Digestive System Notes

Liver

• Largest gland in the body.

• Organized into functional units called lobules.

• Contains hepatocytes.

• Bile duct collects bile from bile canaliculi between sheets of hepatocytes for secretion from the liver via hepatic ducts.

• Functions:

  • Carbohydrate metabolism: Converts glucose to glycogen (glycogenesis).

  • Lipid metabolism:

    • Stores some triglycerides.

    • Breaks down fatty acids to generate ATP.

    • Emulsification.

    • Synthesizes lipoproteins.

    • Synthesizes cholesterol.

    • Uses cholesterol to make bile salts.

  • Protein metabolism: Deaminates amino acids so they can be used for ATP production.

  • Excretion of bilirubin.

  • Synthesis of bile salts: Used in the small intestine for emulsification and absorption of lipids.

Gallbladder

• Stores and concentrates bile.

• The liver produces 250–1,500 ml of bile per day.

• Bile Composition:

  • Bile pigments (bilirubin).

  • Bile salts.

  • Phospholipids (lecithin).

  • Cholesterol.

  • Inorganic ions.

• Some molecules released into the bile are reabsorbed in the small intestine.

• Bile salts are returned to the liver from the ileum as part of enterohepatic circulation.

• About 90%-95% of bile salts are absorbed by the ileum and returned to the liver.

• Enterohepatic circulation: Cycle of bile salt secretion by hepatocytes into bile, absorption by the ileum, and resecretion into bile.

Small Intestine

• Most digestion and absorption of nutrients occur here.

• Secretes intestinal juice (water, mucus, and ions).

• Receives pancreatic juice enzymes and bile to aid digestion.

• Contains brush border enzymes attached to the villi of cells, which are vital for the final stages of digestion.

• Mechanical digestion: Involves segmentation in the duodenum.

• Chemical digestion: Occurs as chyme mixes with pancreatic juice, intestinal juice, and bile.

• Surface area is increased by circular folds, villi, and microvilli to enhance absorption.

• Surface area is approximately 200 m^2, about the size of a tennis court.

• Simple columnar epithelium with goblet cells and absorptive cells.

• Core of lamina propria containing a blood capillary network and lacteal.

• Nutrients absorbed by epithelial cells pass through the wall of a blood capillary or lacteal to enter the blood or lymph, respectively.

• Microvilli form a fuzzy line called the brush border.

Brush Border of Small Intestine

• Brush border enzymes are attached to the microvilli of small intestinal absorptive cells.

• Some enzymatic digestion occurs at the surface of the absorptive cells lining the villi.

• Absorptive cells slough off into the lumen and release enzymes that help digest nutrients in the chyme.

• Monomers for carbohydrates and proteins are formed with the help of brush border enzymes.

Absorption by Small Intestine

• Capillaries absorb monosaccharides and amino acids, while lacteals absorb chylomicrons (fatty acids and monoglycerides).

Digestion and Absorption of Carbohydrates (CHO)

• Starch digestion begins in the mouth with salivary amylase but stops in the stomach.

• 50% of dietary starch is digested before it reaches the small intestine.

• Continues in the small intestine with pancreatic amylase, breaking complex carbohydrates into oligosaccharides.

• Brush border enzymes finish breaking down resulting products and other disaccharides (maltase, sucrase, lactase) in the small intestine.

• Monosaccharides are absorbed across the epithelium into capillaries.

Digestion and Absorption of Proteins

• Begins in the stomach with pepsin, producing short-chain polypeptides (pH 1.5 - 3.5).

• Pancreatic digestive enzymes continue breakdown in the duodenum.

• Some brush border enzymes cleave peptide bonds within the interior of a protein; others remove amino acids from the ends of a peptide.

• Finishes in the duodenum and jejunum.

• Free amino acids are absorbed into capillaries.

Digestion of Fats

• Fat digestion begins in the stomach with lingual lipase and gastric lipase.

• Fat digestion continues in the duodenum when bile emulsifies the fat into emulsified fat droplets.

• Pancreatic lipase acts on the smaller fat droplets to form fatty acids and monoglycerides in the duodenum.

• These move into micelles and are transported to the brush border.

Absorption of Fats

• Inside the absorptive cells, fatty acids and monoglycerides are regenerated into triglycerides, cholesterol, and phospholipids and combined with proteins to form chylomicrons.

• Chylomicrons go through exocytosis into interstitial fluid then diffuse into lacteals.

Digestion and Absorption Summary (CHO, PRO, Triglycerides)

• CHO

  • Mouth: Salivary amylase digests 50% into oligosaccharides and disaccharides.

  • Small Intestine:

    • Pancreatic amylase digests remaining 50% into oligosaccharides and disaccharides.

    • Brush border enzymes further break these down into monosaccharides, which are then absorbed into the absorptive cells via secondary active transport with Na^+ (for glucose and galactose) or facilitated diffusion (for fructose).

• PRO

  • Stomach: Pepsin breaks down proteins into short-chain peptides.

  • Small Intestine:

    • Pancreatic enzymes continue to break down peptides.

    • Brush border enzymes further digest into amino acids, dipeptides, and tripeptides.

    • Amino acids are absorbed into the absorptive cells via secondary active transport with Na^+, while dipeptides and tripeptides are absorbed via secondary active transport with H^+.

• Triglycerides

  • Stomach: Activated lingual lipase and gastric lipase break down triglycerides into short-chain fatty acids and monoglycerides.

  • Small Intestine:

    • Bile emulsifies fats into smaller fat droplets.

    • Pancreatic lipase acts on these droplets to form short-chain fatty acids, long-chain fatty acids, and monoglycerides.

    • Micelles transport these contents to the brush border where they are absorbed into the absorptive cells via simple diffusion.

    • Inside the absorptive cells, they are reformed into triglycerides, cholesterol, and phospholipids and coated with proteins to form chylomicrons, which enter the lacteals.

Summary of Digestive Enzymes

• Saliva

  • Salivary Amylase: Digests starches into maltose, maltotriose, and α-dextrins.

  • Lingual Lipase: Digests triglycerides into fatty acids and diglycerides.

• Gastric Juice

  • Pepsin: Digests proteins into peptides.

  • Gastric Lipase: Digests triglycerides into fatty acids and monoglycerides.

• Pancreatic Juice

  • Pancreatic Amylase: Digests starches into maltose, maltotriose, and α-dextrins.

  • Trypsin, Chymotrypsin, Elastase: Digest proteins into peptides.

  • Carboxypeptidase: Digests peptides into amino acids at the carboxyl-terminal end.

  • Pancreatic Lipase: Digests triglycerides into fatty acids and monoglycerides.

  • Ribonuclease: Digests ribonucleic acid into nucleotides.

  • Deoxyribonuclease: Digests deoxyribonucleic acid into nucleotides.

• Brush Border

  • Enterokinase: Converts trypsinogen to trypsin.

  • α-Dextrinase: Digests α-dextrins into glucose.

  • Sucrase: Digests sucrose into glucose and fructose.

  • Lactase: Digests lactose into glucose and galactose.

  • Maltase: Digests maltose into glucose.

  • Aminopeptidase: Digests peptides into amino acids at the amino-terminal end.

  • Nucleosidases and Phosphatases: Digest nucleotides into nitrogenous bases, pentose sugars, and phosphates.

Large Intestine

• Terminal part of the GI tract involved in water and ion absorption, where feces are formed and released.

• Mechanical digestion involves haustral churning, peristalsis, and mass movement.

• Chemical digestion occurs via bacteria.

• Chyme from the ileum passes through the ileocecal valve into the cecum.

• Concentrates feces by absorbing water, electrolytes, vitamin K, and some B vitamins.

• Produces vitamin K and B vitamins via microbial organisms.

• Defecation reflex expels feces from the body, assisted by the Valsalva maneuver (voluntary contractions of abdominal muscles).

Nervous System Control of Digestion

• Enteric nervous system includes the myenteric plexus (controls smooth muscle) and submucosal plexus (controls glands and endocrine cells).

• Autonomic nervous system regulation:

  • Vagus (X) nerve stimulates peristalsis and secretion (parasympathetic).

  • Contraction of sphincters and inhibition of peristalsis and secretion (sympathetic).