Pancreas or pancreatic gland is considered both as an accessory organ of the digestive system as well as an independent gland by itself. One of the principal identity of the gland is that it is both exocrine and endocrine in nature. It is essentially the organ of glucose metabolism in our body as the major function of the pancreas is to produce and release insulin into the bloodstream. Insulin is a glucose metabolizing hormone, released by the endocrine pancreas which is necessary to keep a check on the serum levels of glucose while it also performs digestive functions through its exocrine segment. Pancreas contains three major types of cells; they are acinar cells, ductal cells and endocrine cells. Pancreatic enzymes necessary for the digestion are released by the acinar cells while the ductal cells help in lining the pancreatic ducts. Ductal cells release watery fluid that helps in transporting the digestive enzymes into the intestine along with it. The most important among all the cells are endocrine cells found in the Islets of Langerhans, which secrete insulin and other hormones. The area where Islets of Langerhance cells found in the pancreas is called the exocrine pancreas because acinar and ductal cells release their secretions into a duct found here ( See image for reference). The exocrine pancreas makes up 85% of the pancreas while the endocrine is hardly 15%. Infection or injury to pancreas is a serious threat because these can cause or increase the risk of diabetes. It is important to understand the gross anatomy, physiology and the mechanism of how the pancreas works. Let us understand these aspects in detail through this post.
The pancreas is a solid accessory organ of the digestive system that lies transversely across the posterior abdominal wall to stretch deep into the epigastrium which is the upper quadrant of the abdomen. The organ is firmly fixed just in front but slightly below the abdominal aorta (a major branch of the aorta) and adjacent to the first and second lumbar segments of the vertebral column. It measures approximately 15 cm long and weighs less than 110 g in a normal adult. It is enclosed inside a thin capsule of connective tissue that helps the pancreas to become lobules. Anatomically, the organ is divided into 4 segments; head, neck, body and tail. The head is close to the first, second, and third portions of the duodenum (the initial segment of the small intestine) while the body lies horizontal to the retroperitoneal space and the tail extends toward the hilum of the spleen. It is shaped like a pear fruit and has access with organs such as liver, spleen and intestine.
Pancrease necessarily needs more blood supply as the gland works as a multifunctional unit. The splenic artery travels just next to the head of the pancreas supplies the left portion of the body of pancreas. The tail of the pancreas is supplied by the pancreatic branches that comes out of the greater pancreatic artery. The superior and inferior pancreaticoduodenal arteries pass along the posterior region of the head of the pancreas supply the head of the pancreas. In the middle portion of the gland, the 2 arteries join together to form the common artery and the process is called anastomosis. The anastomosed artery perfuses the entire body of the pancreas. Similarly, the splenic vein receives the venous blood from the body and neck of the pancreas. Splenic veins lie just beneath the pancreas. The venous blood from the head of the gland drains into the superior mesenteric and portal veins, through the pancreaticoduodenal veins.
Exocrine pancreas and its functions
The exocrine pancreas is drained by a duct called Wirsung, which is channelled into the duodenum at the duodenal papilla adjacent to the common bile duct. Many major blood vessels surround the pancreas with the superior mesenteric artery from the top, the superior mesenteric veins along with the portal vein and the celiac axis makes up a great channel of perfusion. Some of the organs in contact with the pancreas are spleen from its left, adrenal gland from behind, left kidney at the bottom of the gland, and many major blood vessels. The acinus and its duct system make up the functional unit of the exocrine pancreas. The word acinus is derived from the Latin term meaning “berry in a cluster” because they look like a bunch of berries. These cells are regulated and modified by the parasympathetic nervous system while they are inhibited by the sympathetic system. Exocrine pancreas secretes Proteases, pancreatic lipases, and amylase. Proteases are nothing but the Chymotrypsinogen and Trypsinogen which Digest proteins and peptides to single amino acids. Pancreatic lipase helps in digesting triglycerides, monoglyceride and free fatty acids while the Amylase synthesizes Starch and maltose, a type of disaccharides. Additionally, the exocrine pancreas produces other enzymes such as ribonuclease, gelatinase, elastase etc.
Endocrine pancreas and its functions
Having said that the endocrine component of the pancreas comprises of islets of Langerhans that synthesize and release the most important hormones; insulin and glucagon. The 2 hormones are secreted by beta and alpha cells respectively, which gets drained into the bloodstream. The endocrine pancreas also contains delta cell accounts for 4% of the islet cells and secretes the peptide hormone somatostatin. Insulin acts by optimally maintaining the blood sugar while the glucagon help to raise blood sugar whenever necessary. Maintaining proper blood sugar levels is crucial to the functioning of key organs including the brain, liver, and kidneys.
Histology (cell study of the pancreas)
The exocrine pancreas comprises the clusters of lobules, which are drained by ductules. The major portion of the gland is mainly formed by a cluster of cells called the islets of Langerhans, this is where the insulin and glucagon are synthesized. On the othe rhand, the clusters of the exocrine pancreas are made up of a few hundred cells with each cluster located between the lobules. The gland also has the acinus, consisting of a number of acinar cells surrounding a lumen. The acinar cells assist in synthesizing and secreting enzymes. and are typical exocrine glandular cells containing pyramidal epithelial cells arranged in rows. Their apexes join to form the lumen of the acinus and the Zymogen granules containing digestive enzymes which are accommodated within in the acinar cells.
Pancreatic Juice and its composition
An average adult produces bout 1500 mL of pancreatic juice each day. The juice contains water, ions such as HCO3-, Cl-, Na+, and K+, along with a number of proteins. The pH of the pancreatic secretion is around 8.3, which is moderately alkaline. Therefore, pancreatic juice plays a vital role in neutralizing the excessive gastric acids that enter the duodenum with ingested food mixed with the chyme that comes out of the stomach. Furthermore, the juice contains many pancreatic enzymes that aid in the digestion and absorption of fats, carbohydrates, and proteins. The principal pancreatic enzymes are lipases, amylase, deoxyribonuclease, and ribonucleases. All these are secreted by the acinar cells in their active forms. The remaining enzymes are secreted as inactive zymogen cells. Zymogen secretes trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase, and phospholipase A2.
Regulation Pancreatic secretions
Pancreatic secretion is regulated by the consolidated effort of neurological and endocrinal systems(neurohumoral interactions). The secretions are principally controlled by the hormones secretin and cholecystokinin produced by specialized cells of the duodenal mucosa. On the other hand, the secretion of secretin triggered by gastric acid acts chiefly on the pancreatic duct cells to release a highly alkaline pancreatic juice. Another hormone, CCK –cholecystokinin is released by the endocrine pancreas. It is stimulated by the products of protein and fat digestion (peptides, amino acids, and fatty acids). The release of CCK is regulated with the help of cholecystokinin-releasing peptide found in the first part of the small intestine.
Digestive Functions of Pancreatic Juice(exocrine pancreas)
The secretion of pancreatic juice helps the digestive system in many ways. The bulk of bicarbonates along with the pancreatic juice help in neutralizing the acidic chyme released by the stomach which ensures the proper functioning of pancreatic enzymes through the creation of a stable pH. For the healthy function of all the enzymes, normal pH is inevitable. When the pancreatic juice enters the duodenum, inactive trypsinogen becomes active trypsin with the help of the enzyme enteropeptidase, which is found in the intestinal brush border. Pancreatic lipase assists in the management of fat metabolism which prevents obesity. Furthermore, the enzyme amylase aids in the hydrolysis of complex starch into simple maltose (a glucose-glucose disaccharide). The pancreatic amylase act by splitting the long chain of polysaccharides into the smaller dextrins, maltose and malto-trioses. The enzyme carboxypeptidase, an exopeptidase splits peptide bonds adjacent to the carboxyl terminals of peptide chains to release amino acids.