EXAMPLE Lung recoil inwards and chest wall recoil outwards causes - PDF document

Lung compliance and effect of pneumothorax EXAMPLE Lung recoil inwards and chest wall recoil outwards causes negative - intrapleural pressure Reduced pleural pressure → positive transmural pressure → increase - lung volume → reduced airway pressure → inspiration FRC is at most efficient part of compliance curve - Surface tension - van de Waals forces of adjacent water molecules - Two forces: - Circumferential - Tangential (radial) = wall tension - Opposite force = gas pressure - The internal pressure required to keep a sphere inflated. Effect of surfactant on - Laplace ’ s Law: P = 2T/r (T = tension of liquid) alveolar pressure - Therefore smaller alveoli require a higher internal pressure to stay inflated than larger alveoli ! Pulmonary surfactant - = detergent that ↓ surface tension in alveoli - Hydro-philic & -phobic 1. Increases lung compliance Large hydrophilic heads sit on the alveolar surface with hydrophobic tails facing inwards. They sit between water Ⓒ One 2 One Medicine: Pre-clinical revision course 2014 Page ! 10 of ! 22

molecules and interrupt their van de Waals forces, which causes a reduction in the radial (collapsing) and circumferential forces. Surface tension is 70% of lung elasticity, which is 1/ EXAMPLE compliance. Therefore reducing elasticity causes and increase in compliance. Raising the lung’s compliance means that less pressure is required to cause a change in lung volume - therefore the work of breathing is reduced. 2. Stabilises alveoli - Area-dependent effect → greater reduction in surface tension in smaller alveoli. Necessary because Laplace’s Law states that smaller have higher internal pressure therefore prevents smaller emptying into larger. 3. Reduces fluid movement into the alveoli - More compliant → pleural pressure (& transmural pressure) can be lower → reduces suction of fluid into alveoli from pulmonary capillaries ! Resistance in the respiratory system - Resistance = ∆ P/Flow - Airways (80%) + Tissues (10%) - Dynamic resistance causes right shift of compliance curve higher pressure needed to cause volume change - Poiseuille's law : Resistance ∝ 1/r4 - Large airways give majority due to a lower total X-section area (see above graph) - Radius = sum of radii for all of one airway division - Large airways give majority due to: Reynold's number - ↓ total X-section area - Higher number gives higher - More turbulent flow probability of turbulent flow - Two forms of flow : - Higher: - Laminar → low resistance - Higher velocity - Turbulent → high resistance ! - Larger diameter - Branching Bronchoconstriction : - Vagus (some rest tone) & ↑ mucus - Low airway CO 2 - Leukotrienes, histamine, bradykinin ! Bronchodilation : - β 2-adrenoreceptor ligation - Non-adrenergic, non-cholinergic innervation Types of airflow Ⓒ One 2 One Medicine: Pre-clinical revision course 2014 Page ! 11 of ! 22

EXAMPLE Flow limitation during expiration Flow limitation and obstructive airway disease - During expiration pressure highest in alveoli and lowest at mouth - Loss of pressure (energy) due to resistance - Higher flow generated by greater collapsing transmural pressure - equal along airway [inside the thoracic cage] - Airway pressure must be greater than transmural pressure to keep airways open - Theoretical point where: - Transmural (collapsing) pressure = airway pressure - Airway collapses & flow stops - Point is more proximal with: - Increased resistance ( bronchoconstriction , mucus ) - Lower airway pressure ( loss of elastic tissue ) - = Obstructive lung disease: asthma & COPD: chronic bronchitis & emphysema Surfactant increases lung compliance and stabilises alveoli - Turbulent flow increases resistance - predicted by high Reynolds - number COPD causes expiratory limitation due to increased resistance - ! Gas exchange and transport ! Alveoloar gases - CO 2 - Carbon dioxide from venous blood to alveoli: P A CO 2 < P v CO 2 P A CO 2 ∝ V CO2 /V A - - If V CO2 is constant (rest) then P a CO 2 ➙ efficiency of V A - Oxygen - Oxygen: from alveoli into venous blood Ⓒ One 2 One Medicine: Pre-clinical revision course 2014 Page ! 12 of ! 22