Mechanics of Breathing (Part I) PDF

WSUSOM Medical Physiology – Respiratory Physiology Page | 1 of 20 Mechanics of Breathing MECHANICS OF BREATHING Learning Objectives - Upon completion of this session, the student will be able to: 1. Define compliance...

WSUSOM Medical Physiology – Respiratory Physiology Page | 1 of 20 Mechanics of Breathing MECHANICS OF BREATHING Learning Objectives - Upon completion of this session, the student will be able to: 1. Define compliance and identify two common clinical conditions in which lung compliance is higher or lower than normal. 2. Draw a normal pulmonary pressure-volume (compliance) curve (starting from residual volume to total lung capacity and back to residual volume), labeling the inflation and deflation limbs. Explain the cause and significance of the hysteresis in the curves. 3. Define surface tension and describe how it applies to lung mechanics, including the effects of alveolar size and the role of surfactants. Define atelectasis and the role of surfactant in preventing it. 4. Describe the principal components of pulmonary surfactant and explain the role of each component. 5. Describe the effects of airway diameter and turbulent flow on airway resistance. Describe how airway resistance alters dynamic lung compliance. WSUSOM Medical Physiology – Respiratory Physiology Page | 2 of 20 Mechanics of Breathing Lecture Outline I. Mechanics of Breathing A. Lung compliance  Definition and method to measure lung compliance  Determinants of lung compliance  Surface tension and surfactant  Gravity  Disease States B. Chest wall compliance  Determinants of chest wall compliance  Elasticity of wall tissue  Muscle integrity and innervation  Obesity C. Lung and chest wall compliance D. Dynamic lung compliance  Airway resistance and factors that determine airway resistance  Branching pattern of the trachea-bronchial tree  Lung volume  Dynamic compression of the airway  Control of airway smooth muscle  Airflow patterns WSUSOM Medical Physiology – Respiratory Physiology Page | 3 of 20 Mechanics of Breathing Determinants of Lung Compliance  The volume of air that enters or exits the lungs for a given change in pressure (∆V/∆P = compliance) is dependent in part on the properties of the lung and chest wall.  The figure shows the change in lung volume that occurs when intrapleural pressure changes. The upward and downward arrows indicate the inflation and deflation curves, respectively.  The difference between the inspiratory and expiratory curves is referred to as hysteresis which is the result of changes in surface tension created from the air-fluid interface in the alveoli.  Note that the curves flatten at high volumes as the lung tissue reaches its limit of elastic deformation.  Elastic behavior of the lung itself determined by the composition and arrangement of the collagen and elastin fibers of the lung. The construction of the lung is such that inflation of one alveoli tends to augment the inflation of adjacent alveoli (interdependence). These tissue factors account for about 1/3 of the compliance behavior of the lung.  The inspiratory curve is also flat at a very low lung volume because of the presence of increased surface tension.  The majority of static compliance behavior is determined by surface tension. Each alveolus is an air-water interface. Surface tension is a result of unequal attraction between gas molecules and liquid molecules. Water molecules will have more attraction for each other than for air molecules. Thus, there is a tendency to decrease the surface area of the air water interface (to ‘contract’). In an alveolus, this means that surface tension tends to promote deflation (collapse). Quantitatively, surface tension is responsible for 2/3 of the compliance behavior of the lungs.  The slope of the curve at any point is lung compliance. Note that we typically breath at approximately 50 % of total lung capacity. WSUSOM Medical Physiology – Respiratory Physiology Page | 4 of 20 Mechanics of Breathing Determinants of Lung Compliance – Surface Tension  The diagram on the top right shows that in addition to the smooth muscle that surrounds the small airway, the alveoli are surround by elastic fibers that allows the alveoli to expand and contract as air flows in and out this structure.  The figure also shows the presence of alveolar pores (bottom left). These pores serve to equalize pressure. Macrophages are also present at the level of the alveoli to phagocytize particulate matter that may be drawn into the alveoli.  Lastly, note the membrane that separates the alveoli from the pulmonary capillaries. The respiratory membrane consists of a single layer of squamous epithelium, type-I cells, surrounded by a basal lamina. Interspersed among the type-I cells are cuboidal type-II cells. These type II cells secrete surfactant which serves to reduce surface tension and promote alveolar inflation which will be addressed below. WSUSOM Medical Physiology – Respiratory Physiology Page | 5 of 20 Mechanics of Breathing Determinants of Lung Compliance – Surface Tension  Surface tension is a force that reduces the exposed surface of a liquid to the smallest possible area.  Surface tension is created at an air-liquid interface through the attraction of molecules in a liquid to each other. The molecules at the surface do not have other liquid molecules above them (see left figure above).  Molecules at the surface have an unbalanced attraction to molecules below the surface and next to them, but not above them. Thus, molecule at the surface experience a net pull downward. This attraction creates a surface film.  A subsurface liquid molecule has equal molecular attraction in all directions so it sees no "net" pull.  The presence of surface tension is clearly evident when a paper clip is placed on the surface of a column of water. Instead of sinking as one might anticipate the paper clip remains at the surface because of the presence of surface tension (see right figure above). WSUSOM Medical Physiology – Respiratory Physiology Page | 6 of 20 Mechanics of Breathing Determinants of Lung Compliance – Surface Tension The importance of surface tension at a gas-liquid interface is illustrated by the two static compliance curves illustrated above. The graph shows the effect of lung inflation with air versus saline.  The air inflated lung requires large positive pressures and exhibits hysteresis (i.e. the path followed during expiration is different from that taken during inspiration).  At low lung volumes, a greater pressure is required to produce a volume change when the lung is inflated with air versus saline. This implies that the air-fluid interface on the alveolar surface affects lung expansion.  The saline-inflated lung requires much less positive pressure and exhibits no hysteresis. Note that the slope of the P-V curve (i.e. compliance) is greater during saline inflation. This is the result of very low surface tension.  The diagram above does not show the compliance curve of a lung that does not contain surfactant. A lung without surfactant would be less compliant than the normal compliance curve shown above, and would be shifted to the right. WSUSOM Medical Physiology – Respiratory Physiology Page | 7 of 20 Mechanics of Breathing Determinants of Lung Compliance – Surfactant  Surface tension is a force generated by the physical - chemical nature of the interaction between the air- fluid interface in the alveoli.  This force has the effect of collapsing alveoli to the smallest possible surface area. This inward directed force accounts for approximately 2/3 of the inward elastic recoil of the lung.  This relationship is described by LaPlace’s Law (P = 2T/r) where T is the surface tension and r is the radius.  Surfactant gives the alveoli a variable surface tension, so that at small volumes the surface tension is low.  Surfactant is secreted by type II epithelial cells in the alveoli of normal lungs. The major component of surfactant is primarily a phospholipid (dipalmitoyl phosphatidylcholine). This molecule has a hydrophobic portion and a hydrophilic portion. This serves to orient the molecule at the air-water interface.  By reducing surface tension surfactant i) decreases surface tension ii) decreases work of breathing iii) increases compliance iv) stabilizes alveolar size v) dries the alveoli. WSUSOM Medical Physiology – Respiratory Physiology Page | 8 of 20 Mechanics of Breathing How does surfactant promote alveolar stability?  The diagram above shows the effect of surfactant on alveolar stability. To understand the diagram, we must re-consider Laplace’s Law: P = 2T/R  P is the intra-alveolar pressure, T is the tension of the alveolus (resistance that acts to preserve the integrity of the surface), and R is the radius of the alveolus.  In the example shown at the top right of the figure (labelled without surfactant), the alveoli are assumed to be without surfactant. Thus, the surface tensions are equal. Based on the above equation, you can see that the smaller alveolus will have a larger intraluminal pressure. Because gas flows from regions of high pressure to low pressure, gas will flow from the smaller alveolus to the larger alveolus. (One alveolus will be deflated or collapsed). When alveoli are collapsed, they are said to be atelectic.  The example on the bottom right of the figure shows the effects of surfactant on surface tension. First, look at the alveolus on the right. Note that the presence of surfactant decreased the surface tension. The result of this alteration is a significant decrease in the intra-alveolar pressure from 2 cmH2O to 1 cmH2O. The importance of this change is evident when you assess the pressure in the larger alveolus (left). In the larger alveolus, the intraluminal pressure is 1 cm H2O. Thus, gas will not flow from the smaller alveolus to the larger. Thus, deflation (atelectasis) is prevented. WSUSOM Medical Physiology – Respiratory Physiology Page | 9 of 20 Mechanics of Breathing Determinants of Lung Compliance – Surfactant  This figure on the left shows the effect of different alveolar sizes on surfactant concentration at the air- water interface.  Both alveoli have approximately the same amount of surfactant. Note that as the alveolus becomes smaller (right), the surface area of the alveolus is reduced and the layer of surfactant is concentrated at the air-water interface.  In the more expanded alveolus, surfactant is ‘spread out’. Thus, you would expect the alveoli to have different surface tensions because of the coverage of the alveolar surface with surfactant.  Given that the reduction in surface tension is dependent on the concentration of the surfactant on the surface, the figure on the right shows that surface tension is reduced the most when a monolayer film of surfactant molecules is squeezed close together in expiration. The surfactant film is produced when molecules enter the surface, a process called "adsorption."  During inspiration, the molecules in the film separate as the alveolar surface expands. During expiration, the molecules in the film are squeezed together as tightly as possible, and in fact, some are "squeezed out" of the film because there is no room for them. However, these "squeezed out" molecules remain "associated" with the film and they re-enter the film during the next inspiration, a process called "respreading." WSUSOM Medical Physiology – Respiratory Physiology Page | 10 of 20 Mechanics of Breathing Total lung compliance  As stated previously, compliance of the lung is defined as the change in lung volume divided by the change in lung distending pressure (CL = ∆V/∆P).  The example in the above figure displays normal lung compliance. A change in intrapleural pressure from -3 to – 8 cmH2O is evident. Therefore, the change in pressure is 5 cmH2O. In response to the change in pressure, the change in volume is 1 L (1000 ml). Therefore, lung compliance is 1 L/5 5 cmH2O = 0.2 L/cmH2O. WSUSOM Medical Physiology – Respiratory Physiology Page | 11 of 20 Mechanics of Breathing Pulmonary Compliance & Gravity  Gravity has an impact on the intrapleural pressure at functional residual capacity (i.e. resting position of the lung and chest wall immediately prior to inspiration).  Gravitational forces tend to push the upright lung downward and toward the chest wall. Therefore, at the base of the lung, the intrapleural space volume is less compared to the top of the lung. As was pointed out previously, if we decrease volume within an enclosed space the pressure will increase. Thus, the intrapleural pressure at the base of the lung is less negative as compared to the apex of the lung (i.e. – 2 cmH2O vs. – 9 cmH2O).  As a consequence of the difference in intrapleural pressure, the pressure across the lung (i.e. transpulmonary pressure) will also vary at the bottom compared to the top of the lung (i.e. + 2 cmH2O vs. + 9 cmH2O). Thus, because the transpulmonary pressure across the alveoli is greater at the top of the lung, the alveoli are more distended compared to the bottom of the lung at functional residual capacity (see lung shaded in purple above).  As a result of this difference in distension, the compliance of the alveoli differ at the top compared to the bottom of the lung. The alveoli at the top of the lung are less compliant because they are closer to their limit of elastic deformation at functional residual capacity.  As a result, during inspiration the change in volume is less at the top compared to the bottom of the lung for a given change in transpulmonary pressure (see graph on the right). Thus, the lung is less compliant at the apex compared to the base. WSUSOM Medical Physiology – Respiratory Physiology Page | 12 of 20 Mechanics of Breathing Pulmonary Compliance & Disease States  Compliance is increased by destruction of the elastin and collagen content of the lung. This occurs in obstructive lung disease (i.e. emphysema).  Several factors also decrease compliance including i) high lung volume ii) respiratory diseases (restrictive diseases) iii) alveolar edema iv) deficiency of surfactant.