The gallbladder is 8–10 cm (~3–4 in) long and is nested in a shallow area on the posterior aspect of the right lobe of the liver. This muscular sac stores, concentrates, and, when stimulated, propels the bile into the duodenum via the common bile duct. It is divided into three regions. The fundus is the widest portion and tapers medially into the body, which in turn narrows to become the neck. The neck angles slightly superiorly as it approaches the hepatic duct. The cystic duct is 1–2 cm (less than 1 in) long and turns inferiorly as it bridges the neck and hepatic duct.

The simple columnar epithelium of the gallbladder mucosa is organized in rugae, similar to those of the stomach. There is no submucosa in the gallbladder wall. The wall’s middle, muscular coat is made of smooth muscle fibers. When these fibers contract, the gallbladder’s contents are ejected through the cystic duct and into the bile duct (image). Visceral peritoneum reflected from the liver capsule holds the gallbladder against the liver and forms the outer coat of the gallbladder. The gallbladder's mucosa absorbs water and ions from bile, concentrating it by up to 10-fold.

The gallbladder is a thin-walled, pear-shaped pouch. It is about 7 to 10 centimeters (about 2.7 to 3.9 inches) long, up to 5 centimeters (about 2 inches) across at its widest point, and is located in an indentation under the liver. The gallbladder stores and concentrates bile from the liver. The bile then helps to digest and absorb fats from food in the duodenum, the first section of the small intestine.

Every day the cells of the liver produce about 800 to 1,000 milliliters (about 27 to 34 fluid ounces) of yellow, brownish or olive-green bile. The liver cells then pass the bile on through small canals to the bile duct, which leads from the lower surface of the liver into the duodenum. After about 4 to 6 centimeters (about 1.6 to 2.4 inches), a small duct leading to the gallbladder branches off of the larger bile duct before it connects to the intestine.

The bile produced by the liver flows directly into the small intestine during a meal. Between meals, when there is no fat that needs to be digested, most of the bile flows into the gallbladder instead, where it is concentrated and stored. The gallbladder usually holds about 30 to 80 milliliters (about 1 to 2.7 fluid ounces) of fluid. When we eat fatty foods, the gallbladder contracts and squeezes bile through the bile duct bit by bit. The bile is mixed into the semi-digested food in the duodenum.

Bile is mainly made up of water, but also has bile salts, cholesterol, certain fats (lecithin) and bile pigments in it. The most important bile pigment, bilirubin, is made when old red blood cells are broken down in the liver. It is bilirubin that makes urine yellow and stool brown.

Bile salts break down larger fat globules in food into small droplets of fat. Smaller fat droplets are easier for the digestive enzymes from the pancreas to process and break down. Bile salts also help the cells of the intestine to absorb these broken-down fats.

The salivary glands, liver, gallbladder, and pancreas are not part of the digestive tract, but they have a role in digestive activities and are considered accessory organs.

Salivary Glands

Three pairs of major salivary glands (parotid, submandibular, and sublingual glands) and numerous smaller ones secrete saliva into the oral cavity, where it is mixed with food during mastication. Saliva contains water, mucus, and enzyme amylase. Functions of saliva include the following:

• It has a cleansing action on the teeth.
• It moistens and lubricates food during mastication and swallowing.
• It dissolves certain molecules so that food can be tasted.
• It begins the chemical digestion of starches through the action of amylase, which breaks down polysaccharides into disaccharides.

Liver

The liver is located primarily in the right hypochondriac and epigastric regions of the abdomen, just beneath the diaphragm. It is the largest gland in the body. On the surface, the liver is divided into two major lobes and two smaller lobes. The functional units of the liver are lobules with sinusoids that carry blood from the periphery to the central vein of the lobule.

The liver receives blood from two sources. Freshly oxygenated blood is brought to the liver by the common hepatic artery, a branch of the celiac trunk from the abdominal aorta. Blood that is rich in nutrients from the digestive tract is carried to the liver by the hepatic portal vein.

The liver has a wide variety of functions and many of these are vital to life. Hepatocytes perform most of the functions attributed to the liver, but the phagocytic Kupffer cells that line the sinusoids are responsible for cleansing the blood.

Liver functions include the following:

• secretion
• synthesis of bile salts
• synthesis of plasma protein
• storage
• detoxification
• excretion
• carbohyrate metabolism
• lipid metabolism
• protein metabolism
• filtering

Gallbladder

The gallbladder is a pear-shaped sac that is attached to the visceral surface of the liver by the cystic duct. The principal function of the gallbladder is to serve as a storage reservoir for bile. Bile is a yellowish-green fluid produced by liver cells. The main components of bile are water, bile salts, bile pigments, and cholesterol.

Bile salts act as emulsifying agents in the digestion and absorption of fats. Cholesterol and bile pigments from the breakdown of hemoglobin are excreted from the body in the bile.

Pancreas

The pancreas has both endocrine and exocrine functions. The endocrine portion consists of the scattered islets of Langerhans, which secrete the hormones insulin and glucagon into the blood. The exocrine portion is the major part of the gland. It consists of pancreatic acinar cells that secrete digestive enzymes into tiny ducts interwoven between the cells. Pancreatic enzymes include anylase, trypsin, peptidase, and lipase. Pancreatic secretions are controlled by the hormones secretin and cholecystokinin.