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This is a sample clip. Sign in or start your free trial. JoVE Core Biology. Previous Video Next Video. Next Video Embed Share. Accessory digestive organs secrete enzymes and other substances into the primary digestive tract to facilitate digestion.
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Save to playlist. Filter by:. The liver is the largest internal organ of the human body; it weighs around 1. Located in the upper right quadrant of the abdomen, just below the diaphragm, it has over documented functions. Its major role in digestion is the production of bile, which is concentrated and stored in the gall bladder Fig 2.
In a healthy person, the liver releases around ml of bile into the duodenum each day. This is done in small amounts as the gall bladder contracts in response to CCK Hundt et al, Bile salts play an important role in chemical digestion in the duodenum. They lower the surface tension of large fat globules, causing them to collapse into smaller droplets Fig 3.
This process is called emulsification, as it creates an emulsion of tiny fat droplets with a larger surface area for subsequent breakdown by fat-digesting enzymes lipases. The digestion of fats by bile and lipases also allows the efficient absorption of fat-soluble vitamins such as vitamins A, D, E and K. The pancreas is a small, elongated gland around 15cm long and weighing around g; it is located in the loop of the duodenum and is covered by the lower portion of the stomach Fig 2.
The pancreas is an important organ in the endocrine and digestive systems, playing key roles in both the regulation of blood-glucose levels and digestion. The endocrine portion of the pancreas consists of tiny clusters of cells called islets of Langerhans, which produce several key hormones involved in controlling blood-sugar levels and regulating appetite. It consists of thousands of acini, small berry-like structures that secrete pancreatic juice into tiny ducts. These connect and eventually fuse before discharging their secretions into the central pancreatic duct, which itself fuses with the common bile duct before entering the duodenum at the major duodenal papilla Fig 2.
Around As explained by Agrawal and Aoun, , pancreatic juice consists mostly of:. One of its key roles is to neutralise acidity in the chyme coming through from the stomach. This is essential to prevent autodigestion and ulceration of subsequent sections of the GI tract. Beyond pancreatic juice, the pancreas also produces several enzymes that continue, in the duodenum, the chemical digestion started in the mouth and stomach.
Pancreatic lipase Pancreatic lipase is the most active of the gut lipases. Like salivary and gastric lipases see part 1 and part 2 , it breaks down triglyceride fats into fatty acids and glycerol, working mostly on the emulsified fat droplets created by bile salts.
For maximal activity on fat droplets, pancreatic lipase requires the presence of the small protein co-enzyme colipase, which is itself produced by the pancreas Ross et al, Weight-loss drugs such as orlistat inhibit pancreatic lipase, thereby reducing the digestion and absorption of fats. However, the use of such drugs is often associated with side-effects such as abdominal bloating and diarrhoea, as undigested fats collect in the colon and irritate it Qi, ; Al-Suwailem et al, Pancreatic amylase Carbohydrate digestion starts in the mouth with salivary amylase see part 1.
Like salivary amylase, pancreatic amylase attacks the glycosidic bonds between adjacent glucose molecules in polysaccharides, breaking down starch into maltose. As it catalyses starch digestion in the duodenum lumen inside space of the duodenum , it is also called luminal amylase Williams, Pancreatic amylase can function over a broad neutral-to-alkaline pH range.
Amylases are most efficient in an environment with a neutral-to-alkaline pH. The activity of salivary amylase slows down in the acidic environment of the stomach, but the enzymatic digestion of carbohydrates resumes in the alkaline environment of the duodenum. Protein digestion starts in the stomach, where pepsin attacks the peptide bonds of large molecules, breaking down proteins into smaller chains of amino acids termed polypeptides see part 2.
The next stage of protein digestion relies on several pancreatic proteases:. Trypsin is the major protease present in pancreatic juice. To avoid autodigestion and damage to the pancreatic acini and ducts, it is initially secreted as the inactive precursor or zymogen trypsinogen.
Once in the duodenal lumen, trypsinogen is converted into trypsin by the action of the enzyme enteropeptidase or enterokinase , which is produced by the mucosal cells of the duodenum and jejunum. Trypsin then catalyses the activation of the other pancreatic zymogens into their active forms chymotrypsin, carboxypeptidase and elastase Goodman, Carboxypeptidase — an exopeptidase — catalyses the removal of single amino acids from the ends of protein and polypeptide molecules, gradually reducing their length.
Trypsin, chymotrypsin and elastase — all endopeptidases — attack the peptide bonds in the central portions of proteins and polypeptides. This results in the generation of smaller chains of amino acids called peptides Fig 4 , which are subsequently digested in the jejunum and ileum by the action of intestinal peptidase see part 4.
Pancreatic nucleases All food consumed by humans comes from plant, animal, fungal or bacterial sources. As all cells, whatever their origin, contain deoxyribonucleic acid DNA and ribonucleic acid RNA , the human body can digest these by breaking them down into their building blocks, which are called nucleotides. DNA is composed of four major nucleotide bases: adenine, cytosine, guanine and thymine; RNA has the same bases, except that thymine is replaced by uracil Knight and Andrade, The digestion of nucleic acids allows some of the nucleotide bases to be recycled and used as building blocks for human DNA synthesis during cell division, and for RNA during the process of transcription that precedes protein synthesis.
Figure 3: The pancreas has a head, a body, and a tail. It delivers pancreatic juice to the duodenum through the pancreatic duct. These acinar cells secrete enzyme-rich pancreatic juice 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, the islets of Langerhans. These vital cells produce the hormones pancreatic polypeptide, insulin, glucagon, and somatostatin. 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. 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 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.
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 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 vagal stimulation prompts the secretion of pancreatic juice. Usually, the pancreas secretes just enough bicarbonate to counterbalance the amount of 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. 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.
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 fold. 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. Figure 4. The gallbladder stores and concentrates bile, and releases it into the two-way cystic duct when it is needed by the small intestine.
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.
Visceral peritoneum reflected from the liver capsule holds the gallbladder against the liver and forms the outer coat of the gallbladder. 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. Answer the question s below to see how well you understand the topics covered in the previous section.
Skip to main content. Module 7: The Digestive System. Search for:. Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder Learning Objectives By the end of this section, you will be able to: State the main digestive roles of the liver, pancreas, and gallbladder Identify three main features of liver histology that are critical to its function Discuss the composition and function of bile Identify the major types of enzymes and buffers present in pancreatic juice.
Critical Thinking Questions Why does the pancreas secrete some enzymes in their inactive forms, and where are these enzymes activated? Describe the location of hepatocytes in the liver and how this arrangement enhances their function. Show Answers The pancreas secretes protein-digesting enzymes in their inactive forms. If secreted in their active forms, they would self-digest the pancreas. The hepatocytes are the main cell type of the liver. They process, store, and release nutrients into the blood.
Radiating out from the central vein, they are tightly packed around the hepatic sinusoids, allowing the hepatocytes easy access to the blood flowing through the sinusoids. Licenses and Attributions. CC licensed content, Shared previously.
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