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Questions and Answers
What is the primary function of centroacinar cells in the pancreas?
What is the primary function of centroacinar cells in the pancreas?
Which duct is responsible for delivering pancreatic secretions into the duodenum?
Which duct is responsible for delivering pancreatic secretions into the duodenum?
Which type of cells comprises the majority of the pancreatic mass?
Which type of cells comprises the majority of the pancreatic mass?
What effect does sympathetic innervation have on pancreatic function?
What effect does sympathetic innervation have on pancreatic function?
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Which hormone is primarily recognized as a regulator of pancreatic growth?
Which hormone is primarily recognized as a regulator of pancreatic growth?
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How much pancreatic juice can the human pancreas secrete in a day?
How much pancreatic juice can the human pancreas secrete in a day?
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What is the composition variation of pancreatic juice primarily dependent on?
What is the composition variation of pancreatic juice primarily dependent on?
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What results from starvation or parenteral feeding concerning pancreatic tissue?
What results from starvation or parenteral feeding concerning pancreatic tissue?
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Which cation concentrations in pancreatic juice remain constant regardless of the secretion rate?
Which cation concentrations in pancreatic juice remain constant regardless of the secretion rate?
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What is a key characteristic of pancreatic juice compared to saliva?
What is a key characteristic of pancreatic juice compared to saliva?
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What distinguishes the activation of proteolytic enzymes in pancreatic juice?
What distinguishes the activation of proteolytic enzymes in pancreatic juice?
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Which statement accurately describes the anion concentrations in pancreatic juice secretion?
Which statement accurately describes the anion concentrations in pancreatic juice secretion?
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Which pancreatic enzyme is secreted in its inactive form and requires activation?
Which pancreatic enzyme is secreted in its inactive form and requires activation?
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What is the primary source of enzymes found in pancreatic juice?
What is the primary source of enzymes found in pancreatic juice?
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Which type of pancreatic enzyme primarily acts on proteins?
Which type of pancreatic enzyme primarily acts on proteins?
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Which small proteins in pancreatic juice play a role in regulating enzyme activity?
Which small proteins in pancreatic juice play a role in regulating enzyme activity?
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Study Notes
Exocrine Function of the Pancreas
- The pancreas is a large compound gland, located parallel and underneath the stomach, with an internal structure similar to the salivary glands.
- Digestive enzymes are secreted by pancreatic acini.
- Centroacinar cells are the initial components of the duct system, draining the acinus. They appear to be the primary sites of electrolyte and water secretion.
- The intercalated ducts converge to form intralobular ducts, which further drain into interlobular ducts. These empty into one of two main ducts that deliver the secretion to the duodenum.
- The major pancreatic duct (duct of Wirsung) enters the duodenum at the ampulla of Vater, alongside the common bile duct, both surrounded by the sphincter of Oddi.
- Acinar cells comprise approximately 80% of the pancreas by volume. Ductule and centroacinar cells make up about 4%.
- Endocrine cells of the pancreas account for about 2% of its mass and are found in islets of Langerhans, distributed throughout the pancreatic parenchyma.
Pancreatic Vascular Supply
- The pancreas receives blood supply from the celiac trunk, splenic artery, and superior mesenteric artery.
- Venous drainage is via the splenic vein, the superior mesenteric vein and the portal vein.
Regulation of Pancreatic Mass
- Starvation or parenteral feeding leads to pancreatic atrophy.
- Cholecystokinin (CCK), a hormone from the intestinal mucosa, is a major regulator of pancreatic growth.
- Exogenous administration of CCK promotes pancreatic growth.
Composition of Pancreatic Juice
- The human pancreas secretes roughly 1 liter of pancreatic juice per day. This contains electrolytes and proteins; concentrations vary with stimulation.
- Major anions are chloride (Cl-) and bicarbonate (HCO3−).
- Major cations are sodium (Na+) and potassium (K+).
- Enzymes represent more than 90% of the proteins in pancreatic juice.
- Electrolytes and enzymes are secreted primarily by ductular and acinar cells, respectively.
Electrolytes
- Pancreatic juice is isotonic with plasma at all secretion rates.
- Water moves freely across pancreatic epithelia due to an osmotic gradient generated by active transport of solutes (mainly electrolytes).
- Cations and anions in the secreted juice are balanced.
Pancreatic Juice Concentrations
- The concentrations of Cl− and HCO3− ions change dramatically with secretion rate.
- The sum of their concentrations remains relatively constant.
Enzymatic Components of Pancreatic Secretion
- Pancreatic juice contains a variety of enzymes required for digesting all foodstuffs.
- All pancreatic enzymes are synthesized and secreted by acinar cells.
- Digestive enzymes are initially secreted as inactive precursors (proenzymes). These zymogens are activated in the duodenal lumen.
- Amylase, ribonuclease, and lipolytic enzymes (except phospholipase A2) are secreted in their active forms.
- Prophospholipase A2 is activated by trypsin in the duodenum.
Pancreatic Zymogens and Activators
- A list of pancreatic enzymes, their zymogen forms, and the enzymes activating them is provided.
Pancreatic Inhibitors
- Pancreatic juice contains trypsin inhibitor, a protein that combines with trypsin in a one-to-one ratio to form an inactive complex.
- This inhibitor prevents premature activation of trypsin within the pancreas. Its concentration is usually lower than the concentration of trypsinogen to prevent that activation.
Colipase
- Colipase is a small polypeptide in pancreatic secretions.
- It enhances pancreatic lipase activity.
- By anchoring the lipase to lipid-water interfaces of oil droplets, it prevents bile salt-induced inhibition.
Minor Pancreatic Proteins
- A minor proportion of pancreatic proteins (<10%) are not involved in digestion.
- IgA (secretory type) and lactoferrin are two examples.
- These proteins may act as antibacterial agents.
Synthesis of Digestive Enzymes
- Describe the steps involved in the synthesis of digestive enzymes within pancreatic acinar cells. Briefly describes the cellular process.
- Zymogen granules contain a full complement of pancreatic enzymes. These enzymes are secreted in a constant proportion
Regulation of Pancreatic Secretion
- Several factors regulate pancreatic secretion : neural and hormonal
- Secretagogues like secretin and CCK are important hormonal regulators of pancreatic secretion.
- Secretin stimulates bicarbonate and water secretion (by duct cells). CCK stimulates enzyme secretion (by acinar cells).
Phases of Pancreatic Secretion
- Cephalic, gastric, and intestinal phases of pancreatic secretion, including stimuli, regulatory pathways, and quantities of enzyme secretion during each phase.
Pancreatic Bicarbonate Secretion
- The pancreas secretes bicarbonate to neutralize acidic chyme entering the duodenum, helping pancreatic enzymes function optimally.
- Bicarbonate secretion is substantial
- Acidic chyme can damage the duodenal mucosa in the absence of bicarbonate.
Pancreatic Enzyme Secretion
- The pancreas secretes an abundance of enzymes in comparison to the ingested nutrients’ digestive requirements. This is needed to ensure proper digestion and absorption of nutrients
- The pancreas produces a large amount of enzymes relative to the required amount to ensure complete hydrolysis of ingested nutrients.
Clinical Correlations (Chronic Pancreatitis)
- Chronic pancreatitis involves progressive loss of exocrine and endocrine tissue because of replacement by fibrous tissue.
- Often associated with alcoholism or biliary tract disease.
- Calcification can appear in extreme cases, observable via x-rays.
- Severe exocrine loss results in malabsorption and weight loss.
- Malabsorption affects proteins, triglycerides, fat-soluble vitamins, and trace elements.
Clinical Correlation (Acute Pancreatitis)
- Acute pancreatitis is a serious and potentially life-threatening condition characterized by autodigestion of the pancreas.
- Biliary tract problems (e.g. gallstones), alcohol abuse, or certain medications are potential causes.
- Alcohol and gallstones are frequently implicated in cases of acute pancreatitis.
Pancreatic Secretions and Blood Flow
- Pancreatic blood flow doubles after a meal due to similar mechanisms as in salivary glands (changes in metabolites, osmolality, kinins).
- This increased blood flow is critical for increased enzyme secretion.
- Increased capillary hydrostatic pressure is induced by arteriolar dilation to increase fluid flow into the ducts and optimize secretions. Increased interstitial oncotic pressure and reduced hydrostatic pressure further encourage secretion.
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