Pancreatic Phase and Secretion Quiz

Pancreatic Phase

• The gastric phase accountsfor about 10% of pancreatic secretion.
• The intestinal phase is initiated by entry of food into the stomach, where pepsin and gastric lipases break down proteins and fats.
• Entry of low pH gastric chyme into the duodenum stimulates the intestinal phase.
• In the intestinal phase, pancreatic response is regulated by secretin, CCK, and neural influences.

Pancreatic Secretion

• Secretin is released from S cells in the intestine in response to low pH chyme.
• Secretin stimulates bicarbonate release from pancreatic duct cells.
• CCK stimulates pancreatic enzyme secretion through both neural and hormonal pathways.
• CCK binds to CCK-1 receptors on pancreatic acinar cells to stimulate enzyme release.

Control of Bile Secretion

• Secretin stimulates watery, alkaline bile secretion from bile ducts.
• CCK causes gallbladder contraction and bile flow to the duodenum.
• Secretin expands the volume of bile entering the duodenum by stimulating bicarbonate and water secretion.
• Bile acids facilitate lipid digestion and absorption.

Pancreas

• The pancreas has both exocrine and endocrine functions.
• Acinar cells secrete digestive enzymes.
• Pancreatic duct cells secrete fluid and bicarbonate ions.
• Endocrine cells are assembled in islets.
• The pancreas is innervated by sympathetic and parasympathetic nerves.

Pancreatic Juice

• Pancreatic juice is an isotonic fluid secreted throughout the day.
• The rate of secretion is 0.2-0.3 ml/min without stimulation and 3 ml/min after stimulation.
• The pH of pancreatic juice is 8-9, slightly alkaline.

Pancreatic Secretion Phases

• The cephalic phase accounts for about 20-25% of pancreatic secretion.
• The cephalic phase is initiated by sensory inputs like sight, smell, taste, and long-chain fatty acids.
• The cephalic phase is under the control of the vagus nerve.

Liver

• The liver is the largest gland and solid organ in the body.
• It holds about 13% of the total blood supply.
• About 60% of the liver is made up of hepatocytes.
• The liver can regenerate.

Bile

• Bile is a physiological aqueous solution produced and secreted by the liver.
• It's composed of conjugated bilirubin, bile salts, cholesterol, phospholipids, proteins, ions, and water.
• Bile travels through a series of ducts, joining in the common hepatic duct, and flows into the gallbladder.
• Bile salts are synthesized from cholesterol catabolism.

Bile Secretion

• Bile secretion is modulated by cholangiocytes.
• Secretin enhances bile flow.
• Bombesin, vasoactive intestinal polypeptide (VIP), acetylcholine, somatostatin, gastrin, insulin, and endothelin inhibit bile flow.

Liver

• Largest gland and solid organ in the body
• Weighs approximately 1.8 kg in men and 1.3 kg in women
• Holds around 13% of total blood supply at any given time
• Performs over 500 functions
• Approximately 60% of the liver is composed of liver cells called hepatocytes
• Hepatocytes have an average lifespan of 150 days
• Liver has regenerative capabilities, observed after surgical removal or destruction of a part of the liver

Bile

• Physiological aqueous solution produced and secreted by the liver
• Hepatocytes produce bile by secreting conjugated bilirubin, bile salts, cholesterol, phospholipids, proteins, ions, and water into their canaliculi
• Bile travels through a series of ducts, joining in the common hepatic duct
• Flows through this duct into the gallbladder, where it is concentrated and stored
• Cholesterol catabolism by hepatocytes results in the synthesis of the two major primary bile acids, conjugated by the liver with aminoacid taurine or glycine, known as bile salts

Bile Secretion

• Bile is subsequently modified by ductular epithelial cells as it passes through the biliary tree
• These cells, cholangiocytes, dilute and alkalinize the bile through hormone-regulated absorptive and secretory processes
• Cholangiocytes have receptors that modulate the bicarbonate-rich ductular bile flow, regulated by hormones: secretin, somatostatin, cystic fibrosis transmembrane conductance regulator (CFTR), and chloride-bicarbonate exchanger

Control of Bile Secretion

• Secretin stimulates watery alkaline bile secretion from the bile ducts
• Food in the duodenum causes release of CCK (cholecystokinin), leading to gallbladder contraction and bile flow to the duodenum
• Secretin expands the volume of bile entering the duodenum by stimulating biliary and pancreatic ductular cells to secrete bicarbonate and water in response to the presence of acid in the duodenum
• In the small intestine, bile acids facilitate lipid digestion and absorption; approximately 5% of these bile acids are eventually excreted
• Most bile acids are efficiently reabsorbed from the ileum, secreted into the portal venous system, and returned to the liver through enterohepatic recirculation

Pancreas

• Has both exocrine and endocrine functions
• Acinar cells comprise 75-90% of the gland and release digestive enzymes into ducts that empty into the duodenum
• Pancreatic duct cells secrete fluid and bicarbonate ions, neutralizing the acidity of gastric contents entering the duodenum
• Endocrine cells of the pancreas are assembled in islets scattered throughout the gland
• The pancreas is innervated by sympathetic and parasympathetic nerves

Pancreatic Juice

• Transparent isotonic fluid (1-1.5 l/day)
• Secreted throughout 24 hours
• Rate of secretion: 0.2-0.3 ml/min without stimulation, 3 ml/min after stimulation
• pH of 8-9, slightly alkaline

Pancreatic Secretion

• Approximately 20-25% of total pancreatic exocrine secretion occurs during the cephalic phase
• This phase is initiated by sensory inputs like sight, smell, and taste of food, and long-chain fatty acids with receptors in the oral cavity
• The cephalic phase is controlled by the vagus nerve
• Sensory inputs arising from anticipation of food are integrated in the dorsal vagal complex (located in the brainstem) and transmitted to the exocrine pancreas via the vagus nerve

Gastric Phase

• Accounts for approximately 10% of pancreatic secretion
• Initiated by food entering the stomach, where pepsin and gastric lipases catabolize proteins and fats into peptides and triglycerides and fatty acids
• Salivary amylase continues carbohydrate digestion
• Peptic digests of proteins stimulate the intestinal phase

Intestinal Phase

• Pancreatic response is primarily regulated by the hormones secretin and CCK, and neural influences including the enteropancreatic reflex
• Entry of low pH gastric chyme into the intestine stimulates release of secretin from S cells into the blood
• Secretin's main action is stimulating bicarbonate release from pancreatic duct cells, but also directly affects acinar cells and potentiates enzyme secretion
• CCK stimulates enzyme secretion through neural and hormonal pathways, a key physiologic regulator of intestinal phase pancreatic digestive enzyme secretion

CCK Mechanisms

• CCK binds CCK-1 receptors on pancreatic acinar cells, stimulating enzyme release
• CCK binds CCK-1 receptors on capsaicin-sensitive C-type vagal afferent fibers
• Stimulation of vagal afferent nerves generates a signal sent to the medial nucleus tractus solitarius (NTS) in the brain stem, transmitted via cholinergic postganglionic vagal efferent fibers to the pancreas and other target organs
• Acetylcholine released from the efferent nerve endings binds M3 muscarinic receptors on the pancreatic acinar cells, causing the release of pancreatic enzymes