Pancreas

Pancreas

The pancreas lies posterior to the stomach, extend-ing laterally from the duodenum toward the spleen. It is an elongated, tadpole-shaped organ, about 15 cm in length and weighing about 80 g or 3 ounces. The broad head of the pancreas lies within the loop that is formed by the duodenum of the intestine. The thin-ner body of the pancreas extends toward the spleen, whereas the tail appears short and bluntly rounded and abuts the spleen (FIGURE 24-15).

The pancreas is retroperitoneal, firmly bound to the posterior abdominal cavity wall. It not only acts as an endocrine gland, but also has an exocrine ­function— it secretes the digestive juice known as pancreatic juice. Pancreatic acinar cells produce pancreatic juice and make up most of the pancreas. Smaller pancreatic tubes form larger ones, and eventually the main pan-creatic duct, connected with the duodenum at the same place where the bile duct, from the liver and gallbladder, joins the duodenum (Figure 24-14). A smaller accessory pancreatic duct empties into the duodenum, proximal to the main pancreatic duct. The movement of pancreatic juices is controlled by a hepa-topancreatic sphincter.

The secretions of the pancreas are regulated mostly by hormones from the duodenum. When chyme arrives in the duodenum, the hormone secre-tin is released, triggering pancreatic secretion of a watery buffer solution with a pH between 7.5 and 8.8. This secretion contains bicarbonate and phos-phate buffers that help to raise the pH of the chyme. Another duodenal hormone, cholecystokinin, stimu-lates production and secretion of pancreatic enzymes.

These enzymes are able to digest carbohydrates,­ fats, nucleic acids, and proteins. Pancreatic amylase digests carbohydrates, splitting starch or glyco-gen molecules into disaccharides or double sugars. ­Pancreatic lipase digests fat, breaking triglycerides into fatty acids and glycerol. Two nucleases are also contained in ­pancreatic juice. These enzymes break down nucleic acids into nucleotides.

Proteolytic or protein-splitting enzymes such as trypsin, chymotrypsin, and carboxypeptidase split bonds between certain combinations of amino acids in proteins. Complete digestion of proteins requires dif-ferent enzymes because no single enzyme can split all possible combinations of amino acids. The clusters of secretory acinar cells surrounding ducts in the pancreas are known as acini. They are filled with rough endo-plasmic reticulum and have deeply staining zymogen granules, which are tiny structures that store proteo-lytic enzymes in cells. These enzymes are inactive until activated by other enzymes in the small intestine. For example, inactive trypsinogen is activated into trypsin when it contacts the enzyme called enterokinase.

Lightly staining pancreatic islets are scattered among the acini. They are actually small endo-crine glands that release glucagon and insulin. Both these hormones are important in the metabolism of carbohydrates.

Pancreatic Juice

Pancreatic juice is released by nervous and ­endocrine system regulation. It is clear in appearance and con-tains mostly water as well as enzymes, bicarbon-ate ions, and lesser amounts of other electrolytes­. Between 1,200 and 1,500 mL of pancreatic juice is produced every day. The component of pancre-atic juice that is rich in enzymes is produced by the acinar cells. In the smallest pancreatic ducts, the epithelial cells lining them release bicarbonate­ ions, making pancreatic juice alkaline, with a pH near 8.

When gastric juice secretion is stimulated, the ­pancreas is stimulated to release digestive enzymes There is a nearly exact balance between HCl ­production in the stomach and bicarbonate ions production in the pancreas. As bicarbonate is secreted into the pancreatic juice, hydrogen ions enter the blood. The pH of venous blood flowing to the heart is basically unchanged, because acidic blood from the pancreas neutralizes the alkaline blood draining from the stomach.

As acidic chyme enters the duodenum, the pep-tide hormone secretin is released from the duodenal mucous membrane into the bloodstream. Secretin stimulates pancreatic juice with high concentrations of bicarbonate ions to be released, neutralizing acidic chyme. Fats in chyme stimulate cholecystokinin release from the intestinal wall, which travels through the bloodstream to the pancreas. Pancreatic juice then secreted has a high concentration of digestive enzymes.

In the duodenum, enteropeptidase activates trypsinogen to become trypsin. Enteropeptidase is an intestinal brush border protease. Once trypsin is activated, it then activates more trypsinogen as well as the pancreatic proteases known as procarboxy-peptidase and chymotrypsinogen. Once active, they are respectively known as carbodypeptidase and chy-motrypsin. Pancreatic enzymes that are secreted in active form include amylase, lipases, and nucleases. However, these require ions or bile to be present in the intestinal lumen to work effectively. Proelastase is an inert precursor protein of elastase, which is the enzyme that catalyzes the hydrolysis of elastin.

Pancreatic alpha-amylase is a carbohydrase, which is a type of enzyme that breaks down certain types of starches and is nearly identical to salivary amylase. Pancreatic lipase breaks down specific com-plex lipids. It releases fatty acids and other products that are easily absorbed. Nucleases break down DNA and RNA. Proteolytic enzymes break down certain proteins. These enzymes include proteases and pep-tidases. Proteases break down large protein com-plexes. ­Peptidases break down small peptide chains into individual amino acids.

1. Describe the exocrine functions of the pancreas.

2. Explain the actions of pancreatic amylase and pancreatic lipase.

3. Identify the types of fluids found in the pancreatic, cystic, and bile ducts.