Gastric Glands and G Cells

• Gastric glands contain enteroendocrine cells
• Enteroendocrine cells are unicellular glands found in the epithelium
• These cells secrete hormones and paracrine factors

G cells

• G cells are a type of enteroendocrine cell
• G cells secrete gastrin
• G cells are primarily located in the pyloric glands and duodenum

Gastrin

• Gastrin is a hormone produced by G cells.
• Gastrin ramps up digestive processes
• Gastrin increases stomach and intestinal motility
• Gastrin relaxes the ileocecal valve

Gastrin's Effects

• Gastrin stimulates chief cells to secrete pepsinogen
• Gastrin stimulates parietal cells to secrete hydrochloric acid (HCl)
• Gastrin stimulates ECL cells to secrete histamine
  • ECL cells include enterochromaffin-like cells and enteroendocrine cells
• Gastrin causes parietal cells to produce more HCl

Acid-Reducing Drugs: H2 Blockers

• Examples of H2 blockers include Pepcid, Tagamet, Axid, and Zantac
• H2 blockers bind to H2 receptors
• H2 receptors are for histamine and located on parietal cells

Acid-Reducing Drugs: PPIs

• PPIs are proton pump inhibitors
• Examples of PPIs include Nexium, Prevacid, Omeprazole, and Prilosec
• PPIs inhibit the H+/K+ ATPase pump

Cephalic Phase

• Cephalic phase is parasympathetic, promoting "rest and digest"
• Information from the environment stimulates G cells

Gastric Phase

• Gastric phase involves responses to information from the stomach
• Gastric phase includes motility
• Gastrin stimulates HCl production
• Gastrin stimulates enzyme production

Intestinal Phase

• The presence of a little chyme triggers the intestinal phase
• Duodenum is part of the instestinal phase
• The stomach is temporarily inhibited during the intestinal phase
• Digestion occurs in the duodenum
• The stomach is then reactivated

Intestinal Phase: Gastrin Release

• Initial brief release of gastrin occurs
• Release of gastrin is by the duodenum
• Brief increase in gastric activity

Intestinal Phase: Inhibiting Gastric Activity

• The enterogastric reflex is initiated in the duodenum
• Stretch receptors, osmoreceptors, and chemoreceptors are involved
• CNS response includes increased sympathetic and decreased parasympathetic activity
• Results in decreased gastric motility and secretion

Intestinal Phase: Hormones

• Hormones from the duodenal enteroendocrine cells are released
• Secretin and CCK are released
• Secretin and CCK delay gastric emptying
• Secretin and CCK inhibit gastric secretion
• CCK stands for cholecystokinin

Biliary Tree

• The biliary system has many branches inside the liver
• The common bile duct is also known as the bile duct

Common Bile Duct

• The hepatopancreatic ampulla controls the flow of bile and pancreatic juices
• Old names for the hepatopancreatic ampulla include Ampulla of Vater and Sphincter of Oddi

Liver Functions

• Liver filters and processes nutrient-rich blood
  • Liver detoxifies, stores glucose as glycogen, and stores vitamins
• Liver processes hemoglobin and stores iron
• Liver performs metabolism, such as converting NH3 to urea
• Liver produces plasma proteins and bile constantly

Gall Bladder

• Stores and concentrates bile by absorbing water
• The gall bladder can contract

Liver Gross Anatomy

• The liver has 4 primary lobes: right, left, caudate, and quadrate

Liver Micro-Anatomy

• Lobules are the functional units
  • Central vein
• There is a network of vessels and bile ducts

Liver Cells and Structures

• Hepatocytes comprise liver cells
• The portal triad consists of the portal venule
  • branch of hepatic portal vein
  • blood from Gl tract and hepatic arteriole
  • branch of hepatic artery proper, and bile duct

Liver Sinusoids and Canals

• Sinusoids are a part of the liver
• Bile canaliculi are little channels for bile

Kupffer Cells

• Kupffer cells are hepatic macrophages

Circulation in Liver Lobule

• Blood flow and bile flow occur in opposite directions
• Blood goes to the inferior vena cava
• Blood goes to Hepatic veins
• Blood comes from the celiac trunk

Bile Composition

• Water constitutes the majority of bile
  • Liver bile is ~97% water
• Gallbladder bile is ~85% water
• Electrolytes, bile pigments (like bilirubin), bile salts, and lecithin are also in bile

Bile salts

• Bile salts are made from cholesterol and are amphipathic
• Bile salts aid emulsification and form micelles
  • Fats are broken down into smaller droplets

Bile salts: Emulsification process

• Emulsification is the process of breaking down fats into smaller droplets and forming micelles
• This process increases surface area for digestion

Enterohepatic Circulation of Bile Salts

• Bile salts are reabsorbed and reused
• About 5% of bile salts are excreted in feces

Bile Pigments

• Bile pigments, like bilirubin, are excreted
  • Bilirubin is converted to stercobilin, which gives stool brown color
  • This process is carried out by intestinal bacteria
• Lack of bile results in white/grey, fatty feces

Pancreas

• The pancreas has endocrine functions, producing insulin and glucagon
• Is has digestive functions, producing digestive enzymes and bicarbonate
• The pancreas consists of a head, neck, body, and tail

Pacreas: Micro-anatomy

• Acinar glands are composed of acini -Acinar cells secrete digestive enzymes
• Duct cells secrete HCO3-

Pancreatic Enzymes

• Some pancreatic enzymes are secreted as zymogens
• Some pancreatic enzymes are active enzymes, such as:
• Pancreatic amylase - digests Starch
• Pancreatic lipase - digests Triglycerides
• Ribonuclease - digests RNA
• Deoxyribonuclease - digests DNA

Pancreas Zymogens

• Proteases are secreted as zymogens
  • Trypsinogen turns into trypsin.
  • Proelastase turns into elastase.
  • Chymotrypsinogen turns into chymotrypsin
• Proteins are broken down into peptides
• Procarboxypeptidase turns into carboxypeptidase
  • Which Removes AAs from the carboxy end of peptides

Activation of Zymogens

• Propeptide cleavage occurs in the duodenum

• Enteropeptidase helps with activation

• Enteropeptidase, also known as enterokinase, is a duodenal cell transmembrane protein

• Trypsinogen is converted to its active form, trypsin

• Chymotrypsinogen converts to chymotrypsin

• Proelastase converts to elastase

• Procarboxypeptidase converts to carboxypeptidase