Cell Membrane
39 Pulmonary Mechanics
January 09, 2008.
Divisions of lung air, ventilation rates How is air moved into and out of the lungs? 1. The main muscle of breathing is the diaphragm. It is a dome-shaped sheet of muscle separating the thoracic and abdominal cavities. The diaphragm is controlled by the phrenic nerve, from spinal segments C3, C4, and C5. It is inserted into the lower ribs and moves downwards as it contracts – forces the abdominal contents down and forward, and increases the thoracic volume. NOTE: As the diaphragm descends, it flattens and becomes less effective at producing a downward motion (and inspiration). o The diaphragm starts contracting in the horizontal direction (pulls the ribs in). o It is the vertically oriented sides of the diaphragm muscle that produce the downward motion, not the flat horizontal centre section. 2. Inspiration is assisted by the external intercostal muscles, which pull the ribcage up and forward. They increase the thoracic cavity volume like a “bucket handle.”
3. During heavy breathing induced by exercise, accessory muscles may assist inspiration. The Scalene elevates the first two ribs. The Sternomastoids elevate the sternum. NOTE: Inspiration is an active process requiring muscular work. NOTE: Expiration is usually passive, due to the recoil of the elastic system. o When breathing is heavy or under voluntary control, expiration may be active, using the abdominal muscles to force the abdomen in and the diaphragm up, and the internal intercostal muscles to force the rib cage inwards. How does the movement of the diaphragm and rib cage expand the lungs? We can model this process as a balloon in a box. How to expand a balloon – need high pressure inside balloon relative to outside balloon o Increase pressure inside balloon Balloon is stretchy – resists expansion Vectors of contraction – produce an inward pressure to resist the expansion force of the pressure inside the balloon Reach equilibrium between outward force of expansion of balloon and inward pressure of the stretchiness o Lower pressure outside (intrapleural pressure), keep pressure inside balloon constant Balloon expands Expanding the box expands the balloon. However, the air inside the box (and outside the balloon) also expands (air is compressible), so some of the box volume change is not transmitted to the balloon. This problem can be remedied by replacing the air in the box with liquid, which is incompressible. So the real system looks more like this: The outer surface of the lungs is covered by a pleural membrane as is the inner surface of the thoracic cavity. The space between is the intrapleural space. The intrapleural space is filled with a few ml of fluid, which lubricates the sliding tissues as the lungs expand. Since liquid is incompressible, the lungs follow the volume changes of the thorax, even though the lungs are not fastened to the thoracic wall.
1
39 Pulmonary Mechanics
January 09, 2008.
Divisions of lung air, ventilation rates o If move chest wall out and diaphragm down, the lung volume must expand because the volume in intrapleural space cannot change. Pneumothorax An accidental puncture of the chest wall (e.g. knife wound) can allow air to enter the intrapleural space. This condition is known as a pneumothorax (air = pneumo inside the thorax), and illustrates a further point about pulmonary mechanics.
The pneumothorax condition illustrates some important points. 1. The resting volumes of the lungs and thorax. When a pneumothorax occurs, the lungs shrink to their minimum volume (like an empty balloon), and the thoracic walls spring outwards to their resting position (about 1 L more than normal). 2. The intrapleural fluid coupling of lungs and thoracic wall. Because there is no liquid coupling the lungs and thoracic wall, moving the thoracic walls with the respiratory muscles has no effect of the lungs. NOTE: The quick remedy for pneumothorax is a wet dressing on the wound to act as a one-way valve (lets gas out, but no gas in) and positive-pressure at the mouth to inflate the lungs (increase the pressure into the lungs to expand the lungs and force air out of intrapleural space). 3. The intrapleural pressure is normally below atmospheric pressure (negative). The opposing forces (pressures) of the lungs and thoracic wall create a negative pressure. Atmospheric pressure is used as a reference zero pressure, so the intrapleural pressure is negative with respect to atmospheric pressure (about -5 cmH2O). 4. The tug-of-war between the lungs pulling in and the thoracic wall pulling out is in equilibrium and sets the resting volume of the lungs (FRC). No muscles are needed to maintain FRC (relaxed). Need to maintain large FRC by maintaining elastic properties of lungs. How do we specify the elastic properties of the lungs? Suppose that we tested the elastic properties of the lungs in an experiment on the lungs alone, out of the thoracic cavity:
2
39 Pulmonary Mechanics
January 09, 2008.
Divisions of lung air, ventilation rates NOTE: To expand the lung volume, the intrapleural pressure decreases below atmospheric pressure (becomes more negative). o (maintain high pressure inside lungs relative to the pressure in the intrapleural space) The slope of the line relating lung volume (V) to the intrapleural pressure (P) defines the recoil or elastic property of the lungs and is called lung compliance (CL). o CL = ΔV/ΔP o Compliance defines to what degree the lungs will comply by changing its volume when subjected to a change in intrapleural pressure. o Normally the lungs are highly compliant (200 ml/cmH2O); even when coupled to the thoracic wall, the compliance of the system is still high (100 ml/cmH2O). NOTE: the recoil of the lungs is inversely proportional to the lung compliance. o Higher compliance, less recoil The compliance of the lungs is critical to breathing. Too low a compliance o Lungs are hard to inflate. The work of breathing is increased. o Recoil is increase – FRC is decreased o Example – pulmonary fibrosis (damaged lung tissue from smoke inhalation grows back as scar tissue), where the thoracic wall is pulled in (the lungs win the tug-of-war) and inspiration is difficult. Too great a compliance. o Lungs easy to inflate. o Little recoil once the lungs are inflated. o Expiration is difficult because expiration is produced by the passive recoil of the lungs. Also, the thoracic wall wins the tug-of-war and the lung and thoracic volume expands so that the diaphragm flattens and becomes inefficient. Expiration takes a long time, and inspiration occurs before expiration finishes – volume in lungs continually increases Recoil increases – get the same amount of volume out as we put in o Example – pulmonary emphysema (one of the possible terminal stages of death by smoking). These people have barrel chests because of the expanded volume. What factors determine lung compliance? 1. Elastin fibre network. Makes the lung tissue “stretchy.” Functions like a knitted garment, “stretchy” not just because of the fibres, but also because of the “interaction” effect, which gives the whole garment the ability to deform. These fibres become more easily stretched with age – lung compliance increases with increasing age (like old rubber bands that no longer “snap back”). 2. The surface tension of the fluid lining the alveoli tends to shrink the alveolar volume. The surface tension if the fluid lining the alveoli is shown here as vectors. The tendency of these force vectors is to shrink the volume of the alveolus. The pressure inside the alveolus increases. o It is this pressure that must be overcome to inflate the alveolus. When this effect for all alveoli is added together, the effect is to make the lung tissue behave elastically. Part of the work of breathing is the effort it takes to overcome the elastic forces of the system, but there is another factor. Pulmonary Airflow In order to move air in and out of the lungs through the airways, there must be a pressure gradient. Air moves from high pressure to low pressure. o During inspiration: intra-alveolar pressure < atmospheric pressure at the mouth. o During expiration: intra-alveolar pressure > atmospheric pressure at the mouth. intra-alveolar = inside the alveolar space intrapleural = inside the pleural space, outside the alveolar space
3
39 Pulmonary Mechanics
January 09, 2008.
Divisions of lung air, ventilation rates Types of Airflow Laminar the flow is linearly proportional to the pressure the constant of proportionality is the airway resistance RA. o RA = ΔP/Flow Turbulent the airway resistance is not a constant, dependent on the airway geometry, but varies directly with flow itself. Turbulent flow is very energy costly.
Intermediate In the human airway, the flow is partly turbulent at the bifurcations, but usually laminar.
NOTE: Laminar flow is silent. Turbulent flow is noisy and can be heard through a stethoscope. Airway Resistance to Flow The relation between the pressure gradient from the mouth to the alveolar gas exchange volume and flow through the airway can be shown as:
Since the flow in the human airway is mostly laminar, the Hagan-Poiseuille law applies: o RA ∝ 1/Diametre4 o Normal RA is 0.03 cm H2O/L/min Airway diameter is extremely important in determining airway resistance to flow. o At first glance, it might seem that the airway resistance was mostly due to the terminal airways (alveolar sacs and lobules) which are very narrow, but remember that they are also very numerous and the total flow is shared between them. Airway resistance at the terminal airways are not large Flow is divided so many times, that the flow at smaller diameters is smaller o In other words, the total diameter of all the terminal airways is very large, so most of the airway resistance is produced at the segmental bronchi level. However, the law illustrate that even small changes in airway diameter can have a large effect on flow. o A small mucus plug or the small change in airway diameter during normal breathing affects airway resistance to flow (increases RA – hard to breathe). The work of Breathing The respiratory muscles generate pressures to overcome: o 1. the elastic forces of the lungs and thoracic cavity. o 2. the airway resistance to flow. When the effects of respiratory disease on lung compliance and/or airway resistance produce changes in respiratory muscle effort that overwhelm the respiratory muscles, outside help must be sought in the form of positive pressure ventilation. o The lungs are rhythmically inflated by a powerful machine. o When even this intervention fails to provide sufficient gas exchange, the last resort is a lung transplant.
4
January 09, 2008.
Divisions of lung air, ventilation rates How is air moved into and out of the lungs? 1. The main muscle of breathing is the diaphragm. It is a dome-shaped sheet of muscle separating the thoracic and abdominal cavities. The diaphragm is controlled by the phrenic nerve, from spinal segments C3, C4, and C5. It is inserted into the lower ribs and moves downwards as it contracts – forces the abdominal contents down and forward, and increases the thoracic volume. NOTE: As the diaphragm descends, it flattens and becomes less effective at producing a downward motion (and inspiration). o The diaphragm starts contracting in the horizontal direction (pulls the ribs in). o It is the vertically oriented sides of the diaphragm muscle that produce the downward motion, not the flat horizontal centre section. 2. Inspiration is assisted by the external intercostal muscles, which pull the ribcage up and forward. They increase the thoracic cavity volume like a “bucket handle.”
3. During heavy breathing induced by exercise, accessory muscles may assist inspiration. The Scalene elevates the first two ribs. The Sternomastoids elevate the sternum. NOTE: Inspiration is an active process requiring muscular work. NOTE: Expiration is usually passive, due to the recoil of the elastic system. o When breathing is heavy or under voluntary control, expiration may be active, using the abdominal muscles to force the abdomen in and the diaphragm up, and the internal intercostal muscles to force the rib cage inwards. How does the movement of the diaphragm and rib cage expand the lungs? We can model this process as a balloon in a box. How to expand a balloon – need high pressure inside balloon relative to outside balloon o Increase pressure inside balloon Balloon is stretchy – resists expansion Vectors of contraction – produce an inward pressure to resist the expansion force of the pressure inside the balloon Reach equilibrium between outward force of expansion of balloon and inward pressure of the stretchiness o Lower pressure outside (intrapleural pressure), keep pressure inside balloon constant Balloon expands Expanding the box expands the balloon. However, the air inside the box (and outside the balloon) also expands (air is compressible), so some of the box volume change is not transmitted to the balloon. This problem can be remedied by replacing the air in the box with liquid, which is incompressible. So the real system looks more like this: The outer surface of the lungs is covered by a pleural membrane as is the inner surface of the thoracic cavity. The space between is the intrapleural space. The intrapleural space is filled with a few ml of fluid, which lubricates the sliding tissues as the lungs expand. Since liquid is incompressible, the lungs follow the volume changes of the thorax, even though the lungs are not fastened to the thoracic wall.
1
39 Pulmonary Mechanics
January 09, 2008.
Divisions of lung air, ventilation rates o If move chest wall out and diaphragm down, the lung volume must expand because the volume in intrapleural space cannot change. Pneumothorax An accidental puncture of the chest wall (e.g. knife wound) can allow air to enter the intrapleural space. This condition is known as a pneumothorax (air = pneumo inside the thorax), and illustrates a further point about pulmonary mechanics.
The pneumothorax condition illustrates some important points. 1. The resting volumes of the lungs and thorax. When a pneumothorax occurs, the lungs shrink to their minimum volume (like an empty balloon), and the thoracic walls spring outwards to their resting position (about 1 L more than normal). 2. The intrapleural fluid coupling of lungs and thoracic wall. Because there is no liquid coupling the lungs and thoracic wall, moving the thoracic walls with the respiratory muscles has no effect of the lungs. NOTE: The quick remedy for pneumothorax is a wet dressing on the wound to act as a one-way valve (lets gas out, but no gas in) and positive-pressure at the mouth to inflate the lungs (increase the pressure into the lungs to expand the lungs and force air out of intrapleural space). 3. The intrapleural pressure is normally below atmospheric pressure (negative). The opposing forces (pressures) of the lungs and thoracic wall create a negative pressure. Atmospheric pressure is used as a reference zero pressure, so the intrapleural pressure is negative with respect to atmospheric pressure (about -5 cmH2O). 4. The tug-of-war between the lungs pulling in and the thoracic wall pulling out is in equilibrium and sets the resting volume of the lungs (FRC). No muscles are needed to maintain FRC (relaxed). Need to maintain large FRC by maintaining elastic properties of lungs. How do we specify the elastic properties of the lungs? Suppose that we tested the elastic properties of the lungs in an experiment on the lungs alone, out of the thoracic cavity:
2
39 Pulmonary Mechanics
January 09, 2008.
Divisions of lung air, ventilation rates NOTE: To expand the lung volume, the intrapleural pressure decreases below atmospheric pressure (becomes more negative). o (maintain high pressure inside lungs relative to the pressure in the intrapleural space) The slope of the line relating lung volume (V) to the intrapleural pressure (P) defines the recoil or elastic property of the lungs and is called lung compliance (CL). o CL = ΔV/ΔP o Compliance defines to what degree the lungs will comply by changing its volume when subjected to a change in intrapleural pressure. o Normally the lungs are highly compliant (200 ml/cmH2O); even when coupled to the thoracic wall, the compliance of the system is still high (100 ml/cmH2O). NOTE: the recoil of the lungs is inversely proportional to the lung compliance. o Higher compliance, less recoil The compliance of the lungs is critical to breathing. Too low a compliance o Lungs are hard to inflate. The work of breathing is increased. o Recoil is increase – FRC is decreased o Example – pulmonary fibrosis (damaged lung tissue from smoke inhalation grows back as scar tissue), where the thoracic wall is pulled in (the lungs win the tug-of-war) and inspiration is difficult. Too great a compliance. o Lungs easy to inflate. o Little recoil once the lungs are inflated. o Expiration is difficult because expiration is produced by the passive recoil of the lungs. Also, the thoracic wall wins the tug-of-war and the lung and thoracic volume expands so that the diaphragm flattens and becomes inefficient. Expiration takes a long time, and inspiration occurs before expiration finishes – volume in lungs continually increases Recoil increases – get the same amount of volume out as we put in o Example – pulmonary emphysema (one of the possible terminal stages of death by smoking). These people have barrel chests because of the expanded volume. What factors determine lung compliance? 1. Elastin fibre network. Makes the lung tissue “stretchy.” Functions like a knitted garment, “stretchy” not just because of the fibres, but also because of the “interaction” effect, which gives the whole garment the ability to deform. These fibres become more easily stretched with age – lung compliance increases with increasing age (like old rubber bands that no longer “snap back”). 2. The surface tension of the fluid lining the alveoli tends to shrink the alveolar volume. The surface tension if the fluid lining the alveoli is shown here as vectors. The tendency of these force vectors is to shrink the volume of the alveolus. The pressure inside the alveolus increases. o It is this pressure that must be overcome to inflate the alveolus. When this effect for all alveoli is added together, the effect is to make the lung tissue behave elastically. Part of the work of breathing is the effort it takes to overcome the elastic forces of the system, but there is another factor. Pulmonary Airflow In order to move air in and out of the lungs through the airways, there must be a pressure gradient. Air moves from high pressure to low pressure. o During inspiration: intra-alveolar pressure < atmospheric pressure at the mouth. o During expiration: intra-alveolar pressure > atmospheric pressure at the mouth. intra-alveolar = inside the alveolar space intrapleural = inside the pleural space, outside the alveolar space
3
39 Pulmonary Mechanics
January 09, 2008.
Divisions of lung air, ventilation rates Types of Airflow Laminar the flow is linearly proportional to the pressure the constant of proportionality is the airway resistance RA. o RA = ΔP/Flow Turbulent the airway resistance is not a constant, dependent on the airway geometry, but varies directly with flow itself. Turbulent flow is very energy costly.
Intermediate In the human airway, the flow is partly turbulent at the bifurcations, but usually laminar.
NOTE: Laminar flow is silent. Turbulent flow is noisy and can be heard through a stethoscope. Airway Resistance to Flow The relation between the pressure gradient from the mouth to the alveolar gas exchange volume and flow through the airway can be shown as:
Since the flow in the human airway is mostly laminar, the Hagan-Poiseuille law applies: o RA ∝ 1/Diametre4 o Normal RA is 0.03 cm H2O/L/min Airway diameter is extremely important in determining airway resistance to flow. o At first glance, it might seem that the airway resistance was mostly due to the terminal airways (alveolar sacs and lobules) which are very narrow, but remember that they are also very numerous and the total flow is shared between them. Airway resistance at the terminal airways are not large Flow is divided so many times, that the flow at smaller diameters is smaller o In other words, the total diameter of all the terminal airways is very large, so most of the airway resistance is produced at the segmental bronchi level. However, the law illustrate that even small changes in airway diameter can have a large effect on flow. o A small mucus plug or the small change in airway diameter during normal breathing affects airway resistance to flow (increases RA – hard to breathe). The work of Breathing The respiratory muscles generate pressures to overcome: o 1. the elastic forces of the lungs and thoracic cavity. o 2. the airway resistance to flow. When the effects of respiratory disease on lung compliance and/or airway resistance produce changes in respiratory muscle effort that overwhelm the respiratory muscles, outside help must be sought in the form of positive pressure ventilation. o The lungs are rhythmically inflated by a powerful machine. o When even this intervention fails to provide sufficient gas exchange, the last resort is a lung transplant.
4
