Lecture 19: Mechanics of Breathing - PDF

Lecture 19: Mechanics of Breathing Dr. Ann Revill [email protected] Office: Dr. Arthur G. Dobbelaere Science Hall 380E Review: Top Hat Question Join code: 437994 2 ...

Lecture 19: Mechanics of Breathing Dr. Ann Revill [email protected] Office: Dr. Arthur G. Dobbelaere Science Hall 380E Review: Top Hat Question Join code: 437994 2  Lecture objectives By the end of this lecture, you will be able to: 1. Describe the lung volumes and lung capacities 2. Describe key muscles and mechanics involved in inspiration and in expiration 3. Describe how the lung and chest wall are coupled 4. Describe the clinical scenario of a pneumothorax 5. Discuss the mechanisms that facilitate or inhibit inhalation and exhalation: lung compliance, surface tension, airway resistance 6. Compare and contrast obstructive and restrictive lung diseases 7. Identify the factors that increase or decrease lung compliance 8. Explain factors that prevent alveolar collapse 9. Determine physiological and pathological factors that influence airway resistance 3 How are lung volumes measured? Lung volumes are measured by a spirometer 4  Lung Volumes Used when? Exercise Inspiratory reserve volume (IRV), 3000 ml Tidal volume (VT), 500 ml Expiratory reserve volume (ERV), 1200 ml Residual volume (RV), 1200 ml Can RV be exhaled? No Can’t be measured by spirometry Costanzo 5-2 5  Lung Capacities (2 or more lung volumes) Inspiratory Vital capacity = Total lung capacity = ERV + IRV + VT capacity IRV + VT = IRV + VT + ERV + RV Functional residual capacity = ERV + RV Resting volume of the lung Volume of air in the lungs at the end of a normal passive expiration Costanzo 5-2 6  What’s the relationship between pressure and volume? At a constant temperature, pressure exerted by a gas is inversely proportional to the gas volume That is, as volume increases, pressure decreases (Boyle’s Law) Vol = 1/2 Control: Vol = 2 2 Pressure = _____ Vol = 1 1/2 Pressure = _______ Pressure = 1 Sherwood 13-10 7 Respiratory muscles Sherwood 13-11 8  Inspiration What is the most important muscle of inspiration? diaphragm – Contraction Pushes abdominal contents downwards Lifts ribs upwards and outwards External intercostal contraction supports rib movement upward and outward What does contraction do to: – Intrathoracic volume? increases – Intrathoracic pressure? decreases – Due to the pressure gradient, air flows into lungs During eupnea (quiet breathing), diaphragm is sufficient During heavy exercise, accessory muscles (sternocleidomastoid, scalenus) are recruited 9  How does the volume of chest cavity increase for eupnea? Before inspiration Inspiration External “pump Intercostals External handle” intercostal analogy contraction Sternum Diaphragm Diaphragm contraction Sherwood 13-12 10  How long does inspiration last? Air moves into alveoli until: – air pressure in alveoli equals pressure of atmosphere Palv = Patm – if Palv = Patm there’s no movement of air Because there is no pressure gradient to act as the driving force 11  Expiration Definition? During eupnea, expiration is passive – diaphragm and external intercostals relax Stretched lungs recoil – What happens to volume? decreases – Intra-alveolar pressure > atmospheric pressure – Air moves out of lungs, down pressure gradient During active expiration: – abdominal muscles contract, force diaphragm upward – internal intercostals contract, depress ribs & sternum 12  Passive Expiration Active Expiration Internal intercostal contraction flattens ribs and sternum internal External intercostal intercostal contraction relaxation Diaphragm relaxation Abdominal contraction Relaxed Return of diaphragm, abdominals ribs and sternum to rest position restores Abdominal thoracic cavity to contraction pushes Sherwood 13-12 preinspiratory size diaphragm upwards 13  What happens to cause exhalation? Phrenic nerve stops firing, so diaphragm muscle relaxes – it’s all passive at rest Elasticity of thorax and lung → ↓size of alveoli – this ↑ Palv When Palv > Patm air moves from alveoli to atmosphere – until Palv = Patm 14  What determines lung volume? Story is complicated because no muscles attach to lung surface How are changes in thorax volume linked to changes in lung volume? Lungs and thoracic wall are coupled: – due to intrapleural fluid & cohesion between parietal and visceral pleura Diaphragm Sherwood 13-5 15  What determines lung volume? Thoracic cavity size is > unstretched lung size Airway Lung elastic recoil due to Lung wall Pleural cavity collagen and elastic fibers Lungs favors lung collapse Thoracic wall Elastic forces of the thoracic wall favor thorax expansion Intrapleural pressure – pressure in pleural cavity – less than atmospheric (~4 mmHg) Transmural Transmural Lung and thoracic wall coupling pressure gradient pressure gradient results in transmural pressure gradient: Lungs expand when thorax expands Transmural P = Palv - Pintrapleural Sherwood 13-6 16  Breathing cycle at rest Inspiration Expiration 500 ✓ 1. Inspiration: Volume (ml) Add volume of breath Intra-alveolar pressure < atmospheric pressure 0 ✓ 2. Expiration: intra-alveolar pressure > Intra-alveolar atmospheric pressure Atmospheric pressure ✓ 3. End inspiration (or pressure expiration): pressures equal ✓ 4 &5. Intrapleural pressure Pressure (mmHg) always < intra-alveolar Intrapleural pressure pressure. Therefore, lung is always stretched (even during expiration). Sherwood 13-14 17 What happens if intrapleural pressure equilibrates with atmospheric pressure? Pneumothorax Sherwood 13-9 18  Lung compliance Compliance = ΔV/ΔP The elasticity of the respiratory system Lung compliance refers to the change in lung volume for a given pressure change – High compliance = easy to expand lung Lung compliance is decreased: – with deficiency of surfactant (alveolar surface tension increases) – at high lung volumes – pulmonary congestion Lung compliance is increased: – at lower lung volumes – advancing age 19  Lung compliance Compliance = ΔV/ΔP Why does deflation limb differ from inflation limb? – surface tension higher during inflation than deflation – exact mechanism unclear, but it involves redistribution of surfactant (disrupts ST). By filling lung w/ saline, completely eliminate surface tension forces to measure the compliance of the lung. – Note profound increase in compliance! Costanzo 5-7 20  Lung volumes and compliance Lung compliance increased Lung compliance decreased -low lung volumes -high lung volumes Higher V slope i.e. compliance slope i.e. compliance Lower V 21 Lung Compliance and Lung Disease Pulmonary disorders divided into obstructive and restrictive diseases Restrictive disease: increased fibrous tissue – Example: pulmonary fibrosis (one example: asbestos exposure) – Lung compliance? decreased Obstructive disease: airways obstructed – Examples: Chronic Obstructive Pulmonary Disease: Emphysema, chronic bronchitis asthma cystic fibrosis 22  Lung Compliance and Disease Fibrosis: restrictive disease – Stiffening of lung tissues – Compliance? Associated with decreased Volume slope of V/P - + Airway pressure Costanzo 5-11 23  Normal Lung Emphysemic Lung 1. Looking at the structural differences, do you think there is more or less elastic tissue in the emphysemic lung? Less, alveoli have collapsed 2. What do you think has happened to lung compliance? Less elastic tissue, compliance increased 24  Lung Compliance and Disease Emphysema: obstructive disease – Loss of elastic lung fibers – Compliance? Volume Associated with increased slope of V/P - + Airway pressure Costanzo 5-11 25  Alveolar Surface Tension Alveoli are lined with a thin film of fluid Fluid molecules are more attracted to each other than to air This produces surface tension – tendency to minimize alveolar size (i.e. collapse alveoli) – tendency to resist inflation (i.e. reduce compliance) 26  What is the challenge with surface tension? Large alveolus Collapsing pressure (P) on alveolus is directly related to surface tension (T) and inversely related to alveolar radius (r): P = 2T r For the same pressure, which alveolus has lower collapsing pressure? small alveolus – Large alveolus Which alveolus has better surface area to volume ratio for gas exchange? – Small alveolus Costanzo 5-12 What’s the solution? 27  Tendency of alveoli to collapse is countered by two factors: Alveolar surfactant: – complex mixture that disrupts intermolecular forces between water molecules (detergent-like) – reduces work of breathing and increases compliance by reducing surface tension forces Interdependence of alveoli: – balance of forces between alveoli – each alveolus is supported by adjacent alveoli 28  Tendency of alveoli to collapse is countered alveolar surfactant Unique combination of a No surfactant phospholipid, lipids & proteins – Produced by type II alveolar epithelial cells – can be deficient in premature infants Surfactant Reduces surface tension (T) in alveoli – Surface tension is inversely proportional to [surfactant] on alveolar walls Increasing [surfactant] on alveolar Same r walls decreases surface tension forces T causes P (T), which decreases the collapsing pressure (P) Costanzo 5-12 29 What happens to the pressure-volume relationship in premature infants? Lung volume (%) Saline Air Premature infant Pressure (mmHg) 30  Tendency of alveoli to collapse is countered by interdependence of alveoli Interconnected alveoli Alveolus starts to collapse Collapsing alveolus pulled open Collapsing alveolus Stretched surrounding alveoli stretches surrounding recoil, pulling collapsing alveoli alveolus open Sherwood 13-17 31  Airway resistance Like the cardiovascular system: V = air flow (ml/min) V = ΔP ΔP = pressure gradient (mmHg) R R = Airway resistance (cm H20/L/s) R determined by Poiseuille equation: R=8 ηL π r4 – R = resistance – η = viscosity of inspired air – L = length of airway – r = airway radius Primary determinant of resistance is? radius In healthy individuals, radius is large enough that resistance remains low 32  What can change airway resistance? ANS determines smooth muscle tone and therefore, airway radius – PNS constricts bronchial smooth muscle – SNS relaxes bronchiole smooth muscle Lung volume and air viscosity will also influence airway resistance 33 Airway resistance in pulmonary disease For obstructive disease (asthma, chronic bronchitis, emphysema), airway resistance increases due to: 1. thickening of airway walls chronic bronchitis, asthma reduces lumen diameter 2. excessive secretions in airway lumen All obstructive diseases 3. loss of elastic elements Emphysema contribute to loss of airway support and causes premature collapse of airways 34  Loss of lung elasticity leads to premature lung collapse Interconnected alveoli Alveolus starts to collapse Alveolus continues to collapse Fewer surrounding alveoli Not enough elastic recoil get stretched to re-inflate alveolus 35