BIOL 221 Pancreas: Endocrine & Exocrine Functions

• BIOL 221 covers aspects of the digestive system.

Pancreas Overview

• The pancreas is approximately 12.5 cm long.
• It is situated posterior to the stomach's greater curvature.
• The pancreas connects to the duodenum via two ducts.
• The hepatopancreatic ampulla is formed where the larger duct merges with the common bile duct.
• The smaller, accessory pancreatic duct directly enters the duodenum.

Endocrine and Exocrine Functions of Pancreas

• The endocrine portion makes up only 1% of the pancreas and consists of Islets of Langerhans cells.
• These cells secrete hormones directly into the bloodstream, including:
  • Glucagon: Stimulates the liver to convert glycogen into glucose.
  • Insulin: Promotes glucose uptake by organs and its conversion to glycogen.
  • Somatostatin: An inhibitory hormone that suppresses pituitary and gastrointestinal hormone release.
• The exocrine pancreas, is made of clusters of cells called acini.
• The pancreas produces roughly 1200ml of fluid daily that contains water, salts, NaHCO3, and enzymes.
• NaHCO3 creates an alkaline pH that inactivates pepsin, which is optimal for pancreatic enzyme function.

Proteolytic Pancreatic Enzymes

• The secreted enzymes include trypsinogen, chymotrypsinogen, and procarboxypeptidase.
• These enzymes break down the peptide bonds in proteins, resulting in smaller peptides.
• Protein-digesting enzymes are secreted in an inactive form.
• Intestinal enterokinase activates trypsinogen through proteolytic cleavage, subsequently activating chymotrypsinogen and procarboxypeptidase.

Other Pancreatic Enzymes

• Amylase: Breaks down starch into sugars
• Pancreatic lipase: Digests lipids by splitting triglycerides into monoglycerides and fatty acids
• Deoxyribonucleases and Ribonucleases: Cleave the phosphodiester bonds of DNA and RNA, releasing nucleotides

Regulation of Pancreatic Secretion

• Nervous regulation involves the parasympathetic fibers of the vagus nerve.
• Hormonal regulation has two mechanisms:
  • Duodenal pH below 4.5 stimulates the small intestine to release secretin.
    • Secretin prompts the pancreas to release a buffered HCO3- solution and stimulates the liver to secrete HCO3- into the bile.
  • Fat and protein in chyme stimulate the small intestine to release cholecystokinin.
    • Cholecystokinin stimulates the pancreas to release enzymes and relaxes the sphincter of Oddi, facilitating the release of bile and pancreatic juices.

Pancreatitis

• Enzymes accumulate and trypsinogen can become activated if the pancreatic duct becomes blocked.
• This leads to autodigestion of pancreatic cells, causing acute pancreatitis.
• Pancreatitis is most common in alcohol abusers and in those with gallstones or pancreatic tumors.

Small Intestine Structure and Function

• Completes digestion.
• Absorbs nutrients.
• Transports indigestible products to the large intestine.
• Its structure consists of the duodenum, jejunum, and ileum.

Regional Variations of the Small Intestine

• The duodenum contains Brunner's glands in the submucosa that secrete alkaline mucus, a feature unique among GI tract regions other than the esophagus.
• The jejunum has a thicker, more vascular wall and is more active than the ileum.
• The ileum features Peyer's patches, concentrations of lymphoid tissue involved in immune response, containing B-lymphocytes, T-lymphocytes, and macrophages.

Adaptations for Increased Absorption

• The small intestine increases its absorptive area through three adaptations:
  • Plicae circulares: Circular folds involving the mucosa and submucosa that increase the surface area 2x.
  • Villi: Finger-like projections covered with simple columnar epithelium, with a core of lamina propria (connective tissue), increasing the surface area 20x.
  • Microvilli: Extensions of the cell surface that absorb nutrients, increasing the surface area 10x.

Small Intestine Secretions

• Alkaline mucus is secreted by:
  • Goblet cells: Occur throughout the small intestine.
  • Brunner's glands: Found in the duodenum's submucosa, they secrete viscous mucus which protects the duodenal wall from digestion by gastric juices.
• Crypts of Lieberkuhn: Secrete a watery fluid neutral in pH, without enzymes that villi rapidly reabsorb, along with digestive products.
• Brush border enzymes are digestive enzymes:
  • Intestinal lipase that cleaves triglycerides into monoglycerides and fatty acids.
  • Aminopeptidases that cleave amino acids from peptides.
  • Sucrase, lactase, and maltase:
    • Which break down sucrose into glucose and fructose.
    • Lactose into glucose and galactose.
    • Maltose into 2 glucose molecules.

Regulation of Small Intestine Secretions

• Parasympathetic innervation stimulates secretion.
• Mechanical stimulation and irritants stimulate mucus release.

Fluid Absorption in the Small Intestine

• Approximately 9.3L of fluid enters the small intestine daily, with 2.3L ingested and 7.0L secreted.
• The small intestine absorbs about 8.3L of fluid.
• Some fluid, 0.5-1L, enters the large intestine, where most of it is absorbed.
• A small amount is excreted in the feces.

Absorption of Nutrients

• Monosaccharides, amino acids, and short-chain fatty acids are absorbed by villi.
• Fructose: absorbed via facilitated diffusion
• Glucose and galactose: absorbed via active co-transport using ATP with Na+.
• Amino acids: absorbed via active transport and co-transport.
• Short-chain fatty acids: absorbed via simple diffusion.
• The nutrients then move out of the cells via diffusion/facilitated diffusion into capillaries.

Fat Digestion

• Bile salts break down fat globules into tiny droplets through emulsification.
• Emulsification speeds up fat digestion.
• Lipase hydrolyzes triglycerides into a monoglyceride and two free fatty acids.
• Monoglycerides and fatty acids diffuse into epithelial cells of the small intestine to reassemble into triglycerides.
• The triglycerides are combined with phospholipids and cholesterol and surrounded by protein coat to form chylomicrons.
• Chylomicrons exit the cells via exocytosis and enter the lymphatic lacteals in the submucosa, eventually reaching the bloodstream.

Absorption Notes

• Newborns can absorb substantial amounts of undigested proteins, including antibodies from mother's milk.
• Adults typically break down and absorb proteins as amino acids.
• Foreign food proteins are antigenic and do not enter the blood. However, Clostridium botulinum's heat-resistant toxin is an exception, which interferes with muscle function through neuromuscular communication, potentially causing respiratory failure.

Malabsorption in the Small Intestine

• Symptoms can include diarrhea, weight loss, weakness, vitamin deficiencies, anemia, and demineralization of bones.
• Causes may include:
  • Surgical removal of portion of the small intestine.
  • Obstruction of lacteals by tumor.
  • Decreased bile production due to liver disease.
  • Celiac disease: Gluten causes destruction of microvilli and entire villi.
  • Stress.