Gallbladder and Biliary Tract
The gallbladder is a small, pear-shaped, muscular storage sac that holds bile. Bile flows out of the liver through the left and right hepatic ducts, which come together to form the common hepatic duct. This duct then joins with a duct connected to the gallbladder, called the cystic duct, to create the common bile duct. The common bile duct enters the small intestine at the sphincter of Oddi (a ring-shaped muscle), a few inches below the stomach.
About half the bile secreted between meals is diverted through the cystic duct and into the gallbladder, where bile is stored. In the gallbladder, up to 90% of the water in the bile is absorbed into the bloodstream, making the remaining bile very concentrated. The rest of the bile produced by the liver flows directly through the common bile duct into the small intestine. When food enters the small intestine, a series of hormonal and nerve signals trigger the gallbladder to contract and the sphincter of Oddi to relax and thus open. Bile then flows from the gallbladder into the small intestine to mix with food contents and perform its digestive functions.
After bile enters and passes down the small intestine, about 95% of the bile salts are reabsorbed into the bloodstream through the wall of the lower small intestine. The liver then extracts these bile salts from the blood and resecretes them back into the bile. The bile salts in the body go through this cycle about 10 to 12 times a day. Each time, small amounts of bile salts escape absorption and reach the large intestine, where they are broken down by bacteria. Some bile salts are reabsorbed in the large intestine; the rest are excreted in the stool.
Although the gallbladder is useful, it is not necessary. Thus, if the gallbladder is removed (for example, in someone with cholecystitis), bile is able to move directly from the liver to the small intestine.
Bile consists of bile salts; electrolytes (dissolved charged particles, such as sodium and bicarbonate); bile pigments, such as bilirubin; cholesterol; and other fats (lipids). Bile is responsible for the elimination of certain waste products from the body--particularly pigment from destroyed red blood cells and excess cholesterol--and assists in the digestion and absorption of fats. Bile salts increase the solubility of fats and fat-soluble vitamins to aid in their absorption from the intestine. Hemoglobin (the protein that carries oxygen in the blood) from destroyed red blood cells is converted into bilirubin (the main pigment in bile) and excreted in bile as a waste product.
Gallstones may obstruct the flow of bile from the gallbladder, causing pain (biliary colic) or inflammation. Gallstones may also migrate from the gallbladder to the bile duct, where they can block the normal flow of bile to the intestine, which results in jaundice, a yellowish discoloration of the skin and the whites of the eyes. The flow of bile can also be blocked by tumors and by other less common causes.
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