L 29

Respiratory System Lecture Notes

• Learning Outcomes:
  • Describe factors affecting airflow resistance
  • Describe factors affecting work of breathing, focusing on lower airway resistance
  • Outline gas laws related to pulmonary diffusion
  • Describe factors affecting net gas diffusion across the membrane
  • Describe factors affecting pulmonary gas diffusion and their clinical significance
  • Describe pulmonary oxygen diffusion capacity at rest and during exercise

Work of Breathing

• Pressures: Pleural and alveolar

• Resistance: Airways

• Compliance: Ability to expand

• Surface Tension: Surfactant

• Components of Resistance:

  • Alveolar surface tension
  • Elastic resistance
  • Airway resistance

Work of Inspiration

• Performed by respiratory muscles to stretch elastic tissues of the chest wall and lungs
• Compliance (65%):
  • Work to expand lungs against lung and chest elastic forces
  • Elastin and collagen fibers
  • Surface tension
• Tissue resistance (7%):
  • Work to overcome the viscosity of lung and chest wall structures
• Airway resistance (28%):
  • Work to overcome airway resistance
  • Increases during bronchiole constriction

Factors Affecting Airflow Resistance

• Parasympathetic:
  • Constriction of bronchioles
  • Mild to moderate constriction
  • Vagus nerves release acetylcholine
  • Action through M1, M2, and M3 receptors
  • Increased intracellular Ca2+ and smooth muscle contraction
• Sympathetic:
  • Dilation of bronchioles
  • Norepinephrine and epinephrine
  • Relatively weak direct control
  • Release from adrenal medulla
  • Stimulation of beta-adrenergic receptors leads to bronchodilation

Cough Reflex

• Receptors in airways stimulated by irritants
• Activation of vagovagal reflex
• Bronchoconstriction
• Rapid inspiration
• Epiglottis and vocal cords close tightly
• Abdominal muscles contract forcefully
• Expiratory muscles contract forcefully
• Pulmonary pressure rises rapidly
• Vocal cords and epiglottis open widely to expel air

Airway Resistance: Effect of Airway Radius

• Resistance determined by Poiseuille's formula: R = 8VL/πr⁴
  • R = peripheral resistance
  • V = viscosity
  • L = length of tube
  • r = radius of tube
• Viscosity and length do not change under normal conditions
• Diameter of airways is the major factor
• Minor radius change greatly affects airway resistance

Airway Resistance

• Intrapleural Pressure:
  • Negative pressure keeps airways open during inspiration, decreasing resistance
  • Positive pressure during forced expiration collapses some small airways, increasing resistance
  • Excessive mucus increases resistance
  • Exercise increases resistance

Gas Laws Applicable to Pulmonary Diffusion

• Charles' Law: Volume of gas is directly proportional to temperature (at constant pressure)
• Boyle's Law: Pressure of gas is inversely proportional to volume (at constant temperature)

Partial Pressures of Gases

• Total pressure: Sum of all component gases in a mixture
• Dry air: 21% O2, 78% N2, 0.04% CO2, and 0.096% other inert constituents
• Barometric pressure: 760 mm Hg at sea level
• Water vapor: Reduces partial pressures; 47 mm Hg inside the body
• Alveolar partial pressures: PO2 = 149 mm Hg, PN2 = 564 mm Hg, and PCO2 = 0.3 mm Hg

Respiratory Membrane

• Total surface area: 70 square meters
• Components:
  • Alveolar epithelium
  • Capillary endothelium
  • Thin fluid layer with surfactant
  • Alveolar wall
  • Interstitial space
  • Basement membranes of alveoli and capillaries
  • Capillary endothelium
• Average thickness: 0.6 μm

Factors Affecting Diffusion: Diffusion Coefficient

• Diffusion depends on Fick's Law: D_a (P₁-P₂) x A x S / d x Mol wt
  • D_a = diffusion coefficient
  • P₁-P₂ = pressure gradient
  • A = total surface area
  • S = solubility
  • d = thickness of membrane
  • Mol wt = molecular weight of the gas
• Factors affecting diffusion:
  • Pressure gradient
  • Total surface area
  • Thickness of the membrane
  • Solubility of the gas
  • Molecular weight of the gas

Factors Affecting Diffusion: Diffusion Coefficient

• Pressure gradient: Larger gradient, faster diffusion
• Solubility: Higher solubility, faster diffusion
• Molecular weight: Lower molecular weight, faster diffusion
• Surface area: Larger surface area, faster diffusion
• Thickness: Thicker membrane, slower diffusion

Diffusing Capacity

• Definition: Volume of gas that diffuses across the respiratory membrane per minute with a 1 mm Hg pressure difference
• O2 diffusing capacity: 23 ml/min/mm Hg
• CO2 diffusing capacity: 20 times faster than O2

Changes in O2 Diffusion During Exercise

• Increased blood flow through pulmonary capillaries
• More time for PO2 to reach equilibrium
• Increased surface area of respiratory membrane
• Increased ventilation and blood flow stretch alveolar and capillary walls
• Further enhancement from dormant capillaries opening

Explore the essential functions and factors affecting the respiratory system in this detailed quiz. Learn about airflow resistance, gas diffusion laws, and the mechanics of breathing. Perfect for students studying respiratory physiology.