BIOM2012 Respiratory Physiology L2 PDF

BIOM2012 - Systems Physiology: Respiratory System L2 Dr. Jacky Suen [email protected] Credit: A/Prof Stephen Anderson and Dr. Hardy Ernst Learning Objectives Pulmonar...

BIOM2012 - Systems Physiology: Respiratory System L2 Dr. Jacky Suen [email protected] Credit: A/Prof Stephen Anderson and Dr. Hardy Ernst Learning Objectives Pulmonary Ventilation and Respiratory mechanics Gas exchange Gas transport Blood pH regulation Control of Respiration Physical factors influencing ventilation ? ? ? 1. Airway resistance Friction from air moving against walls or airways Depends mostly on the diameter of the airway low resistance high resistance Bronchoconstriction Mucous, secretion Fluid, Oedema  Airways Resistance Large lumen, high flow velocity, increased branching contribute to turbulence & therefore resistance Overall, the greatest resistance occurs in the medium-sized bronchi. Resistance in large bronchi is low because of their large diameters, whereas resistance in small bronchi is low because of their large total cross-sectional area. discover.hubpages.com  Airways Resistance The most important causes of increased airway resistance include bronchospasm, secretions, and mucosal oedema, as well as volume-related and flow-related airway collapse. Pathophysiology – asthma involves increased airway resistance discover.hubpages.com  Airways Resistance Resistance of bronchioles can be varied by contraction of smooth muscle Bronchoconstriction Bronchodilation PSNS acetylcholine via muscarinic SNS - no direct innervation receptors but beta2-receptors Anticholinergics (atropine) Beta-agonists (salbutamol) Leukotrienes, histamine and bradykinin Antihistamines (fexofenadine) Leukotrine antagonists (montelukast) Dynamic Small Airway Closure 2. Lung compliance and elastic recoil Compliance Change in volume per unit change in pressure Ability for lungs to be stretched a measure of elastic resistance Elastic recoil (Lung elasticity) Ability for lungs to rebound A key driving force during expiration  Pressure - Volume characteristics A simple way to consider lung characteristics is to remove lungs from the body…. As the transpulmonary pressure increases, lung volume increases pressure – volume curve represents both elastic and airway resistance properties of the lungs often simplified to single line Pressure-volume curve for isolated lungs. From Levitzky MG. Pulmonary Physiology McGraw-Hill.  Lung Compliance Many diseases affect lung compliance and it is a useful clinical measure Pathophysiology Fibrosis stiffer, less compliant Emphysema increased compliance Compliance determines 65% of work of breathing. If a lung has low compliance, it requires more work for breathing Representative static pulmonary compliance curves. TLC = total lung capacity, RV = residual volume From Levitzky MG. Pulmonary Physiology McGraw-Hill.  Compliance vs Elastance Lung compliance is important because it is inversely proportional to elastance Compliance denotes the ease with which something can be stretched or distorted Elastance refers to the tendency for something to oppose stretch or distortion, as well as to its ability to return to its original configuration after the distorting force is removed. High compliance = less elastic recoil; low compliance = more elastic recoil. Factors affecting compliances: surface tension, elastic fibres, surfactant 3. Alveolar surface tension Water molecules are charged and attracted to each other Tendency to collapse alveoli Resist alveoli inflation  Surface Tension Note different curve for inspiration vs expiration. This is when excised lungs filled with air. Dynamic resistance in inspiration & expiration is not linear or equal. Elastic recoil lungs alone no liquid-air Elastic recoil lungs interface and surface tension forces hysteresis liquid-liquid fluid only curve Hysteresis…. lung does not act as a perfect elastic system Pressure-volume curves for excised cat lungs inflated with air or saline From Levitzky MG. Pulmonary Physiology McGraw-Hill. Law of LaPlace Magnitude of inward-directed pressure (P) in a bubble (alveolus) = 2 x Surface tension (T) Radius (r) of bubble (alveolus) Sherwood Fig 13.14  Pulmonary surfactant The alveolar surface contains a component that lowers the elastic recoil due to surface tension this component increases compliance more than just an air-water interface, decreasing the work required during inspiration  Pulmonary surfactant Pulmonary surfactant is synthesised by type II alveolar cells, and is a lipoprotein complex ie. mixture of lipids and proteins It is amphilic ie. both hydrophilic and hydrophobic Effectively surfactant stabilises the alveoli and increases lung dipalmitoyl phosphatidylcholine (DPPC) molecules on fluid surface compliance Pathophysiology New-Born Respiratory Distress Syndrome (NRDS), lack surfactant Acute Respiratory Distress Syndrome (ARDS), inactivation surfactant  Surface tension & Surfactant Elastic recoil lungs alone Elastic recoil lungs deflation and surface tension forces inflation Deficit surfactant eg infant NRDS increase opening pressure difficult to inflate less deflation stability Sherwood Fig 13.14 Why does it matter? Infant Respiratory Distress Syndrome (IRDS) Premature infants (before week 28) or newborn with inadequate surfactant production Alveoli collapse during expiration Dyspnea, fatigue from re-inflating alveoli Treatment: Surfactant spray Mechanical ventilation – positive pressure to keep alveoli open At risk of bronchopulmonary dysplasia  Opposing Forces Acting on Lung Forces keeping the alveoli Forces promoting alveolar open collapse (recoil) Summary: Do you understand the various factors affecting ventilation? Can you explain the importance of pulmonary surfactant? Do you know the difference between compliance and elastance? A healthy person cannot fully empty his/her lungs because: A. Alveolar surface tension B. Dynamic small airway closure C. Lung elasticity D. All of the above Which of the following does not help keeping the alveoli open? A. Pulmonary elasticity B. Intrapleural pressure C. Alveolar interdependence D. Pulmonary surfactant E. None of the above