Anatomy and Physiology of the Gallbladder and Bile Ducts

The anatomy of the biliary tree is a little complicated, but it is important to understand. The liver's cells (hepatocytes) excrete bile into canaliculi, which are intercellular spaces between the liver cells. These drain into the right and left hepatic ducts, after which bile travels via the common hepatic and cystic ducts to the gallbladder. The gallbladder, which has a capacity of 50 milliliters (about 5 tablespoons), concentrates the bile 10 fold by removing water and stores it until a person eats. At this time, bile is discharged from the gallbladder via the cystic duct into the common bile duct and then into the duodenum (the first part of the small intestine), where it begins to dissolve the fat in ingested food.

The liver excretes approximately 500 to 1000 milliliters (50 to 100 tablespoons) of bile each day. Most (95%) of the bile that has entered the intestines is resorbed in the last part of the small intestine (known as the terminal ileum), and returned to the liver for reuse.

The many functions of bile are best understood by knowing the composition of bile:

1. Bile Salts (cholates, chenodeoxycholate, deoxycholate): these are produced by the liver's breakdown of cholesterol. They function in bile as detergents that dissolve dietary fat and allow it to be absorbed. Hence, disruption of bile excretion disrupts the normal absorption of fat, a process called malabsorption. Patients develop diarrhea because the fat is not absorbed (steatorrhea) , and develop deficiencies of the fat-soluble vitamins (A, D, E, and K).

2. Cholesterol and phospholipids-while only 4% of bile is cholesterol, the secretion of cholesterol and its metabolites (bile salts) into bile is the body's major route of elimination of cholesterol. Phospholipids, which are components of cell membranes, enhance the cholesterol solubilizing properties of bile salts. Inefficient excretion of cholesterol can cause an increased serum cholesterol. This predisposes to vascular disease (heart attacks, strokes, etc.)

3. Bilirubin-while this comprises only 0.3% of bile, it is responsible for bile's yellow color. Bilirubin is a product of the body's metabolism of hemoglobin, the carrier of oxygen in red blood cells. Disruption of the excretion of this component of bile leads to a yellow discoloration of the eyes and skin (jaundice).

4. Protein and miscellaneous components

Metabolic functions, such as the maintenance of glucose (blood sugar) levels

Synthetic functions, such as the synthesis of serum proteins such as albumin, blood clotting (coagulation) factors, and complement (a mediator of inflammatory responses)

Storage functions, such as the storage of sugar (glycogen), fat (triglycerides), iron, copper, and fat soluble vitamins (A, D, E, and K)

Catabolic functions, such as the detoxification of drugs