layer is referred to as the adventitia. The unattached surface is covered by a serosa or visceral peritoneum consisting of a layer of mesothelium and a thin layer of loose connective tissue.
In addition, deep diverticula of the mucosa, called Roki-tansky-Aschoff sinuses, sometimes extend through the muscularis externa (Fig. 17.18). They are thought to presage pathologic changes. Also, bacteria may accumulate in these sinuses, causing chronic inflammation.
Concentration of the bile requires the coupled transport of salt and water
The epithelial cells of the gallbladder actively transport both Na+ and CI" (and HC03~) from the cytoplasm into junctional complexes;
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