21.7: Accessory Organs in Digestion- Liver, Pancreas, and Gallbladder

By the end of the section, you will be able to:

• State the main digestive roles, the location, and both gross and histological structure of the liver, pancreas, and gallbladder
• Explain the relative positions of the structures within a liver lobule
• Diagram the branching pattern of the bile ducts and pancreatic ducts
• Discriminate between the exocrine pancreas and the endocrine pancreas.

Chemical digestion in the small intestine relies on the activities of three accessory digestive organs: the liver, pancreas, and gallbladder (Figure 21.7.1). 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 Organs. The liver, pancreas, and gallbladder are considered accessory digestive organs, but their roles in the digestive system are vital. (Image credit: "Digestive System Accessory Organs" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by KnowledgeWorks.)

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 (Figure \(\PageIndex{2}\)). 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 \(\PageIndex{2}\): Inferior-anterior view of the liver. The inferior vena cava would be posterior along your spinal column. You can see the hepatic veins exiting the lobes of the liver. From this position, the left lobe is on your left hand side and the right lobe is on the right hand side. Framed between the gallbladder, porta hepatis and the ligamentum Lastly the caudate lobe is anterior to the inferior vena cava and posterior to the porta hepatis. In the porta hepatis you have the hepatic artery and hepatic portal vein entering the liver and the common hepatic duct leaving. (Image credit: "Cenveo - Drawing Liver anatomy and vascularisation - English labels" by Cenveo, license: CC BY, modifications by Jennifer Lange.)

The porta hepatis (“gate to the liver”) is where the hepatic artery and hepatic portal vein enter the liver. These two vessels, along with the common hepatic duct, run behind the lateral border of the lesser omentum on the way to their destinations. As shown in (Figure \(\PageIndex{2}\)), the hepatic artery delivers oxygenated blood from the heart to the liver. The hepatic portal vein delivers partially deoxygenated blood containing nutrients absorbed from the small intestine and actually supplies more oxygen to the liver than do the much smaller hepatic arteries. In addition to nutrients, drugs and toxins are also absorbed. After processing the blood bourne nutrients and toxins, the liver releases nutrients needed by other cells back into the blood, which drains into the central vein and then through the hepatic vein to the inferior vena cava. With this hepatic portal circulation, all blood from the alimentary canal passes through the liver. This largely explains why the liver is the most common site for the metastasis of cancers that originate in the alimentary canal.

Figure \(\PageIndex{3}\): Microscopic Anatomy of the Liver. The liver receives oxygenated blood from the hepatic artery and nutrient-rich deoxygenated blood from the hepatic portal vein. The blood filters through the sinusoids of a liver lobule, where it comes in contact with the hepatocytes, and is collected by the central vein.  Hepatocytes also produce bile, which drains through the branches of the bile duct. (Image credit: "Microscopic Anatomy of the Liver" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of originals by OpenStax, BlueLink, and Cenveo.)

Histology

The liver (Figure \(\PageIndex{4}\)) has three main components: hepatocytes, bile canaliculi, and hepatic sinusoids. A hepatocyte is the liver’s main cell type, accounting for around 80 percent of the liver's volume. These cells play a role in a wide variety of secretory, metabolic, and endocrine functions. Plates of hepatocytes called hepatic laminae radiate outward from the central vein in each hepatic lobule. The liver lobule is a hexagon shape with portal triads on each corner at the perimeter of the hexagon. The portal triad is a distinctive arrangement consisting of three basic structures: a bile duct, a small branch of the hepatic artery (portal arteriole) bringing in oxygenated blood and a branch of the hepatic portal vein (portal venule) delivering nutrient rich blood. The bile duct carries freshly made bile away from hepatocytes toward the gallbladder and duodenum.

Figure \(\PageIndex{4}\): Liver Histology. The cells of the liver are encapsulated into hexagonal shaped lobules. At each corner of the lobule, within the connective tissue septa, is a portal triad (arteriole, venule, bile duct).  An arteriole from the hepatic artery and a venule from the portal vein bring blood to the lobule. The blood drains through a series sinusoids that are lined by hepatocytes and then exists the lobule through the central vein.  (Image Credit: "Pig Liver" by Digital Histology is licensed under CC BY 4.0.) 

Between adjacent hepatocytes, grooves in the cell membranes provide room for each bile canaliculus (plural = canaliculi). These small ducts accumulate the bile produced by hepatocytes. From the hepatocytes, bile flows first into bile ductules and then into bile ducts. The bile ducts unite to form the larger right and left hepatic ducts, which themselves merge and exit the liver as the common hepatic duct (Figure \(\PageIndex{5}\)). This duct then joins with the cystic duct from the gallbladder, forming the common bile duct through which bile flows into the small intestine.

Figure \(\PageIndex{5}\): Bile Ducts. The ducts carrying bile from the liver and the gallbladder unite and release bile into the duodenum. (Image credit: "Bile Ducts" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original from BlueLink.)

Recall that lipids are hydrophobic, that is, they do not dissolve in water. Thus, before they can be digested in the watery environment of the small intestine, large lipid globules must be broken down into smaller lipid globules, a process called emulsification. Bile is a mixture secreted by the liver to accomplish the emulsification of lipids in the small intestine.

Hepatocytes secrete about one liter of bile each day. A yellow-brown or yellow-green alkaline solution (pH 7.6 to 8.6), bile is a mixture of water, bile salts, bile pigments (including bilirubin), phospholipids (such as lecithin), electrolytes, cholesterol, and triglycerides. Some molecules in bile have regions attracted to water molecules and regions attracted to lipids. This results in the large lipid globules being pulled apart into many tiny lipid fragments of about 1 µm in diameter. This change dramatically increases the surface area available for lipid-digesting enzyme activity. This is the same way dish soap works on fats mixed with water.

Hepatocytes work non-stop, but bile production increases when fatty chyme enters the duodenum and stimulates the secretion of the gut hormone secretin. Between meals, bile is produced but conserved. The valve-like hepatopancreatic ampulla closes, allowing bile to divert to the gallbladder, where it is concentrated and stored until the next meal.

A hepatic sinusoid is an open, porous blood space formed by fenestrated capillaries from nutrient-rich hepatic portal veins and oxygen-rich hepatic arteries. Hepatocytes are tightly packed around the fenestrated endothelium of these spaces, giving them easy access to the blood. From their central position, hepatocytes process the nutrients, toxins, and waste materials carried by the blood. Materials such as bilirubin are processed and excreted into the bile canaliculi. Other materials including proteins, lipids, and carbohydrates are processed and secreted into the sinusoids or just stored in the cells until called upon. The hepatic sinusoids combine and send blood to a central vein. Blood then flows through a hepatic vein into the inferior vena cava. This means that blood and bile flow in opposite directions. The hepatic sinusoids also contain star-shaped reticuloendothelial cells (Kupffer cells), phagocytes that remove dead red and white blood cells, bacteria, and other foreign material that enter the sinusoids.​​​​​

Gallbladder

Figure \(\PageIndex{5}\) shows the anterior view of the gallbladder, part of the liver superior to the gallbladder, and the duct connecting the liver to gallbladder, eventually becoming the common bile duct. 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 (Figure \(\PageIndex{5}\)). 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 can accumulate gall stones over time (Figure \(\PageIndex{6}\)), resulting in removal of the gallbladder with the gall stones in some people.

Figure \(\PageIndex{6}\): 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 soft, oblong, glandular pancreas lies transversely in the retroperitoneum behind the stomach. Its head is nestled into the “c-shaped” curvature of the duodenum with the body extending to the left about 15.2 cm (6 in) and ending as a tapering tail in the hilum of the spleen. It performs a curious mix of exocrine (secreting digestive enzymes) and endocrine (releasing hormones into the blood) functions (Figure \(\PageIndex{7}\)). Histology of the pancreas reveals round clusters of acinar cells connected to the pancreatic duct and the bigger clusters of pancreatic islets.

Figure \(\PageIndex{7}\): 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.)

The exocrine part of the pancreas arises as little grape-like cell clusters of serous cells, each called an acinus (plural = acini), located at the terminal ends of pancreatic ducts (Figure \(\PageIndex{7}\)). These acinar cells secrete enzyme-rich pancreatic juice into tiny merging ducts that form two dominant ducts. The main pancreatic duct fuses with the common bile duct at the ampulla (carrying bile from the liver and gallbladder) just before entering the duodenum via a common opening (the main duodenal papilla). The smooth muscle sphincter of the hepatopancreatic ampulla controls the release of pancreatic juice and bile into the small intestine. The second and smaller pancreatic duct, the accessory duct (duct of Santorini), present in approximately 30% of the population, runs from the pancreas directly into the duodenum, approximately 1 inch above the hepatopancreatic ampulla. When present, it is a persistent remnant of pancreatic development.

Figure \(\PageIndex{8}\): Pancreas Histology @ 100x.  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.) 

Scattered through the sea of exocrine acini are small islands of endocrine cells, the pancreatic islets (islets of Langerhans) (Figure \(\PageIndex{8}\) and Figure \(\PageIndex{9}\)). These vital cells produce the hormones pancreatic polypeptide, insulin, glucagon, and somatostatin. These hormones play roles in regulating several metabolic functions.

 

Figure \(\PageIndex{9}\): Pancreatic Islet Histology @ 400x. you can see the serous cells are clumped into the grape light acini. The lighter colored pancreatic islet is also more clear under this magnification.  (Image Credit :"Pancreatic Islet Histology @ 400x" micrograph provided by the Regents of University of Michigan Medical School © 2012.)

Pancreatic Juice

The pancreas produces over a liter of pancreatic juice each day. Unlike bile, it is clear and composed mostly of water along with some salts, sodium bicarbonate, and several digestive enzymes. Sodium bicarbonate is responsible for the slight alkalinity of pancreatic juice (pH 7.1 to 8.2), which serves to buffer the acidic gastric juice in chyme, inactivate pepsin from the stomach, and create an optimal environment for the activity of pH-sensitive digestive enzymes in the small intestine. Pancreatic enzymes are active in the digestion of sugars, proteins, and fats.

The pancreas produces protein-digesting enzymes in their inactive forms. These enzymes are activated in the duodenum. If produced in an active form, they would digest the pancreas (which is exactly what occurs in the disease, pancreatitis). The intestinal brush border enzyme enteropeptidase stimulates the activation of an enzyme that starts the cascade of other enzymes to digest proteins.

The enzymes that digest starch (amylase), fat (lipase), and nucleic acids (nuclease) are secreted in their active forms, since they do not attack the pancreas as do the protein-digesting enzymes.

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Concept Review

Chemical digestion in the small intestine cannot occur without the help of the liver and pancreas. The liver produces bile and delivers it to the common hepatic duct. Bile contains bile salts and phospholipids, which emulsify large lipid globules into tiny lipid droplets, a necessary step in lipid digestion and absorption. The gallbladder stores and concentrates bile, releasing it when it is needed by the small intestine.

The pancreas produces the enzyme- and bicarbonate-rich pancreatic juice and delivers it to the small intestine through ducts. Pancreatic juice buffers the acidic gastric juice in chyme, inactivates pepsin from the stomach, and enables the optimal functioning of digestive enzymes in the small intestine.

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Review Questions

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Critical Thinking Questions

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Glossary

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Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology