In this lesson, you will learn to identify the accessory digestive organs of the pancreas, gallbladder, and liver, and you will investigate their role in digestion. Specifically, this lesson will cover:
The pancreas, gallbladder, and liver are identified as accessory organs. This means they play a role in digestion, but they're separate from the digestive tract. Chemical digestion in the small intestine relies on the activities of three accessory digestive organs: the pancreas, liver, and gallbladder. The digestive role of the liver is to produce bile and export it to the duodenum. Recall that bile is a digestive juice produced by the liver that is important for digestion of lipids. 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.
As you move through the lesson, refer to the diagram below as a visual.
Accessory Organs—The liver, pancreas, and gallbladder are considered accessory digestive organs, but their roles in the digestive system are vital.
2. The Pancreas
Recall that you previously learned about the endocrine functions of the pancreas and its role in regulating blood glucose levels by producing insulin and glucagon. However, it also has an extremely important role as an exocrine organ, secreting various digestive enzymes. Here, you will learn more about the role of the pancreas in digestion.
watch
View the following video to learn more about the pancreas.
The soft, oblong, glandular pancreas lies 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 attached to the spleen.
Exocrine and Endocrine Pancreas—(left) The pancreas has a head, a body, and a tail. It delivers pancreatic juice to the duodenum through the pancreatic duct. (right) The acinus, which is a secretory unit, showing the locations of different secretions.
The exocrine part of the pancreas arises as little grape-like cell clusters, each called an acinus (plural = acini), located at the terminal ends of pancreatic ducts. The acinar cells secrete digestive enzymes. At the junction of the acinus and the duct lies the centroacinar cells. Both the centroacinar cells and the duct cells secrete bicarbonate ions. Together, the secretions of all these cells form an enzyme-rich pancreatic juice that is secreted into tiny merging ducts that form two dominant ducts.
The larger duct fuses with the common bile duct (carrying bile from the liver and gallbladder) just before entering the duodenum via a common opening (the hepatopancreatic ampulla). 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), 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.
Scattered through the sea of exocrine acini are small islands of endocrine cells. These islands are referred to as the islets of Langerhans or pancreatic islets; they are referred to as pancreatic islet cells in the image above. These vital cells produce the hormones pancreatic polypeptide, insulin, glucagon, and somatostatin.
terms to know
Acinus
The cluster of glandular epithelial cells in the pancreas that secretes pancreatic juice in the pancreas.
Pancreatic Juice
The secretion of the pancreas containing digestive enzymes and bicarbonate.
Accessory Duct
The duct that runs from the pancreas into the duodenum. Also called duct of Santorini.
2a. Pancreatic Juice
The pancreas produces over a liter of pancreatic juice each day. Unlike bile, pancreatic juice 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 trypsin from trypsinogen of the pancreas, which in turn changes the pancreatic enzymes procarboxypeptidase and chymotrypsinogen into their active forms, carboxypeptidase and chymotrypsin.
The enzymes that digest starch (pancreatic amylase), fat (pancreatic lipase), and nucleic acids (pancreatic nucleases) are secreted in their active forms because they do not attack the pancreas as do the protein-digesting enzymes.
terms to know
Enteropeptidase
The intestinal brush border enzyme that activates trypsinogen to trypsin.
Trypsin
An enzyme secreted by the pancreas (in the form of trypsinogen) that participates in protein digestion.
Pancreatic Amylase
The enzyme secreted by the pancreas that completes the chemical digestion of carbohydrates in the small intestine.
Pancreatic Lipase
The enzyme secreted by the pancreas that participates in lipid digestion.
Pancreatic Nuclease
An enzyme secreted by the pancreas that participates in nucleic acid digestion.
2b. Pancreatic Secretion
Regulation of pancreatic secretion is the job of hormones and the parasympathetic nervous system. The entry of acidic chyme into the duodenum stimulates the release of secretin, which in turn causes the duct cells to release bicarbonate-rich pancreatic juice. The presence of proteins and fats in the duodenum stimulates the secretion of cholecystokinin (CCK), which then stimulates the acini to secrete enzyme-rich pancreatic juice and enhances the activity of secretin. Parasympathetic regulation occurs mainly during the cephalic and gastric phases of gastric secretion when vagus nerve stimulation prompts the secretion of pancreatic juice.
Usually, the pancreas secretes just enough bicarbonate to counterbalance the amount of hydrochloric acid (HCl) produced in the stomach. Hydrogen ions enter the blood when bicarbonate is secreted by the pancreas. Thus, the acidic blood draining from the pancreas neutralizes the alkaline blood draining from the stomach, maintaining the pH of the venous blood that flows to the liver.
IN CONTEXT
Pancreatic Disorder: Pancreatitis
As you have learned, the pancreas creates enzymes to help digest food. In some cases, these enzymes may damage the pancreas and cause inflammation. Pancreatitis is the term used to describe inflammation of the pancreas.
Illustration of Healthy and Inflamed Pancreases
Pancreatitis can be acute (sudden and short-term) or chronic (long-lasting, in which the pancreas does not heal).
Acute pancreatitis can result in a range of symptoms, such as abdominal pain, fever, nausea, and vomiting. However, in most cases, acute pancreatitis improves quickly. If pancreatitis is mild, it will usually improve with treatment, but more severe cases can be life-threatening. In particular, chronic pancreatitis causes severe damage to the pancreas, which means the body can no longer produce many essential enzymes and hormones and can even result in conditions such as diabetes. In the United States, acute pancreatitis accounts for approximately 275,000 hospital stays, whereas chronic pancreatitis accounts for approximately 86,000 hospital stays (NIDDK, 2017).
Chronic pancreatitis can also cause another pancreatic disorder called pancreatolithiasis, in which stones (also known as calculi) are formed that can block the pancreatic ducts.
3. The Liver
key concept
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, defined by internal features. The liver is connected to the abdominal wall and diaphragm by five peritoneal folds referred to as ligaments. These are the falciform ligament, the coronary ligament, two lateral ligaments, and the ligamentum teres hepatis. The falciform ligament and ligamentum teres hepatis are actually remnants of the umbilical vein and separate the right and left lobes anteriorly. The lesser omentum tethers the liver to the lesser curvature of the stomach.
The porta hepatis (“gate to the liver”) is where the hepatic artery and hepatic portal vein enter the liver. As shown in the image below, 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 bloodborne 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.
did you know
The fact that all blood from the alimentary canal passes through the liver largely explains why the liver is the most common site for the metastasis of cancers that originate in the alimentary canal.
Microscopic Anatomy of the Liver—The liver receives oxygenated blood from the hepatic artery and nutrient-rich deoxygenated blood from the hepatic portal vein.
terms to know
Liver
The largest gland in the body whose main digestive function is the production of bile.
Hepatic Artery
The artery that supplies oxygenated blood to the liver.
Hepatic Portal Vein
The vein that supplies deoxygenated nutrient-rich blood to the liver.
3a. Histology
The liver has three main components: hepatocytes, bile canaliculi, and hepatic sinusoids. A hepatocyte is the liver’s main cell type, accounting for around 80% 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 portal vein in each hepatic lobule.
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 here, 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. This duct then joins with the cystic duct from the gallbladder, forming the common bile duct through which bile flows into the small intestine.
A hepatic sinusoid is an open, porous blood space formed by fenestrated capillaries from nutrient-rich hepatic portal veins and oxygen-rich hepatic arteries.
terms to know
Hepatocytes
Major functional cells of the liver.
Hepatic Lobule
The hexagonal-shaped structure composed of hepatocytes that radiate outward from a central vein.
Bile Canaliculus
The small duct between hepatocytes that collects bile.
Common Hepatic Duct
The duct formed by the merging of the two hepatic ducts.
Common Bile Duct
The structure formed by the union of the common hepatic duct and the gallbladder’s cystic duct.
Hepatic Sinusoid
Blood capillaries between rows of hepatocytes that receive blood from the hepatic portal vein and the branches of the hepatic artery.
3b. Bile
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. Recall that 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, phospholipids (such as lecithin), electrolytes, cholesterol, and triglycerides. The components most critical to emulsification are bile salts and phospholipids, which have a nonpolar (hydrophobic) region as well as a polar (hydrophilic) region. The hydrophobic region interacts with the large lipid molecules, whereas the hydrophilic region interacts with the watery chyme in the intestine. 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.
Bile salts act as emulsifying agents, so they are also important for the absorption of digested lipids.
Fat Emulsification by Bile Salt
While most constituents of bile are eliminated in feces, bile salts are reclaimed by enterohepatic circulation: once bile salts reach the ileum, they are absorbed and returned to the liver in the hepatic portal blood. The hepatocytes then excrete the bile salts into newly formed bile. Thus, this precious resource is recycled.
Bilirubin, the main bile pigment, is a waste product produced when the spleen removes old or damaged red blood cells from the circulation. These breakdown products, including proteins, iron, and toxic bilirubin, are transported to the liver via the splenic vein of the hepatic portal system. In the liver, proteins and iron are recycled, whereas bilirubin is excreted in the bile. It accounts for the green color of bile.
did you know
Bilirubin is eventually transformed by intestinal bacteria into stercobilin, a brown pigment that gives your stool its characteristic color! In some disease states, bile does not enter the intestine, resulting in white (“acholic”) stool with a high-fat content, since virtually no fats are broken down or absorbed.
Hepatocytes work nonstop, 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.
term to know
Bile
An alkaline solution produced by the liver and important for the emulsification of lipids.
4. The Gallbladder
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. 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 tenfold.
Gallbladder—The gallbladder stores and concentrates bile and releases it into the two-way cystic duct when it is needed by the small intestine.
IN CONTEXT
Cholecystectomy
Although the gallbladder serves an important role in storing and concentrating bile, there are times when its removal is necessary to improve an individual’s health. For example, if someone has gallstones (small, hardened masses in their gallbladder or bile ducts), there can be complications, such as inflammation and blockage. Removing the gallbladder can be done to resolve these issues.
Cholecystectomy is the term for gallbladder removal, and it is among the most common medical procedures performed in the United States. In fact, more than 1.2 million are performed annually.
Although gallbladder removal may be medically necessary, there are some common side effects that can result, such as fatty diarrhea. This is caused by the bile continuously draining into the intestines, when it is usually concentrated and released as needed for fat digestion. The result of not having a gallbladder means the bile is not concentrated and released as needed for fat digestion. This results in a laxative effect that usually resolves after a period of a few weeks to months after gallbladder removal. However, a low-fat, high-fiber diet and eating smaller, more frequent meals can help mitigate this issue.
terms to know
Gallbladder
The accessory digestive organ that stores and concentrates bile.
Cystic Duct
The duct through which bile drains and enters the gallbladder.
summary
In this lesson, you explored an overview of the three digestive accessory organs (the pancreas, liver, and gallbladder) and their roles in digestive processes. First, you explored the structure and functions of the pancreas, which has both exocrine and endocrine functions, and learned that it produces pancreatic juice and delivers the pancreatic juice to the duodenum. Then, you examined how pancreatic secretion is regulated by hormones and the parasympathetic nervous system. You also learned how the liver, which is the largest gland in the body, produces and exports bile, which aids in digestion. You examined the histology of the liver and that there are three main components: the hepatocytes, which produce bile; bile canaliculus, which accumulates and moves bile from the liver; and the hepatic sinusoid, which receives blood from the hepatic portal vein and branches of the hepatic artery. You also explored how bile is formed and that it emulsifies lipids in the small intestine. Finally, you learned about the structure and function of the gallbladder, which stores, concentrates, and releases bile.
