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Week 4 Supplemental – Respiratory & Digestive Physiology

Respiratory Physiology

Respiratory Volumes

  • Four primary, non-overlapping air volumes measured with a spirometer
    • Tidal Volume (TV / TC)
    • Air moved in & out during quiet breathing
    • Graph scale showed baseline of 0 up to \approx 6000\,\text{mL} (total scale of the figure) with TV occupying the middle “breathing” plateau
    • Inspiratory Reserve Volume (IRV)
    • Extra volume that can be inhaled after a normal inspiration
    • Occupies the large upper segment of the figure (between \text{TV} and \approx 6000\,\text{mL} line)
    • Expiratory Reserve Volume (ERV)
    • Extra volume that can be exhaled after a normal expiration
    • Shown beneath TV but above residual volume in the diagram
    • Residual Volume (RV)
    • Air remaining after maximal expiration; prevents lung collapse

Respiratory Capacities (sums of volumes)

  • Inspiratory Capacity (IC)
    \text{IC}=\text{TC}+\text{IRV}
  • Functional Residual Capacity (FRC)
    \text{FRC}=\text{ERV}+\text{RV}
  • Vital Capacity (VC)
    \text{VC}=\text{TC}+\text{ERV}+\text{IRV}
  • Total Lung Capacity (TLC) \text{TLC}=\text{TC}+\text{IRV}+\text{ERV}+\text{RV}
    • Represents the absolute maximum air the lungs can contain

Gas Exchange & Dalton’s Law

  • “Pause for more physics” → emphasis on partial pressures
  • Dalton’s Law of Partial Pressures: total pressure of a gas mixture equals the sum of the pressures each gas would exert independently
    P{\text{total}}=\sum P{i}
  • Atmospheric composition (sea level, 760\,\text{mm Hg}):
    • Nitrogen: 78.6\% → 597.4\,\text{mm Hg}
    • Oxygen: 20.9\% → 158.8\,\text{mm Hg}
    • Water vapour: 0.4\% → 3.0\,\text{mm Hg}
    • Carbon dioxide: 0.04\% → 0.3\,\text{mm Hg}
    • Other gases: 0.06\% → 0.5\,\text{mm Hg}
  • Simplified illustration in the slide:
    • Oxygen partial pressure labelled 159\,\text{mm Hg}
    • Nitrogen partial pressure labelled 597\,\text{mm Hg}
    • Combined icon showed 756\,\text{mm Hg} (round-off excluding water & trace gases)

The Alveolar Exchange Surface

  • Respiratory membrane: alveolar epithelium + fused basement membrane + capillary endothelium (ultra-thin ≈ 0.5\,\mu\text{m})
  • Key cellular/structural components (diagram labels)
    • Type I alveolar cells – simple squamous epithelium (gas diffusion)
    • Type II alveolar cells – secrete surfactant
    • Alveolar pores – equalise pressure between neighbouring alveoli
    • Alveolar macrophages – remove debris & pathogens
  • Diffusion driven by \Delta P of each gas
    • O2 moves: alveolus → blood (binds Hb forming O2!\cdot!Hb)
    • CO2 moves: blood (dissolved/\text{HCO}3^-) → alveolus for expiration

Digestive Physiology

Accessory Organ – Pancreas (Exocrine)

  • Secretory units: acini (cluster of acinar cells)
  • Enzymes & zymogens released into pancreatic duct → merges with common bile duct
    • Trypsin(ogen)
    • Chymotrypsin(ogen)
    • (Pro)carboxypeptidase
    • Amylase
    • Lipase
    • Nuclease
  • Pancreatic juice: enzymatic cocktail + bicarbonate to neutralise acidic chyme

Accessory Organ – Liver

  • Supplied by hepatic portal circulation (nutrient-rich blood from GI tract)
  • Functions
    • Processes & stores nutrients
    • Glycogen (glucose storage)
    • Iron (ferritin)
    • Lipids (lipoprotein synthesis)
    • Metabolises drugs & poisons (detoxification via cytochrome P450)
    • Breaks down heme → bilirubin (excreted in bile)
    • Produces bile (bile salts, bilirubin, cholesterol)
    • Synthesises most plasma proteins (albumin, clotting factors, complement)

Chemical Digestion – Macromolecules & Bonds

  • Proteins → amino acids / di- & tripeptides
    • Hydrolysis occurs at peptide bonds (\ce{–C(=O)–NH–})
  • Carbohydrates → monosaccharides
    • Amylose, amylopectin (starch) broken down to maltose then glucose
    • Lactose → glucose + galactose
  • Triglycerides
    • Split into fatty acids, monoacylglycerides & glycerol
  • Nucleic acids
    • Yield pentose sugar, phosphate group, nitrogenous bases
    • Bases
    • Pyrimidines: Cytosine (C), Thymine (T, DNA only), Uracil (U, RNA only)
    • Purines: Adenine (A), Guanine (G)
    • Components linked by phosphodiester bonds

Enzymatic Breakdown – Where & How

  • Carbohydrates
    • Mouth / oesophagus: salivary amylase
    • Small intestine
    • Pancreatic amylase
    • Brush-border enzymes: \alpha-dextrinase, lactase, maltase, sucrase
  • Proteins
    • Stomach: pepsin (optimum low pH)
    • Small intestine
    • Pancreatic: trypsin, chymotrypsin, carboxypeptidase, elastase
    • Brush-border peptidases (dipeptidase, aminopeptidase, etc.)
  • Triglycerides
    • Mouth: lingual lipase
    • Stomach: gastric lipase
    • Small intestine: pancreatic lipase (activity enhanced by bile emulsification)
    • Absorption products enter lacteals (lymphatic capillaries in villi)
  • Nucleic acids
    • Small intestine: pancreatic DNase, RNase → nucleotides
    • Brush-border: nucleosidase & phosphatase → nitrogenous bases + sugars + \text{PO}_4^{3-}

GI Tract Journey – "Start to Finish"

  1. Stomach phase

    • Bolus becomes chyme via peristaltic mixing & gastric juice (HCl + pepsin)
    • Limited absorption: alcohol, aspirin
    • Gastric emptying time: 2–4\,\text{h} (regulated release through pyloric sphincter)

  2. Small intestine phase (duodenum → jejunum → ileum; transit \sim 3–5\,\text{h})

    • Bile + pancreatic juice enter through hepatopancreatic (Oddi) sphincter
    • Motility patterns: segmentation & migrating motility complex
    • Structural amplifiers: circular folds, villi, microvilli ("brush border")
    • Outcome: nearly all nutrients + 90\% of water absorbed

Hormonal Regulation of Digestion

  • Gastrin (G-cells; stomach, SI, pancreas)
    • ↑ gastric juice secretion & motility
    • Relaxes ileocecal valve
    • Triggers large-intestine mass movements
  • Secretin (duodenum)
    • Stimulates pancreatic bicarbonate release (neutralises acid)
  • Gastric Inhibitory Peptide (GIP) / Glucose-dependent insulinotropic peptide (duodenum)
    • ↓ gastric secretion & motility
  • Somatostatin (δ-cells; stomach & pancreas)
    • Global inhibitor: ↓ gastric secretion, motility, pancreatic secretion, intestinal blood flow
  • Cholecystokinin (CCK) (duodenum)
    • Stimulates bile release (gallbladder contraction) & pancreatic enzyme secretion

Pancreatic Islets – Endocrine Summary

  • Alpha (α) cells → glucagon (raises blood glucose)
  • Beta (β) cells → insulin (lowers blood glucose)
  • Delta (δ) cells → somatostatin (paracrine inhibition)
  • PP / F cells → pancreatic polypeptide (self-regulates pancreatic secretions)
  • G cells → gastrin (echoes gastric G-cells)

Integration & Significance

  • Lung volumes & capacities provide diagnostic metrics (e.g.
    • ↓ VC in restrictive disease, ↑ RV in obstructive disease)
  • Dalton’s law underpins O2/CO2 diffusion gradients across respiratory membrane
  • Digestive accessory organs (liver & pancreas) supply the enzymes/bile salts that finalise macromolecule breakdown, enabling absorption in SI
  • Hormonal feedback loops fine-tune motility & secretion, ensuring chyme is properly processed before advancing through GI tract
  • Structural specialisations (villi, microvilli, acini, islets) illustrate the theme of form matching function throughout respiratory & digestive systems