Module 11: Digestive Physiology

Module 11: Digestive PYSO

• Types of Digestion

  • Mechanical Digestion: Physically breaks down food into smaller particles, increasing surface area for enzyme action.
  • Chemical Digestion: Series of hydrolysis reactions breaking down macromolecules into simpler monomers (carried out by enzymes from salivary glands, stomach, pancreas, and small intestine, with pancreas playing a major role).

• Digestive Tract Results

  • Polysaccharides into monosaccharides
  • Proteins into amino acids
  • Fats into monoglycerides and fatty acids
  • Nucleic acids into nucleotides

• General Anatomy - Digestive Tract

  • 30 ft long muscular tube from mouth to anus.
  • Includes the stomach and intestines.

• Accessory Organs

  • Teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
  • Supporting digestion mechanically or chemically.

• Saliva

  • Begins chemical digestion (starch and fat).
  • Dissolves food for taste bud exposure.
  • Moistens food, forming bolus.
  • Mostly water plus enzymes.

• Salivary Amylase

  • Enzyme initiating starch digestion in the mouth
  • Breaks down carbohydrates

• Lingual Lipase

  • Enzyme activated by stomach acid, digesting fats after swallowing.
  • Breaks down triglycerides (lipids)

• Mucus

  • Binds and lubricates food into a bolus.

• Lyzozyme

  • Enzyme killing bacteria.

• Immunoglobulin

  • Fights foodborne pathogens.

• Swallowing (Deglutition)

  • Reflex regulated by medulla and vagus nerve (CNX).

• Bolus

  • Food mass formed by tongue compression.
  • Travels through pharynx and esophagus.
  • Moves down via peristalsis (muscle contractions).
  • Passes through the lower esophageal sphincter into the stomach.

• Stomach

  • Begins chemical digestion of protein and fat.
  • Temporary food storage. -Chyme is a pasty mixture of semi-digested food in the stomach.
  • Contains stomach acid and the enzyme pepsin, initiating the chemical breakdown of protein in the stomach.
  • Most chemical digestion occurs after chyme enters the small intestine.
  • Pyloric sphincter controls chyme passage into the small intestine.
  • Mechanically compresses food due to its muscle layers (longitudinal, circular, and oblique).

• Glands of the Stomach

  • Gastric pits (lined w/ simple columnar epithelium) secrete gastric secretions (water, HCl, pepsin).
  • Pepsin is an enzyme released from glands at gastric pit bottoms.
  • Various pit types exist.
  • Mucous cells coat and liquify bolus to allow enzyme access.
  • Parietal cells produce HCl, intrinsic factor, and ghrelin.
  • Chief cells secrete gastric lipase and pepsinogen.
  • Hydrochloric acid (gastric juice) activates pepsin

• Gastric Glands

  • Predominant gland in stomach
  • Contains chief, parietal, mucous neck, and G cells ( pyloric region).

• Functions of Hydrochloric Acid

  • Activates pepsin
  • Breaks down connective tissues
  • Kills pathogens
  • Inactivates salivary amylase

• Pepsinogen

  • Inactive enzyme precursor
  • Becomes active pepsin in presence of HCl in the stomach.
  • Digests protein in the stomach.

• Gastric Lipase

  • Another enzyme released by chief cells.
  • Has a minor role (10-15%) in digesting fat
  • Most fat digestion happens in the small intestine.

• Digestion and Absorption

  • Salivary and gastric enzymes digest protein/starch/fat partially in the mouth and stomach.
  • Stomach largely doesn't absorb any significant nutrients.
  • Stomach protected via mucous coat, tight junctions, and epithelial cell replacement.

• Appetite

  • Complex study area.
  • Gut-brain peptides (short-term) like ghrelin (from parietal cells in the empty stomach, producing hunger sensation) play a role.
  • Long-term ones include leptin (secreted by adipocytes, regulating appetite).

• Appetite Regulation by Hypothalamus

  • Hypothalamus produces neuropeptide Y (NPY - appetite stimulant).
  • Ghrelin stimulates NPY secretion.
  • Key appetite regulators like insulin, PYY, and leptin inhibit NPY secretion.

• Accessory Digestive Organs - Liver

  • Body's largest gland; major storage site for glucose (glycogen).
  • Produces bile salts for fat digestion.
  • Composed of hepatocytes.

• Hepatocytes -Cuboidal cells around central veins of the liver.

  • Absorb blood glucose, amino acids, vitamins, iron.
  • Remove hormones, toxins, bile pigments, and drugs from blood -Release albumin, lipoproteins, clotting factors, angiotensinogen into the blood between meals.
  • Break down stored glycogen, releasing glucose into blood between meals.

• Gross Anatomy of Gallbladder, Pancreas, and Bile Passages

  • Liver produces bile.
  • Gallbladder stores bile between meals.

• The Gallbladder

  • Stores bile, a yellow-green fluid.
  • Bile contains minerals, cholesterol, neutral fats, phospholipids, bile pigments, and bile salts.
  • Important for fat digestion/absorption (bile salts aid fat digestion).

• Pancreas

  • Both exocrine and endocrine gland.
  • Endocrine portion (pancreatic islet cells / islets of Langerhans) releases insulin and glucagon (alpha cells release glucagon, beta cells release insulin).
  • Endocrine portion secretes pancreatic juice (99% of pancreas), an alkaline mixture.
  • This juice contains water, enzymes, zymogens, sodium bicarbonate, and electrolytes.
  • Important for neutralizing stomach acid.

• Pancreatic Zymogens

  • Inactive forms of enzymes.
  • Activated in the small intestine.
  • Examples include Trypsinogen (converted to trypsin by enterokinase, a brush border enzyme), Chymotrypsinogen, Procarboxypeptidase.

• Other Pancreatic Enzymes

  • Include pancreatic amylase (digests starch), pancreatic lipase (digests fats), ribonuclease and deoxyribonuclease (digests RNA and DNA).

• Duodenum

  • Receives chyme, bile, and pancreatic juice.
  • Bile breaks down fats.
  • Pepsin is inactivated.
  • Pancreatic enzymes take over chemical digestion.

• Jejunum

  • Rich blood supply leading to red appearance.
  • Thick muscular walls.
  • Principal site for most digestion and nutrient absorption.

• Ileum

  • Last segment of small intestine, connecting to large intestine at ileocecal junction/ileocecal valve.
  • Ileocecal valve regulates passage of digested food residue into the large intestine.
  • Intestinal villi and lacteals absorb lipids in the ileum lumen.

• Intestinal Motility

  • Segmentation and peristalsis (mixing and movement).

• Carbohydrate Digestion

  • Starch (chain of glucose monomers) is the most digestible carbohydrate.
  • Process begins in the mouth with salivary amylase beginning the hydrolysis of starch into oligosaccharides, then in the duodenum pancreatic amylase converts starch into oligosaccharides and maltose (within 10 minutes).
  • Glucose is absorbed by the absorptive cells of the small intestine via brush border enzymes. (e.g., maltase).

• Protein Digestion

  • Proteases (enzymes that digest proteins) begin protein digestion in the stomach.
  • Pepsin (activated in the stomach) hydrolyzes peptide bonds between tyrosine and phenylalanine in proteins to break them down.
  • Other proteases continue protein digestion process further in the small intestine.

Explore the intricacies of digestion in Module 11, focusing on both mechanical and chemical processes. Understand how food is broken down into essential nutrients and learn about the anatomy of the digestive tract and the role of accessory organs. This quiz will help solidify your knowledge on how digestion functions overall.