Ventilation_The_physics_of_breathing.pdf

Ventilation: Physics of Breathing Dr John P Winpenny Senior Lecturer in Physiology School of Medicine University of St Andrews ([email protected]) Footer: Ventilation: Physics of Breathing 1 Learning Outcomes • • • • • • • • • • Define the terms intrapleural pressure and intrapulmonary pre...

Ventilation: Physics of Breathing Dr John P Winpenny Senior Lecturer in Physiology School of Medicine University of St Andrews ([email protected]) Footer: Ventilation: Physics of Breathing 1 Learning Outcomes • • • • • • • • • • Define the terms intrapleural pressure and intrapulmonary pressure Explain what causes intrapleural pressure Describe the processes involved in quiet inspiration and expiration Describe the processes involved in forced inspiration and expiration Define the term: work of breathing, and list what the work of breathing involves Define airway resistance and explain what determines it Define compliance and describe how it changes in different disease states Explain how alveolar surface tension is reduced in health and be able to describe conditions when it would be raised Describe the respiratory volumes measured by a spirometer Describe how pulmonary function tests can be used What’s the Function of Ventilation? • Function of ventilation is to provide O2 to the tissues and remove CO2 • Function achieved by: – – – – – Pulmonary ventilation (movement of air from atmosphere to alveoli) Regulation of ventilation Matching of pulmonary blood flow to alveolar ventilation Movement of O2 and CO2 between alveoli and blood Transport of O2 and CO2 in blood and body fluids • Non-respiratory functions – Expulsion of foreign bodies – Defence against infection/disease Alveolar Ventilation • • • Pulmonary ventilation renews air in gas exchange areas Alveolar Ventilation is the rate at which new air reaches these areas Some air that is breathed in never reaches gas exchange areas but fills respiratory passages (e.g. nose, pharynx, trachea) – Anatomic dead space air (about 150ml) • Minute (total) ventilation rate (VE) = Freq x VT • Alveolar Ventilation Rate (VA ) = Freq 4200ml/min x (VT – VD) = 12breath/min x (500ml – 150ml) VA, volume of alveolar ventilation per min Freq, frequency of respiration per min VT, tidal volume VD, dead space volume • Alveolar ventilation is one of major factors determining O2 and CO2 concentrations in alveoli Mechanics of Pulmonary Ventilation • Most important muscles that raise rib cage are: – External intercostals – Sternocleidomastoid (lift upward on sternum) – Anterior serrati (lift many ribs) – Scaleni (lift first two ribs) • Most important muscles that lower rib cage are: – Abdominal recti – Internal intercostals Mechanics of Pulmonary Ventilation • Lungs can be expanded and contracted in 2 ways: – Downward and upward movement of diaphragm to lengthen or shorten chest cavity – Elevation and depression of the ribs to increase or decrease anterioposterior diameter of chest cavity • Normal quiet breathing is accomplished entirely by 1st method • During heavy breathing, normal elastic recoil not quick enough so need contraction of abdominal muscles too Static Properties of Lungs • • • • • Lung is elastic structure collapses like a balloon when no force to keep inflated Not attached to chest wall Lung floats in thoracic cavity surrounded by thin layer of pleural fluid that acts as lubricant Chest wall also elastic Lymphatic drainage of excess fluid between lung pleural membrane and pleural surface of thoracic wall leads to suction effect – lungs held against thoracic wall Lungs at FRC Pressure Changes During Ventilation • • • • • • Intrapleural (Pleural) pressure is pressure of fluid in thin space between lung pleura and chest wall pleura – usually slight negative pressure IP pressure varies over length of lungs At start of respiration pleural pressure about -5 cm H2O During inspiration expansion of chest cage pulls lungs outward so negative pressure increases to about -7.5 cm H2O Air sucked into lungs Expiration process reversed Pressure Changes During Ventilation • • • • • • • Alveolar pressure is the pressure of air inside the lung alveoli When glottis open and no air flowing, pressure in all parts of respiratory tree is equal to atmospheric pressure (0 cm H2O) During inspiration and chest wall expansion, alveolar pressure es to about -1 cm H2O Pulls 0.5 L air into lungs During expiration opposite occurs Transpulmonary pressure is the pressure difference between that in the alveoli and that on the outer surfaces of the lungs It is a measure of the elastic forces that tend to collapse the lungs (recoil pressure) Overview of Inspiration • Change in volume leads to change in pressure • Main muscle of inspiration - diaphragm. Contraction flattens domes. Abdominal wall relaxes to allow abdominal contents to move downwards • Role of the external intercostals – with first rib fixed, two movements, forward movement of lower end of sternum, and upward and outward movement of ribs • Increases volume of thorax by about 500 ml – normal tidal volume • Intrapleural pressure drops to approximately -7 mmHg • Decreases intrapulmonary pressure by approximately 1 mmHg • Accessory muscles in forced inspiration – respiratory distress – trapezius 10 Overview of Expiration Quiet expiration: – – – – – – Passive – no direct muscle action normally Cessation (relaxation) of muscle contraction Elastic recoil – drives air out of lungs Thoracic volume decreases by 500 ml Intrapulmonary pressure increases Air moves down pressure gradient Forced expiration: – Contraction of abdominal walls, forces abdominal contents up against diaphragm, and internal intercostals – pull ribs downwards 11 Dynamic Lung Mechanics - Work of Breathing Energy is required to: – contract the muscles of inspiration – in quiet breathing contraction of the diaphragm comprises 75% of energy expenditure – stretch elastic elements – overcome airway resistance – overcome frictional forces arising from the viscosity of the lung and chest wall – overcome inertia of the air and tissues 12 Dynamic Lung Mechanics - Airway Resistance • • • • • • • Most significant non-elastic source of resistance F = ΔP (PB-PA)/R i.e. amount of air that flows is determined by change of pressure divided by resistance Airway Resistance (R) = 8Lƞ / πr4 (Poiseuille Law) So airway radius has greatest effect on airway resistance Turbulent flow – likely to occur with high velocities and large diameter airways Greatest resistance to airflow is found in the segmental bronchi - cross sectional area relatively low and airflow high and turbulent At smallest airways, flow is laminar and resistance is small (there is a large total cross-sectional area due to large number of small airways combined) 13 Compliance of the Lung • • • Static Compliance is the extent to which the lungs will expand for each unit increase in transpulmonary pressure (given time to reach equilibrium) The elastance of the lungs (measure of elastic recoil) is the reciprocal of compliance (E = 1/C). So high compliance means low elastic recoil Compliance diagram opposite is determined by 2 elastic forces: – Elastic forces of the lung tissue itself • determined mainly by elastin and collagen fibres among lung parenchyma • deflated lungs, fibres are contracted and kinked • expanded lungs, fibres become stretched and unkinked – Elastic forces caused by surface tension of fluid that lines alveoli Compliance Changes in Lung Diseases • Pulmonary Fibrosis, a restrictive lung disease – – – – • Emphysema, a chronic obstructive pulmonary disease (COPD) – – – – – – • Disease process causes deposition of fibrous tissue, so lungs become stiff Lung compliance is ed, resulting in smaller than normal changes in lung volume for small changes in transpulmonary pressure Patients breath more shallowly and rapidly es in RV, FRC, TLC Common consequence of cigarette smoking Alveolar and capillary walls progressively destroyed, particularly elastic tissue Lung compliance is ed, resulting in larger than normal changes in lung volume for small changes in transpulmonary pressure However, as airways tend to collapse on expiration, airway resistance is also ed Patients breath more slowly and deeply es in RV, FRC, TLC Chronic bronchitis, also a COPD – – Mucus and airway inflammation produce an  in airway resistance es in RV, FRC, TLC, however, compliance is normal Elastic Forces due to Surface Tension • • • Surface tension is a measure of the force acting to pull a liquid’s surface molecules together at an air-liquid interface In the lungs this results in the alveoli trying to force the air out of them so allowing the alveoli to collapse Law of Laplace states that the pressure (P) within a fluid-lined alveolus is dependent on the surface tension of the fluid (T) and the radius of the alveolus (r) P = (2 x T) / r • So if 2 alveoli are connected together but have different diameters, air will flow from smaller alveoli to larger alveoli Production of Surfactant • • • Lipid components enter Type II cell from bloodstream Secreted by Type II alveolar epithelial cells Surfactant is a complex mixture of phospholipids – Dipalmitoylphosphatidylcholine (DPPC) – proteins (surfactant apoproteins, SPA, SP-B, SP-C & SP-D) – ions (calcium) • • Part of DPPC molecule dissolves in fluid while rest spreads over surface of fluid Alveolar macrophages help in degrading surfactant, Type II cells take up rest and recycle or destroy it Role of Surfactant • • • • • • Surfactant greatly reduces the surface tension of H2O Thus es compliance, so easier to inflate lungs Surfactant reduces pressure by 4.5 times By reducing surface tension minimises fluid accumulation in alveolus Surfactant helps keep alveolus size relatively uniform during respiratory cycle reduces atelectasis – alveolar collapse Pulmonary Volumes and Capacities • Spirometry – method for studying pulmonary ventilation • Tidal volume is volume of air inspired or expired with each normal breath (500ml) Inspiratory reserve volume is extra volume of air that can be inspired over and above normal tidal volume (2500ml) Expiratory reserve volume is max extra volume of air that can be expired by forceful expiration after end of normal tidal expiration (1100ml) Residual volume is volume of air remaining in lungs after most forceful expiration (1200ml) • • • Summary • Breathing involves repeated cycles of inspiration and expiration. • The diaphragm, chest wall, and lungs move as one unit - linked by pleural fluid, which lubricates the visceral and parietal pleurae • At rest, the lung is subject to two opposing forces – Surface tension and elastic elements in lung tissue favour collapse (elastic recoil) – Elastic elements in the chest wall favour expansion and prevent collapse • Airflow between the alveoli and the external atmosphere is driven by pressure gradients. Flow occurs against a resistance that is largely dependent on the internal radius of the airway • Assessment of lung health can be investigated using spirometry and other pulmonary function tests (PFTs) References • Boron, WF & Boulpaep, EL (2017) Medical Physiology (3rd Edition) – – • Guyton & Hall (2020) Textbook of Medical Physiology (14th Edition) – • Chapter 28 Mechanical Properties of the Lung and Chest Wall p372-383 Preston RR & Wilson TE (2013) Lippincott’s Illustrated Reviews: Physiology (1st Edition) – • Chapter 37 Pulmonary Ventilation p471-482 Levy, MN, Koeppen, BM & Stanton, BA (2005) Berne & Levy Principles of Physiology (4th Edition) – • Chapter 26 Organisation of the respiratory system p590-605 Chapter 27 Mechanics of Ventilation p606-627 Chapter 22 Lung mechanics p263-279 Naish, J & Syndercombe Court, D. (2019). 3rd Edition. Medical Sciences – Chapter 13 The Respiratory System p603-642