into and out of the gallbladder. Distal to the junction with the cystic duct, the fused duct is called the common bile duct and extends for about 7 cm to the wall of the duodenum at the ampulla of Vater. A thickening of the muscularis externa of the duodenum at the ampulla constitutes the sphincter of Oddi, which surrounds the openings of both the common bile duct and the pancreatic duct and acts as a valve to regulate the flow of bile and pancreatic juice into the duodenum.
The adult human liver secretes, on average, about 1 L of bile a day.
The bile fulfills two major functions. It is involved in the absorption of fat and is used by the liver as a vehicle for excretion of cholesterol, bilirubin, iron, and copper. The composition of bile and the specific functions of most of its components are listed in Table 18.1. 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 reab sorbed by the gut and transported back to the liver in the portal blood. The bile salts are then reabsorbed and rese creted by hepatocytes. Hepatocytes also synthesize new bile salts to replace those that are lost.  Cholesterol and the 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 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 fow 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. Following stimulation, the gallbladder contracts steadily 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 fbers innervate blood vessels, and increased stimulation in this system causes an increase of vascular resistance, decreased hepatic blood volume, and a rapid increase of serum levels of glucose. The parasympathetic fbers are assumed to innervate the large ducts (those that contain smooth muscle in their walls) and possibly blood vessels; their stimulation promotes glucose uptake and utilization. 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. 18.15). 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 concentrates and stores bile.