12.2: Liver, Pancreas, and Gallbladder — Accessory Digestive System Organs

Chemical digestion in the small intestine depends on the coordinated actions of the liver, gallbladder, and pancreas, which supply bile and digestive enzymes to break down nutrients efficiently.

Master this section and you'll be able to:

• Describe the roles of the liver, gallbladder, and pancreas in releasing bile and pancreatic juice to support chemical digestion in the small intestine.
• Distinguish between the pancreas’s exocrine enzyme secretion and endocrine hormone release.

Chemical digestion in the small intestine relies on the activities of three accessory digestive organs: the liver, pancreas, and gallbladder (see figure below). The digestive role of the liver is to produce bile and export it to the duodenum. The gallbladder primarily stores, concentrates, and releases bile. The pancreas produces pancreatic juice, which contains digestive enzymes and bicarbonate ions, and delivers it to the duodenum.

Figure \(\PageIndex{1}\): Accessory Digestive Organs and Their Ducts. The liver, gallbladder, and pancreas work together to release bile and digestive enzymes into the duodenum (1^st part of the small intestine), where they help break down fats, proteins, and carbohydrates. This diagram highlights the location of each organ and shows how the bile ducts and pancreatic duct empty into the first part of the small intestine to support digestion.

Liver

The liver is the largest gland in the body, weighing about three pounds in an adult. It is also one of the most important organs. In addition to being an accessory digestive organ, it plays a number of roles in metabolism and regulation. The liver lies inferior to the diaphragm in the right upper quadrant of the abdominal cavity and receives protection from the surrounding ribs.

The liver is divided into two primary lobes: a large right lobe and a much smaller left lobe. In the right lobe, some anatomists also identify an inferior quadrate lobe and a posterior caudate lobe, which are defined by internal features. The liver is connected to the abdominal wall and diaphragm by five peritoneal folds referred to as ligaments.  The lesser omentum tethers the liver to the lesser curvature of the stomach.

  
Figure \(\PageIndex{2}\): Liver Blood Flow and Bile Drainage. Blood arrives from two sources: the hepatic arteries, which supply oxygenated blood, and the portal vein, which carries nutrient-rich blood from the digestive tract. Inside the liver, these vessels branch extensively and travel alongside bile ducts. As the liver processes nutrients and detoxifies blood, cleaned blood exits through the hepatic veins and returns to the heart. Bile produced by hepatocytes drains into small ducts that merge into the right and left hepatic ducts and eventually flow toward the gallbladder and small intestine. (Image credit: "Cenveo - Drawing Liver anatomy and vascularisation - English labels" by Cenveo, license: CC BY, modifications by Jennifer Lange.)

The liver is one of the body’s most versatile and essential organs, often described as a metabolic powerhouse because nearly every nutrient absorbed from the digestive tract passes through it for processing. After digestion, the liver adjusts nutrient levels in the blood by storing some molecules, converting others, and releasing them as needed to maintain stable internal conditions. It also detoxifies harmful substances, transforms ammonia into urea, breaks down old red blood cells, and helps recycle iron. With hundreds of chemical reactions occurring at any moment, the liver plays a central role in keeping metabolism balanced and supporting overall health.

The liver performs both exocrine and endocrine functions. Its major exocrine role is the production and secretion of bile, a fluid that contains bile salts needed for the emulsification and absorption of dietary lipids. Bile flows through a duct system and is stored in the gallbladder until it is released into the small intestine during digestion.

In addition to being a huge metabolic and detoxifying organ, the liver quietly behaves like an endocrine gland and helps regulate growth, blood pressure, blood cell production, iron balance, and calcium homeostasis. 

Last but not least, besides its many metabolic, and endo-and exocrine function, the liver also releases other important substances directly into the bloodstream, including plasma proteins such as albumin and clotting factors. 

 
Gallbladder

The gallbladder is a small, muscular sac, about 8–10 cm (3–4 in) long, nestled in a shallow depression on the underside (posterior aspect) of the right lobe of the liver. It stores and concentrates bile produced by the liver, then releases it into the duodenum through the common bile duct when needed for digestion. The gallbladder connects to the liver via the cystic duct, which joins the hepatic duct to form the common bile duct. Structurally, the gallbladder has three regions: the fundus is the widest portion, which tapers into the body; the body narrows into the neck, which bends slightly upward as it approaches the hepatic duct. The cystic duct is a short (1–2 cm) passage that connects the neck of the gallbladder to the hepatic duct.

The inner lining of the gallbladder is a simple columnar epithelium arranged in folds called rugae, similar to the rugae of the stomach. There is no distinct submucosa. The middle layer of the wall is made of smooth muscle; when this muscle contracts, bile is squeezed out through the cystic duct and into the bile duct. The outer surface is covered by visceral peritoneum that reflects from the liver capsule, helping hold the gallbladder tightly against the liver. As bile sits in the gallbladder, its mucosa absorbs water and ions, concentrating the bile by up to about tenfold. Over time, some people develop gallstones within the gallbladder , and in many of these cases the gallbladder is surgically removed along with the stones.

Figure \(\PageIndex{3}\): Gallstones.  Ultrasound image of a stone in the gallbladder.  (Image credit: "Gallstone" by © Nevit Dilmen is licensed under CC BY-SA 3.0, via Wikimedia Commons.)

 
Pancreas

The oblong, glandular pancreas lies transversely in the retroperitoneal space behind the stomach. Its head fits snugly into the c-shaped curve of the duodenum, while the body extends to the left about 15 cm (6 in) and narrows into a tapering tail that reaches the hilum of the spleen. The pancreas performs both exocrine functions (secreting digestive enzymes into ducts) and endocrine functions (releasing hormones into the blood). On histologic examination, you can see round clusters of acinar cells connected to the pancreatic ducts, as well as larger clusters of lighter-staining endocrine cells called pancreatic islets.

The exocrine portion of the pancreas is organized into many small, grape-like clusters of serous cells called acini (singular: acinus) at the terminal ends of the pancreatic ducts. These acinar cells secrete enzyme-rich pancreatic juice into tiny ducts that merge to form two main ducts. The main pancreatic duct joins the common bile duct at the hepatopancreatic ampulla, just before entering the duodenum through the main duodenal papilla. A ring of smooth muscle called the sphincter of the hepatopancreatic ampulla regulates the release of both pancreatic juice and bile into the small intestine. In about 30 percent of people, a second, smaller accessory pancreatic duct (duct of Santorini) carries secretions directly into the duodenum about 2.5 cm (1 in) above the main opening; when present, it is a persistent remnant of pancreatic development.

Scattered among the exocrine acini are small “islands” of endocrine cells called pancreatic islets (islets of Langerhans). These cells release several important hormones into the bloodstream, including insulin, glucagon, somatostatin, and pancreatic polypeptide. Together, these hormones help regulate blood glucose levels and coordinate other metabolic functions throughout the body.

Figure \(\PageIndex{4}\): Exocrine Pancreas. The pancreas is located posterior to the stomach. The acinar and ductal cells secrete enzymes and bicarbonate into the pancreatic duct(s) that empty into the duodenum at the duodenal papilla(e). (Image credit: "Exocrine Pancreas" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original illustration by BlueLink.)

Pancreatic Juice

The pancreas produces more than a liter of clear, watery pancreatic juice each day. This fluid contains salts, sodium bicarbonate, and several digestive enzymes. Sodium bicarbonate gives pancreatic juice its slightly alkaline pH (7.1 to 8.2), which helps neutralize acidic chyme arriving from the stomach, inactivates pepsin, and creates the proper pH for the enzymes that work in the small intestine. Pancreatic enzymes participate in breaking down carbohydrates, proteins, and fats.

Protein-digesting enzymes are released in inactive forms so they do not harm pancreatic tissue. These enzymes are activated only after they reach the duodenum. A brush border enzyme called enteropeptidase triggers the first activation step, which then sets off a cascade that turns the remaining protein-digesting enzymes into their active forms.

Enzymes that digest starch (amylase), fats (lipase), and nucleic acids (nuclease) are released already active, because they do not pose a risk of damaging the pancreas the way active protein-digesting enzymes would.

Figure \(\PageIndex{5}\): Pancreas Histology. This shows that he pancreas is made up of lots of serous cells. However, at this power, you cannot see the acini. (Image Credit: "Pancreas Histology @ 100x" micrograph provided by the Regents of University of Michigan Medical School © 2012.)