Pancreas and Bile Secretion

Pancreas

• Located mostly retroperitoneally, deep to stomach's greater curvature; head encircled by duodenum, tail near spleen
• Exocrine function: secretes pancreatic juice via ducts
  • Acini: clusters of secretory cells producing zymogen granules (inactive digestive enzymes)
• Endocrine function: secretes insulin and glucagon via pancreatic islet cells
• Pancreatic juice composition: watery, alkaline (pH 8) to neutralize stomach acid
  • Electrolytes (primarily HCO₃⁻) and enzymes
    • Proteases (e.g., trypsin): inactive form to prevent self-digestion
    • Amylase (carbohydrates)
    • Lipases (lipids)
    • Nucleases (nucleic acids)

Bile and Pancreatic Secretion

• Bile duct and pancreatic duct unite in duodenum wall forming hepatopancreatic ampulla
• Ampulla opens into duodenum via major duodenal papilla
• Hepatopancreatic sphincter controls bile and pancreatic juice entry
• Regulation: neural and hormonal controls
  • Hormones: cholecystokinin (CCK) and secretin
  • Bile secretion increased by:
    • Enterohepatic circulation returning bile salts
    • Responses to acidic (secretin) and fatty (CCK) chyme by duodenal mucosa cells
  • Hepatopancreatic sphincter closed except during digestion; bile stored in gallbladder

Liver, Gallbladder, and Pancreas Relationship

• Liver's hepatic ducts merge to form common hepatic duct, which joins bile duct
• Gallbladder connects to bile duct via cystic duct
• Pancreas' main pancreatic duct joins bile duct at ampulla
• All structures empty into duodenum at major duodenal papilla

Small Intestine

• Major organ of digestion and absorption (7-13 ft)
• Subdivisions:
  • Duodenum (~10 in): retroperitoneal, curves around pancreas
  • Jejunum (~8 ft): attached posteriorly by mesentery
  • Ileum (~12 ft): attached posteriorly by mesentery, joins large intestine at ileocecal valve
• Structural modifications for absorption:
  • Circular folds: permanent folds increasing surface area (600x)
  • Villi: finger-like projections with capillary beds and lacteals (lymphatic capillaries)
  • Microvilli: cytoplasmic extensions forming brush border with brush border enzymes for final digestion

Small Intestine Histology

• Mucosa and submucosa modifications reflect digestive function
• Intestinal crypts: tubular glands between villi
  • Enterocytes: absorb nutrients and electrolytes with brush border enzymes
  • Goblet cells: secrete mucus
  • Enteroendocrine cells: secrete enterogastrones (e.g., CCK, secretin)
  • Paneth cells: secrete antimicrobial agents
• MALT: protects against microorganisms; Peyer's patches in ileum
• Duodenal glands (submucosa): secrete alkaline mucus to neutralize chyme

Intestinal Juice

• 1-2 L secreted daily in response to distension/irritation
• Composed of water, mucus (duodenal glands and goblet cells)
• Slightly alkaline and isotonic with blood plasma

Digestive Processes in Small Intestine

• Chyme (partially digested) takes 3-6 hours in small intestine for nutrient/water absorption
• Enzymes: imported from liver and pancreas; brush-border enzymes finalize digestion
• Chyme entry regulation: slow entry to prevent osmotic water loss; adjust pH and mix with digestive fluids; controlled by enterogastric reflexes and hormones
• Motility: segmentation (mixes) and peristaltic waves (propulsion)
  • Segmentation most common after meal
  • Peristaltic waves occur every 90-120 minutes; move remnants/bacteria
  • Ileocecal valve controls chyme entry into large intestine

Large Intestine

• Unique features: teniae coli (muscle bands), haustra (pockets)
• Subdivisions: cecum, appendix (lymphoid tissue), colon (ascending, transverse, descending, sigmoid), rectum, anal canal (internal and external sphincters)
• Relationship to peritoneum: some retroperitoneal segments (cecum, parts of colon, and rectum), intraperitoneal segments attached by mesocolons

Appendicitis

• Acute inflammation; usually from blockage (feces) trapping bacteria
• Most common in adolescence
• Symptoms: pain (umbilical area to lower right quadrant), loss of appetite, nausea, vomiting
• Treatment: appendectomy

Large Intestine Histology

• Thicker mucosa (simple columnar epithelium except anal canal; stratified squamous)
• Lacks circular folds, villi, and digestive secretions
• Abundant mucus-producing goblet cells

Bacterial Flora

• 1000+ different types; outnumber human cells 10 to 1
• Enter from small intestine/anus and colonize
• Metabolic functions: fermentation yielding short-chain fatty acids (used for fuel), vitamin synthesis (B complex, K)
• Beneficial bacteria suppress pathogens
• Evidence suggests gut bacteria impact body weight, mood, and disease susceptibility

Digestive Processes in Large Intestine

• Residue remains 12-24 hours
• Absorbs vitamins from bacterial flora, water and electrolytes (NaCl)
• Functions: propel feces to anus; defecation
• Motility: mass movements (3-4 times/day) driven by gastrocolic reflex and fiber
  • Descending and sigmoid colon act as storage reservoir
  • Defecation reflex:
    • Parasympathetic signals, relaxation of internal anal sphincter
    • Conscious control: relaxation of external sphincter
    • Valsalva's maneuver assists

Stool Imbalances

• Diarrhea: rapid transit, insufficient water absorption (e.g., infection, jarring) leading to dehydration and electrolyte imbalance
• Constipation: extended colon transit time, causing hard stool (e.g., insufficient fiber/fluid, immobility)

Mechanisms of Digestion and Absorption

• Digestion: breakdown of food to absorbable molecules. Enzymes (intrinsic/accessory) hydrolyze macromolecules
• Absorption: movement of molecules from lumen to body. Tight junctions ensure passage through cells; lipids passively; other substances by transport
• Most digestion and absorption in small intestine

Digestion of Carbohydrates

• Only monosaccharides are absorbed
• Starch and disaccharides broken down to monosaccharides (glucose, fructose, galactose)
  • Salivary amylase initiates starch digestion
  • Pancreatic amylase further digests starch
  • Brush border enzymes (duodenum) finalize digestion

Digestion of Proteins

• Dietary, enzymatic, and mucosal proteins broken down to amino acids (dipeptides/tripeptides) via pepsin (stomach)
  • Pancreatic proteases cleave to small peptides
  • Brush border enzymes further digest to amino acids

Digestion of Lipids

• Emulsification (bile salts) breaks large fat globules
• Pancreatic lipase digests triglycerides to fatty acids and monoglycerides
• Micelle formation: bile salts coat digestion products for absorption along enterocytes
• Diffusion: lipids leave micelles, re-form triglycerides, enter lymphatic lacteals

Digestion of Nucleic Acids

• Pancreatic nucleases hydrolyze DNA/RNA to nucleotides
• Brush border enzymes break nucleotides into components

Absorption Imbalances

• Lactose intolerance: deficient lactase, lactose remains undigested, osmotic diarrhea and gas
• Malabsorption: impaired nutrient absorption (bacterial infection, antibiotics, celiac disease). Gluten-sensitive enteropathy (celiac disease) due to immune response to gluten

Absorption of Vitamins, Electrolytes, and Water

• Fat-soluble vitamins absorbed via micelles (diffusion)
• Water-soluble vitamins absorbed via active/passive transport
• Vitamin B12 absorbed via intrinsic factor and endocytosis
• Electrolyte absorption (active transport), related to need (e.g. iron, calcium)
• Calcium absorption influenced by vitamin D and PTH
• Water (9 L) absorption mostly in small intestine

Developmental Aspects

• Alimentary canal forms by week 5
• Fetal nutrition via placenta, but GI stimulated by amniotic fluid
• Infants progress to solid foods
• Aging: decline in GI activity, absorption, motility; taste/smell less acute; risk of periodontal disease, fecal incontinence and GI cancers