Digestive System – Comprehensive Exam Notes

Digestive Processes – Overview

• Primary goal of the digestive system: to move essential nutrients into the body’s internal environment so that every cell has access to them.
• Five core mechanisms work together:
  • Ingestion – entry of food into the mouth.
  • Digestion – mechanical + chemical breakdown of food.
  • Motility – muscular contractions that mix & propel.
  • Secretion – release of water, mucus, enzymes, acids, bile, electrolytes.
  • Absorption – transfer of digested nutrients to blood/lymph.
  • Elimination – expulsion of indigestible residues as feces.

Mechanical Digestion

• Converts large food particles → minute particles to expose more surface area for enzymes.
• Three functional outcomes:
  • Churning to mix with digestive juices → facilitates chemical digestion.
  • Mixing ensures contact with intestinal mucosa → promotes absorption.
  • Propulsion moves material along the tract → supports elimination.
• Mastication (chewing): first mechanical step; performed voluntarily by teeth, tongue, cheeks.

Deglutition (Swallowing)

• Complex, coordinated sequence moving bolus mouth→stomach; integrated by cranial nerves V, IX, X, XII.
• Oral Stage (voluntary):
  • Tongue moves bolus posteriorly → oropharynx.
• Pharyngeal Stage (involuntary):
  • Soft-palate seals nasopharynx.
  • Larynx elevates; epiglottis covers glottis.
  • Constrictor contraction + gravity move bolus → esophagus.
• Esophageal Stage (involuntary):
  • Bolus propelled by peristaltic wave ↓ esophagus → lower esophageal sphincter (LES) → stomach.
• Pressure profile (manometry) shows sequential high-pressure zones: UES, mid-esophagus, LES.

GI Motility: Peristalsis & Segmentation

• Peristalsis:
  • Wavelike ring of contraction just behind bolus + relaxation ahead.
  • Propels content one direction.
• Segmentation:
  • Non-propulsive, mixing contractions.
  • Alternate ring contractions “chop” & mix chyme; forwards-then-backwards motion.
• Often alternate to move + mix simultaneously.

Regulation of Motility & Gastric Emptying

• Stomach motility produces propulsion ↔ retropulsion every ~20\ \text{s} → forms chyme.
• Gastric emptying time: 2\text{–}6\ \text{h} depending on volume & composition.
• Controlled by:
  • Hormonal (gastrin ↑; secretin & CCK ↓ emptying).
  • Neural (enteric & autonomic reflexes, vagus).
• Small-intestinal transit: ~5\ \text{h} mouth of duodenum → ileocecal valve; peristaltic rate rises near jejunal end.

Chemical Digestion & Digestive Enzymes

• Chemical change mainly by hydrolysis reactions.
• Enzymes: extracellular protein catalysts acting within GI lumen.
  • Specificity – act on one substrate or bond type.
  • Optimal pH requirement (e.g., pepsin best at \text{pH}\,1–2; trypsin best at \text{pH}\,7–8).
  • Reversible catalysis (forward & backward).
  • Continuously degraded → must be resynthesized.
  • Mostly released as inactive proenzymes/zymogens to prevent autodigestion.

Carbohydrate Digestion

• Carbs = saccharides.
• Polysaccharides (starch/glycogen) → via salivary amylase (mouth) & pancreatic amylase (duodenum) → disaccharides.
• Brush-border enzymes on enterocyte microvilli complete process:
  • Sucrase: sucrose → glucose + fructose.
  • Lactase: lactose → glucose + galactose.
  • Maltase: maltose → 2 glucose.
• Monosaccharides mainly absorbed in upper small intestine by sodium-glucose cotransport (SGLT) or facilitated diffusion (GLUT-5 for fructose).

Protein Digestion

• No proteolysis in mouth.
• Stomach: chief-cell pepsin (activated by low pH) hydrolyzes peptide bonds, producing proteoses/peptones.
• Small intestine:
  • Pancreatic trypsin, chymotrypsin, elastase, carboxypeptidase continue breakdown → oligopeptides.
  • Brush-border & cytosolic peptidases → free amino acids, di- & tri-peptides.
• Absorption:
  • Amino acids by Na\^+-coupled secondary active transport.
  • Small peptides by H\^+-linked cotransport → intracellular peptidases finish hydrolysis.

Fat Digestion & Emulsification

• Large fat globules (hydrophobic) must be emulsified → smaller droplets.
  • Bile salts + lecithin coat droplets (hydrophobic tails toward lipid, hydrophilic heads outward) → micelles.
• Pancreatic lipase + colipase hydrolyze triglycerides:
  • \text{TG} \xrightarrow[\text{lipase}]{} \text{2 FFA} + \text{monoglyceride}.
• Products + cholesterol + fat-soluble vitamins are packed into micelles → diffuse through enterocyte membrane.
• Inside cells: re-esterified → chylomicrons → lacteals → thoracic duct → bloodstream.
• Undigestible residues form component of feces.

Digestive Secretions

• Saliva (parotid, submandibular, sublingual):
  • Water & mucus lubricate food; amylase begins starch digestion; small amount of salivary lipase (function minor/unclear).
• Gastric Juice (≈2\ \text{L/day}):
  • Pepsinogen → pepsin (protein digestion).
  • HCl (parietal cell H\^+/K\^+-ATPase): lowers pH, denatures proteins, kills microbes, activates pepsinogen.
  • Intrinsic factor (IF): B_12 binding & absorption.
  • Mucus & water: protect mucosa, dilute chyme.
• Pancreatic Juice (~1.5\ \text{L/day}):
  • Proteases (trypsin, chymotrypsin, carboxypeptidase).
  • Lipases & colipase.
  • Amylase.
  • Nucleases.
  • Bicarbonate (HCO_3^–) from duct cells neutralizes gastric acid → raises pH to \approx 8.
• Bile (liver → gallbladder):
  • Bile salts & lecithin (fat emulsification).
  • HCO_3^– buffer.
  • Cholesterol, bilirubin, drug metabolites (excretory).
• Intestinal Juice (crypts of Lieberkühn):
  • Versatile mix of water, mucus, HCO_3^– to continue dilution & buffering.

Control of Secretions

• Salivary: purely reflex (parasympathetic) – oral chemical/mechanical stimuli; smell/vision via higher centers.
• Gastric:
  • Cephalic phase (“psychic”): vagus-mediated gastrin & HCl secretion before food enters.
  • Gastric phase: food peptides/buffered pH stimulate G cells → gastrin → parietal & chief cell activation.
  • Intestinal phase: chyme in duodenum modulates gastric output; acid, fat, hypertonicity produce enterogastrones (secretin, CCK, GIP) → inhibit gastric acid/motility.
• Pancreatic secretion:
  • Secretin (acidic chyme) → HCO_3^–-rich juice.
  • CCK (fat/protein) → enzyme-rich juice; also ↓ gastric emptying & ↑ gallbladder contraction.
• Bile: produced continuously; CCK triggers gallbladder contraction; bile salts reabsorbed (enterohepatic circulation) stimulate more secretion.
• Intestinal juice: regulation uncertain; likely local reflex + hormonal.

Absorption Mechanisms & Structures

• Occurs mainly in small intestine; colon reclaims water & electrolytes.
• Routes:
  • Transcellular: across enterocytes (luminal → cytoplasm → basolateral → blood/lymph).
  • Paracellular: between cells via tight-junction pores.
• Key transport modes:
  • Secondary active transport: Na\^+ gradient drives many nutrients (e.g., glucose via SGLT-1, amino acids).
  • Facilitated diffusion: fructose (GLUT-5).
  • Simple diffusion: lipids from micelles.
• Nutrient destinations:
  • Water-soluble (monosaccharides, amino acids) → intestinal capillaries → hepatic portal vein → liver.
  • Chylomicrons (fat) → lacteals → thoracic duct → systemic circulation.
• Anatomical amplifiers: circular folds → villi → microvilli (brush border) provide vast surface area.

Elimination (Defecation) & Motility Disorders

• Defecation reflex triggered by rectal distension:
  1. Stretch receptors → spinal cord (parasympathetic sacral outflow).
  2. Internal anal sphincter relaxes; external sphincter under voluntary control.
• Constipation: slowed colonic transit; hard, dry stool.
• Diarrhea: ↑ small-intestinal motility ↓ absorption → watery stool; risks: dehydration, electrolyte loss.

Numerical / Physiological Reference Points

• Gastric emptying: 2\text{–}6\ \text{h}; chyme ejected ≈ every 20\ \text{s}.
• Small-intestinal transit: ≈5\ \text{h}.
• Optimal pH:
  • Pepsin \approx1\text{–}2.
  • Trypsin \approx7\text{–}8.
• Saliva flow: \approx1\text{–}1.5\ \text{L/day}; pancreatic juice \approx1.5\ \text{L/day}; gastric juice \approx2\ \text{L/day}.

Ethical / Practical Implications

• Proton-pump inhibitors (PPIs) target the H\^+/K\^+-ATPase to lower gastric acidity; necessary for ulcers but prolonged use may impair B_12, Ca\^{2+} absorption.
• Disorders of lactase → lactose intolerance → osmotic diarrhea; highlights enzyme specificity & genetic variation.
• Enterohepatic recycling of bile salts reduces need for de novo synthesis; bile-acid sequestrants used clinically to decrease cholesterol.