THE MECHANICS of RESPIRATION. Introduction

Transcription

1 THE MECHANICS of RESPIRATION Dr. James Duffin Departments of Physiology and Anaesthesia General Learning Objectives: 1. How is air moved into and out of the lungs? 2. What mechanical factors affect the breathing effort? 3. What changes in the mechanical factors cause respiratory failure? Specific Learning Objectives: 1. How is FRC maintained? 2. What is lung compliance, and what factors affect it? 3. What is thoracic wall compliance, and what factors affect it? 4. How do changes in these compliances affect FRC? 5. How do changes in these compliances affect breathing effort? 5. What is airway resistance, and what factors affect it? 6. How do changes in airway resistance affect breathing effort? Introduction Respiratory gas exchange is limited by the ability of the respiratory muscles to ventilate the lungs, and ventilation fails when the muscles cannot cope with the load imposed by the mechanics of the pulmonary system. It is important to understand the mechanics of breathing, not only to diagnose how the system has failed, and prescribe the correct treatment for the failure, but also to be able to provide the most effective temporary respiratory support using a mechanical ventilator. In order to understand the mechanics of respiration it is necessary to construct a mental picture or model of the way in which the mechanical elements are assembled, and this process requires time and effort on the part of the student to achieve comprehension. To this end, these notes present several different ways of thinking about respiratory mechanics, as well as supplementary information (Mechanics Supplementary Notes), so that students may choose a method of understanding suited to their individual aptitudes. The Mechanics of Respiration Page 1

2 There are some guiding principles that should be kept in mind as these notes are studied. The first group relates to the consequences for pulmonary mechanical function of the anatomical structure of the lungs, and the second group describes the changes in the pulmonary mechanical factors that can lead to respiratory failure. They are listed immediately following. The consequences of anatomical structure: 1. The lungs and their thoracic container behave like springs. 2. The airways (and blood vessels) of the lung are collapsible tubes. Mechanical factors in respiratory failure: 1. Increased airway resistance. 2. Decreased lung compliance. 3. Increased lung compliance. The Respiratory Muscles The movement of air into the lungs is produced mainly by the diaphragm; the dome-shaped sheet of muscle separating the thoracic and abdominal cavities. The diaphragm, inserted into the lower ribs, contracts during inspiration forcing the abdominal contents downward and forward increasing the vertical size of the thoracic cavity. The diaphragm is controlled by the phrenic motoneurons in spinal segments C3-C5 whose axons make up the phrenic nerve. Afferent signals also travel in the phrenic nerve, but the diaphragm has few muscle spindles. Note that as the diaphragm descends, it flattens, decreasing its effectiveness. Further contraction results in the rib cage being pulled in rather than the lung volume being expanded. The Action Of The Diaphragm Muscle DIAPHRAGM The dome-shaped diaphragm muscle contracts during inspiration, forcing the abdominal contents downward and forward to increase the volume of the thorax. The Mechanics of Respiration Page 2

3 Expiration is usually due to passive recoil of the system, but in active expiration the abdominal muscles contract, forcing the abdominal contents inward and upward to decrease the volume of the thorax. The diaphragm is assisted by the external intercostal muscles, which pull the ribs upward and forward. This action not only increases the volume of the thoracic cavity, but also prevents the rib cage from being pulled inward as the diaphragm contracts. In infants with their undeveloped intercostal muscles the rib cage is pulled inwards during inspiration (paradoxical motion). A similar motion can be observed in chest injuries where a section of rib cage has broken free (flail chest). INTERNAL The Action of the Intercostal Muscles SPINE The contraction of the external intercostals muscles during inspiration pulls the ribs upward and outward to increase the volume of the thorax. EXTERNAL the volume of the thorax. During a forced expiration, the contraction of the internal intercostals muscles pulls the ribs downward and inward to decrease RIBCAGE SPINE The Ribcage Motion The ribcage rotates on the spine in a motion analogous to that of a bucket handle, thereby changing the diameters of the thorax in both the lateral and anterioposterior directions. The Mechanics of Respiration Page 3

4 In heavy exercise, accessory muscles such as the scalenes that elevate the first two ribs, and the sternomastoids that raise the sternum, may also assist inspiration. Expiration is usually passive, with the recoil of the elastic (compliant) components of the system emptying the lungs to the resting volume, the functional residual capacity (FRC). However, expiration may be assisted during heavy exercise or voluntary control by the abdominal muscles and the internal intercostals, which decrease the volume of the thoracic cavity. Although the expiratory muscles are capable of considerable effort, expiratory flow is limited, and is therefore of little value in overcoming conditions where the system recoil has been reduced (see the Flow-Volume curve in the Mechanics Supplementary Notes). The Balloon-in-a-box Model The lungs may be viewed as balloons inside a box with flexible walls. The walls are the ribcage and diaphragm, which together constitute the thoracic container. The respiratory muscles act on, as well as partly form, the thoracic container walls. The lung balloon is elastic, recoiling to a small volume at rest but expanding as pressure is applied. The box is also elastic, but its resting volume is large. Although this model illustrates the concept that applying pressures to elastic components causes their volume to change, it is a great simplification. One of the faults of this model is that the air inside the box and surrounding the lung balloon expands to a lower density as its pressure is decreased. The change in lung balloon volume is therefore less than the change in the box volume. P = atmospheric pressure The Balloon in a Box Model P P This model of lung inflation demonstrates the pressures that occur when the diaphragm descends. The pressure inside the box, but outside the balloon, decreases below atmospheric pressure as the volume of the box expands according to the ideal gas law. The balloon inflates because the pressure inside the balloon (atmospheric) is greater than the pressure outside the balloon. The Mechanics of Respiration Page 4

5 In the case of the real lungs, which almost fill the thoracic container, both the inside of the thoracic container and the outside of the lungs are covered with a pleural membrane. The narrow space between these pleural membranes, the intrapleural space, is filled with a few ml of liquid. This liquid serves to lubricate the sliding of lung lobes over each other as the lungs expand, and also serves another very important function. Because liquids are incompressible, their volume does not expand when they are subject to decreases in pressure. For this reason the lungs follow the movements of the thoracic container exactly, and the changes in lung volume match those of the thoracic container. The volume of air inside the lungs at rest is the FRC. This volume is the result of a balance of forces (pressures) between the lungs, whose relaxed volume is quite small, and the chest wall and diaphragm composing the thoracic container, whose relaxed volume is greater than the FRC. As a result of this balance of forces (pressures), the intrapleural pressure at rest is less than atmospheric pressure (about -5 cmh 2 O relative to an atmospheric pressure reference of cm H 2 O). P = P = P = -5 P = P = NORMAL PNEUMOTHORAX Intrapleural Pressure Normally the intrapleural pressure is about 5 cm H2O because of the balance of forces between the lungs and thoracic container, but in the case of a pneumothorax, the balance of forces is lost and the intrapleural pressure is zero (atmospheric). The Mechanics of Respiration Page 5

6 If air enters the intrapleural space (e.g. via a puncture wound), then the lungs collapse, the chest wall springs outward and the diaphragm descends as the intrapleural pressure becomes equal to atmospheric pressure. This situation is termed a pneumothorax (air inside the thorax), and the lungs no longer follow the movements of the thoracic container. Instead, the expansion of the thoracic container during inspiration results in air entering the intrapleural space via the puncture wound. During expiration the air exits via the puncture wound, and any build up of pressure in the intrapleural space results in a further compression of the lungs. The immediate treatment for this situation should now be apparent from a consideration of the mechanics learned so far. A wet dressing is applied to the puncture wound to act as a one-way valve, allowing air to leave the intrapleural space but not enter. A positive pressure inflation of the lungs via the mouth can then be used to expand the lungs to normal volume and re-establish gas exchange. The Two-Spring Model Both the lungs and the thoracic wall act as if they were springs. As the previous figure showed, the lung s resting volume is low, and the thoracic container s volume is high, so that when these are coupled by the intrapleural fluid, the system comes to rest at an intermediate volume, the FRC. The lungs are therefore partially filled and the thoracic walls are pulled inwards. This situation can be modelled using two springs. The lung spring is a short, coiled spring with a small resting length, and the thoracic wall spring is a flat spring with a resting position at volume V1. The figure below shows that when the two springs are coupled, the coiled lung spring is stretched and the flat thoracic wall spring is bent to a volume less than V1. The equilibrium position (volume), with the springs pulling equally in opposite directions, is the FRC. The Mechanics of Respiration Page 6

7 coiled lung spring flat thoracic wall spring coiled lung spring flat thoracic wall spring minimum volume V1 FRC PNEUMOTHORAX The Two-Spring Model NORMAL In this model, the horizontal position is related to lung volume. The pneumothorax shows the two springs in their relaxed positions. Normally the springs are coupled and pull in opposite directions, reaching an equilibrium position, the FRC. In this way the lungs are filled to the FRC without any muscular effort expended. Because the springs are pulling in opposite directions it is easy to see that the pressure in the intrapleural fluid space will be less than atmospheric (negative, if relative to atmospheric pressure = ). Maintaining the FRC, rather than allowing the lungs to empty completely during expiration, is an advantage for two reasons. First, the FRC is large with respect to tidal volume and therefore buffers the changes in alveolar gas tensions, which occur during a breath. Second, it is much easier to inflate the lungs when they are partially filled than when they are empty, because in empty lungs the surface tension of the fluid lining the alveoli causes them to stick closed, and extra pressure must be applied in order to inflate them. The FRC is maintained by the balance of forces (pressures) between the lungs and the thoracic wall, and is an important system parameter. If the FRC is too low, then ventilation/perfusion inequalities increases, and the lungs become harder to inflate. If the FRC is too high, the diaphragm becomes flattened and ineffective. It is therefore necessary to understand the factors that determine the strength of the springs in the two-spring model. The Mechanics of Respiration Page 7

8 Lung Compliance The strength of the lung spring is measured by a quantity called compliance. While it is usual to characterize springs in terms of their forcelength relation or spring constant; the lung is characterized by its pressurevolume relation or compliance curve. So: Lung Compliance = Change in Lung Volume Change in Intrapleural Pressure The figure below shows a graph of lung volume on the vertical axis plotted against intrapleural pressure on the horizontal axis. The figure also shows how such a graph might be obtained from excised lungs, where the influence of the thoracic container is absent. As the graph shows, the lung volume does not decrease completely to zero when the intrapleural pressure becomes zero. As the lungs collapse, so do the airways (remember they are collapsible tubes), trapping air within the alveoli before they can empty. While this airway closure occurs at small volumes in young healthy adults, it occurs at higher volumes with increasing age and in some disease states. So, in young healthy adults, the minimum lung volume, which may be achieved by a maximum expiratory effort, is determined by the limit of chest wall motion, but in older adults it may be determined by airway closure. VOLUME 1 % VC 1 % TLC 75 lung INTRAPLEURAL PRESSURE pump -4-2 cm H 2 O The Lung Volume vs. Intrapleural Pressure; Lung Compliance Curve This figure shows the change in lung volume that occurs for changes in intrapleural pressure without the influence of the thorax. The upward and downward arrows indicate the inflation and deflation curves respectively. The curves flatten at high volumes as the lung tissue reaches the limit of its elastic deformation. The slope of the curve at any point is the compliance of the lungs. The Mechanics of Respiration Page 8

9 Inflation of the lungs from their minimum volume requires an effort to separate the walls of fully closed alveoli at first because they are stuck together by the surface tension of the fluid lining the alveoli and airways. It is for this reason that the inflation and deflation curves differ, and this nonlinear phenomenon is termed hysteresis. The lungs do not normally empty completely, and so the deflation curve describes the changes in lung volume between the intrapleural pressures of -2 cm H 2 O and -1 cm H 2 O during normal breathing. In this range the curve approximates a straight line with a slope of about 2 ml/cm H 2 O. This slope is equal to the lung compliance. The lung compliance is determined by two main factors: 1. The elasticity of the lung tissues. 2. The surface tension of the fluid lining the alveoli. The effect of tissue elasticity is easy to understand. The network of supporting tissue in the lungs has elastin fibres in it, and these act as an elastic network, rather like the network of a knitted garment. With increasing age, the elastin fibres decrease in tone, thereby increasing the compliance of the lungs, and decreasing their recoil during expiration. When the lung tissue is damaged and fibrosis occurs, the lungs become stiffer and less compliant, making inspiration difficult. Using the two-spring model, you can see that in the first case the FRC will increase, and in the second case the FRC will decrease. The effect of surface tension is not as easy to understand. Consider the alveoli as fluid-lined bubbles or balloons. As we have all experienced, inflating a balloon becomes easier the bigger it gets. La Place's law applies, as it does for bubbles or fluid-lined alveoli. Pressure in the Bubble = 4 X Surface Tension of Film Radius of Bubble Since the surface tension of the bubble film is constant, the equation states that the pressure inside the bubble decreases as the size of the bubble increases. If such a situation applied to the alveoli, small alveoli would be harder to expand than large alveoli, and the small alveoli would tend to empty into the larger alveoli via connecting airways. This situation would lead to areas of ventilation/perfusion mismatch, and consequent hypoxemia. The Mechanics of Respiration Page 9

10 However, this situation does not occur in normal lungs due to the presence of surfactant (surface active agent), which modifies the surface tension of the fluid lining the alveoli. Alveolar cells secrete surfactant into the fluid lining, interposing its molecules between those of water, and lowering the surface tension. During the time taken for a breath, the amount of surfactant in an alveolus remains constant, so that as the alveolus expands the surfactant is spread more thinly, and its effect in reducing the surface tension becomes less. Thus, the surface tension of the fluid lining the alveoli varies directly with the radius (see the figure below). A substitution into La Place's law of a surface tension, which is a function of radius, shows that the pressure inside an alveolus does not vary much with alveolar size. r P varies as T/r But, T varies as r T P therefore: P is approx. constant as r varies LaPlace s Law for the Alveolus P = pressure inside the alveolus T = the surface tension of the fluid lining r = radius of the alveolus Nevertheless, there is a pressure generated by the surface tension effect, and it contributes to the tendency of the lungs to collapse. When the lungs are inflated, this pressure must be overcome, and so this is one of the factors determining the compliance of the lungs (see the figure below). In situations where surfactant production is deficient (e.g. respiratory distress syndrome in premature babies), not only is there an overall increase in the pressure required to inflate the lungs because of the lost surface tension reduction effect of surfactant, but there is also ventilation/perfusion mismatching due to the effect of La Place's law, which now applies. The Mechanics of Respiration Page 1

11 VOLUME ml SALINE AIR INFLATION PRESSURE cm H 2 O The Effect of Surface Tension Volume vs. pressure, compliance curves for lungs inflated with either air or saline. When the lung is inflated with saline there is no surface tension and the compliance is higher and there is no hysteresis. Thoracic Wall Compliance Consider the chest cavity without the lungs. The chest wall and the diaphragm form a container whose walls are composed partly of respiratory musculature and partly of other supporting structures. This thoracic wall is also an elastic structure characterized by its compliance. The pressure inside this container is the intrapleural pressure, and the volume of the container will vary with it. As the intrapleural pressure increases, the volume in the thoracic container will increase, and vice-versa. The figure below shows the relation between intrapleural pressure and the thoracic container volume (measured in terms of lung volume). VOLUME 1 % VC V1 1 % TLC 75 thorax wall INTRAPLEURAL PRESSURE pump 5-2 cm H 2 O The Thoracic Volume vs. Intrapleural Pressure; Compliance Curve This figure shows the change in thoracic wall volume that occurs for changes in the intrapleural pressure when the lungs are not present. When intrapleural pressure is zero, the relaxed volume is V1. The curve flattens at low volumes as the limits of wall collapse are reached. The slope of the curve at any point is the thoracic wall compliance. The Mechanics of Respiration Page 11

12 The resting volume for the thoracic container is large and called V1. Positive pressures (above atmospheric) inflate the thoracic container, while negative pressures (below atmospheric) deflate the thoracic container. In the region of quiet breathing between -2 cm H 2 O and -1 cm H 2 O intrapleural pressure, the curve approximates a straight line with a slope of 2 ml/cm H 2 O. This slope is the thoracic wall compliance, and is approximately equal to the lung compliance. The thoracic wall compliance is determined by two main factors: 1. The elasticity of the wall tissues. 2. The elasticity of external pressures applied to the wall. The elasticity of the tissues composing the thoracic wall is relatively unchanged until advanced age, but may be affected in extreme obesity. The second factor includes situations such as wrapping the chest with elastic bandages. Applying constant external pressures, like immersion in water or changing position (upright vs. supine alters the weight of the abdominal contents against the diaphragm) do not change the thoracic wall compliance, but do change the resting volume (V1). The Two-Compliance Model The two-compliance model is a more accurate representation of respiratory system mechanics than the two-spring model. This model allows us to determine the FRC and the muscular effort necessary to breathe. It also shows how these factors change with alterations in the compliance of the lungs, the compliance of the thoracic wall, and the resting volume of the thoracic wall (V1). The figure below shows the model that is produced by plotting both lung and thoracic wall compliance curves on the same graphical axes. The compliance curves show the intrapleural pressure for any particular lung or thoracic volume (both are measured in lung volume units). Since the resting volume of the thoracic container is high at V1, and that for the lungs is low, it can be seen that they are pulling in opposite volume directions (as the two-spring model demonstrates). The two-compliance system comes to rest at the FRC where the curves cross each other, because at this point the intrapleural pressure required to inflate the lungs and deflate the thoracic container is the same (about -5 cm H 2 O). The Mechanics of Respiration Page 12

13 1 % VC 5 V1 FR C 1 % TLC Elastic Muscular Effort The length of the horizontal lines joining the compliance curves in the graph shows the pressure that the respiratory muscles must generate in order to move the coupled lung and thorax wall system away from its equilibrium point (FRC). -2 cm H 2 O 2 Inspiratory effort Expiratory effort To displace the two-compliance model from its FRC equilibrium position, the respiratory muscles must generate changes in the intrapleural pressure by deforming the thoracic container. The model shows how much intrapleural pressure change the muscles must apply to reach any particular volume. During quiet breathing, as lung volume increases, the intrapleural pressure required to expand the lungs becomes more negative. Graphically, the horizontal distance between the vertical axis and the lung compliance curve shows the intrapleural pressure necessary to inflate the lungs. The thoracic wall supplies part of this pressure as it attempts to reach V1, its resting volume. Graphically, the horizontal distance between the vertical axis and the thoracic wall compliance curve shows this pressure exerted by the thoracic wall. The respiratory muscles supply the rest of the pressure to inflate the lungs. This muscle-generated pressure is shown graphically as the horizontal distance between the two compliance curves (see the previous figure). When tidal volume exceeds V1, the thoracic wall compliance no longer aids lung expansion but opposes it, so that the pressure change supplied by the respiratory muscles is the sum of the pressures required to inflate both the lungs and the thoracic container. The muscle-generated pressure to inflate the lungs is therefore still shown graphically as the horizontal distance between the two compliance curves. During a forced expiration, when lung volume decreases below FRC, the pressure required from the respiratory muscles is that required to decrease the thoracic container volume further away from its resting volume The Mechanics of Respiration Page 13

14 V1, minus the pressure assistance provided by the lungs trying to collapse. Graphically, this pressure is still shown by the horizontal distance between the two compliance curves (dotted lines). The graphical approach of the two-compliance model not only provides an estimate of FRC and respiratory effort under normal circumstances, but also predicts the changes occurring in disease states. In the figure below the effect of a decrease in lung compliance (e.g. fibrosis) is shown. Because the lungs are stiffer (lower compliance), they are harder to inflate, and the FRC is reduced. 1 1 % VC % TLC 5 V1 FRC FRC 75 5 Decreased Lung Compliance A decreased lung compliance (dotted curve) decreases the FRC and requires an extra muscular effort to breathe (horizontal dotted lines) 25-2 cm H 2 O 2 An Algebraic approach The two-compliance model can also be considered as two compliances in series (in analogy with capacitors in an electrical circuit), so that algebraically the two compliances combine to form one equivalent compliance of about 1 ml/cm H 2 O as shown below (also see the relaxation pressure-volume curve in the Supplementary Notes). 1 = C equivalent C lungs C thoracic wall The Mechanics of Respiration Page 14

15 Most textbooks use a model of respiratory mechanics with a single equivalent compliance to explain the work of breathing. While this simplification may be justified because it is difficult to determine the individual compliances of the thoracic wall and lungs in patients, the twocompliance model offers greater insight into the energetics of breathing (see Work of Breathing in the Mechanics Supplementary Notes). Airway Resistance While the compliances of the system determine its static behaviour, the frictional resistance to motion determines its dynamic behaviour. The most significant of these is the resistance to airflow through the airways of the lungs. Airway resistance is defined as follows: Airway Resistance = pressure difference between mouth & alveolar space total air flow through the airway The flow of gases through tubes like the airways of the lungs may be laminar, turbulent or transitional between the two as shown in the figure below. Laminar Flow Air flows in a tube in concentric layers (laminae) with the fastest flow in the centre and the slowest at the sides. Turbulent Flow Air flows in confused eddies. Although the general flow is in one direction, local flows may be in any direction. Transitional Flow Air flows in layers (laminae) mostly, but with eddies (minor turbulence) at tubing bifurcations. The Mechanics of Respiration Page 15

16 In laminar flow the pressure loss, which must be supplied by the respiratory muscles, is directly proportional to the flow, so that airway resistance is independent of flow. But in turbulent flow (e.g. asthma) the pressure loss is proportional to the square of the flow, so that airway resistance becomes a function of flow. In the normal human airway, because of its bifurcations, the flow is partly turbulent and partly laminar. Laminar flow is silent, and turbulent flow is noisy (can be heard with a stethoscope; see the Mechanics Supplementary Notes). Because airway flow is mostly laminar, the predominant factor determining airway resistance is the airway diameter. The Hagan-Poiseuille law applies so that: Airway resistance is proportional to: 1 (Airway Diameter) 4 Since airway resistance is an inverse function of the diameter to the fourth power, it is highly sensitive to changes in diameter such as those produced by excessive mucus secretions and bronchoconstriction. CROSS-SECTIONAL AREA (square cm) glottis TLC RV DISTANCE FROM MOUTH (cm) The Cross-sectional Area of the Airways A pulse of sound is applied at the mouth of the subject (open glottis). A sophisticated computer analysis of the echoes provides a measure of the total cross-sectional area of the airways at varying distances from the mouth. The graph shows that the area increases towards the alveoli, but only when the lungs are expanded (TLC) not when they are empty (RV); a consequence of the effect of intrathoracic pressure on airway diameter. The airways are collapsible tubes! The Mechanics of Respiration Page 16

17 Although it might at first appear that the terminal airways would offer the most resistance because they are narrow, the total flow is shared between many airways at this point, so that the total airway diameter available for flow actually increases towards the periphery of the lungs (see the figure above). This increasing total airway diameter means that the velocity of flow decreases too, and particles settle out. The chief site of airway resistance is the medium sized bronchi. The pressure loss due to airway resistance must be supplied by the respiratory muscles, as well as the pressure required to overcome the system compliance. It is these two factors that determine the muscular effort to breathe. The figure below shows the combined elastic and resistive forces (pressures), which must be overcome by muscular effort during quiet breathing (see also the Work of Breathing in the Mechanics Supplementary Notes). lung INSPIRATION thorax wall % VC lung EXPIRATION thorax wall Intrapleural Pressure cm H 2 O Muscular Effort of Quiet Breathing The lung and thoracic wall compliance curves for quiet breathing plotted together allow prediction of the muscular effort needed to breathe. The intrapleural pressure and lung volume follow the heavy curved lines in the direction of the arrows. The horizontal lines between the two compliance curves show the muscular effort causing the increase in volume during inspiration and slowing (braking) the decrease in lung volume during expiration. Note the added muscular effort required to overcome the airway resistance (left bulge) and tissue movement friction (right bulge) in inspiration compared to the elastic muscular effort alone. The Mechanics of Respiration Page 17

18 Although the diagram above shows how to predict the muscular forces from the compliance curves for the thorax wall and lungs and the airway resistance, it is useful to picture how these pressures are applied to an anatomical view. The figure below shows how those forces (pressures) are applied to the system. It also demonstrates how the airways (which are collapsible tubes) are held open by these pressures during normal breathing. PI = -5 PA = 5 End Expiration Airway opening pressure = 5 lung % VC 52 thorax wall PM = -1 PI = -8 PA = -2 6 During Inspiration Airway opening pressure = Intrapleural Pressure cm H 2 O lung thorax wall PM % VC =3 PM = Intrapleural Pressure cm H 2 O 32 The Mechanics of Respiration Page 18

19 PI = -7 PA = End Inspiration Airway opening pressure = 7 lung PM % VC 52 thorax wall PM = Intrapleural Pressure cm H 2 O 32.5 PI = -5 PA = 1 During Expiration Airway opening pressure = 5.5 lung % VC 52 thorax wall =5 PM = Intrapleural Pressure cm H 2 O 32 Pressures of the Respiratory Cycle The pressures exerted by the lungs and thoracic wall due to their compliances are shown as vertical arrows with their numerical values taken from their respective compliance curves. PI = intrapleural pressure; PA = intra-alveolar pressure; PM = muscle exerted pressure. Notice that the thoracic wall compliance and muscle pressures sum to equal the negative intrapleural pressure, as well as the algebraic sum of the lung compliance and intra-alveolar pressures. The horizontal arrows denote that the pressure exerted on the airway walls is such as to keep them open. The Mechanics of Respiration Page 19

20 Conclusion At this point in your studies you should have gained an understanding of how the lungs are actually ventilated, and the mechanical characteristics that affect the breathing effort. It should also be apparent why an increase in airway resistance, a decrease in lung compliance, and an increase in lung compliance can lead to respiratory failure. Since the subject requires the construction of conceptual models by the student in order to obtain such an understanding, it may be necessary to read these notes again. The Mechanics Supplementary Notes may offer additional insight, as well as extending the student's knowledge of respiratory mechanics. Further help is available from general physiology textbooks and specialised texts such as "Applied Respiratory Physiology" by J.F. Nunn (Butterworths). The Mechanics of Respiration Page 2