Anatomy of the Biliary System
The gallbladder is a pear-shaped, muscular, pouch-like organ that is roughly three to six inches long and sits under the liver in the upper right region of the abdomen. Some of the bile produced by the liver flows directly into the small intestine, but about 50% flows into the gallbladder where it is stored and concentrated until needed to facilitate fat digestion. Between meals, the gallbladder is relaxed, but during meals, signals from the digestive tract stimulate contractions of the gallbladder and bile is released, through a system of ducts, into the intestine. In addition to helping with digestion, bile carries waste products from the liver to be eliminated through the digestive tract (JHM 2018; Zakko 2018; Housset 2016).
Bile, a greenish-yellow fluid produced in the liver, is made of bile salts, cholesterol, phospholipids (a type of fatty molecule), electrolytes, bilirubin, and water (JHM 2018; Hundt 2018). When excess cholesterol is present in bile, it can crystalize. Sludge forms when cholesterol crystals mix with mucus in the gallbladder. Over time, these crystals can aggregate and form stones (Jones, Ghassemzadeh 2018; Tanaja 2018).
Gallstones that are more than 70% cholesterol are known as cholesterol stones; if cholesterol content is between 30% and 70%, they are called mixed stones; and gallstones with a cholesterol content less than 30% are pigment stones (Molvar 2016). Calcium salts, such as calcium bilirubinate and calcium phosphate, are concentrated in pigment gallstones, making these types of gallstones easy to visualize on an X-ray. Calcium, usually as calcium carbonate, is present in smaller amounts in cholesterol and mixed gallstones (Njeze 2013; Portincasa 2012; Kaufman 1994). As many as 90% of gallstones in people living in Western societies are cholesterol and mixed gallstones that form in the gallbladder (Mayo Clinic 2017; Molvar 2016; Shaffer 2018).
Pigment stones usually form when there is excess bilirubin (ie, yellow pigment produced during the breakdown of red blood cells) in the bile. Conditions such as liver cirrhosis and certain blood disorders can lead to excess bilirubin in the bile and contribute to the formation of black pigment gallstones (Tanaja 2018; Mayo Clinic 2017). Parasitic infections and anatomical narrowing of bile ducts are major causes of brown pigment stones (Shaffer 2018).
Gallbladder Motility and Cholesterol Stone Formation
In the early stages of cholesterol sludge and gallstone formation, the muscular function of the gallbladder weakens, reducing the gallbladder's ability to respond to signals. Lack of bile flow due to low gallbladder motility then further contributes to sludge and stone formation by allowing time for increased cholesterol concentration, crystallization, and aggregation (Tanaja 2018; Jones, Ghassemzadeh 2018; Chen 2015).
Oxidative stress contributes to gallstone development. The process of gallstone formation and growth has been shown to be enhanced in the presence of oxidized bilirubin, which is found in gallstones (Sanikidze 2016; Lichtenberg 1988). People with gallstones often have high levels of oxidized lipids in their blood, and gallbladders removed due to gallstones show high levels of oxidative damage as well as low levels of antioxidant enzymes (Geetha 2002).
Complications of Gallstones
A biliary blockage due to trapped gallstones can lead to acute complications such as (Zakko 2018; Tanaja 2018; Shaffer 2018; Zimmer 2015):
- Cholecystitis: inflammation of the gallbladder
- Choledocholithiasis: one or more gallstones lodged in the common bile duct
- Cholangitis: infection with inflammation in the bile ducts due to an obstruction
- Pancreatitis: inflammation of the pancreas due to obstruction of the pancreatic duct
- Ileus: a rare and dangerous complication wherein the GI tract no longer properly propels ingested food material through the GI tract via peristaltic action. A large gallstone may cause Ileus.
In some cases, gallstones cause chronic inflammation in the gallbladder (chronic cholecystitis), leading to thickening of the gallbladder walls (Jones, Ferguson 2018).