Pulmonary Mechanisms Flashcards
(46 cards)
Explain the piston model and how this related to the lungs as a breathing apparatus?
To help identify the forces involved in breathing, consider a simple model of a piston of a certain mass (M), moving down a cylinder. In this model, the piston is attached to the upper cylinder wall by a pair of springs. To move the piston down the cylinder, the applied force must be sufficient to initiate and accelerate the piston down the cylinder at some velocity. In the piston-cylinder model, the three main forces that oppose movement are 1) elastase of the springs, 2) frictional resistance between piston and cylinder wall, and 3) inertia of the piston. These same opposing forces are important in respiratory mechanics as previewed in the table below.
The respiratory muscles must general enough force to? Need what to do this? Where does this come from? How can the force of the respiratory muscles be changed? What are accessory muscles?
The lung resembles a reciprocating bellows pump powered by the respiratory muscles. For breathing, the respiratory muscles must generate sufficient force to overcome the forces that oppose movement of the lung-chest cage. Unlike the sinoatrial node that generates rhythmic contraction of the heart, the respiratory muscles lack inherent rhythmicity. They depend upon motor nerve impulses to initiate contraction and relaxation. The nerve impulses initiating contraction of the respiratory muscles originate in the medullary region of the brain stem. The force generated by the respiratory muscles (rate or strength of contraction) can be increased by increasing the frequency of discharge in individual motor units, activating additional motor units, or by calling upon the accessory muscles of respiration. Accessory muscles refer to muscles not normally used during eupneic (resting) breathing. The respiratory muscles can be divided functionally into two groups: the muscles responsible for inspirationand those involved in expiration.
What are the muscles of inspiration? Expiration?
The muscles of inspiration include the diaphragm, the external intercostals, and the accessory muscles. The diaphragm and external intercostals are the most important muscles of breathing. During inspiration, contraction of the diaphragm and external intercostal muscles enlarges the chest cavity. This action leads to an expansion and stretching of the lung. As the lung inflates, potential energy is stored in the stretched elastic structures of the lung such that with relaxation of inspiratory muscles, expiration occurs from elastic recoil of the lung-chest cage apparatus. Thus, at rest, expiration is normally considered to be passive because muscle contraction is not required.
What is the role of the diaphragm? structure? origin and insertion?
Besides being the principal muscle of inspiration, the diaphragm also separates the thoracic from the abdominal cavity to convert the thorax into a closed chamber. The diaphragm is composed of two muscular hemidiaphragms, joined by a membranous portion at the midline. The origin and insertion of the diaphragm is on the interior surface of the lower ribs and sternum. The center portion of the diaphragmnormally curves upward when relaxed at end-expiration. During contraction, the central portion of the diaphragmbecomes more flattened, similar to a piston moving down a cylinder. In an adult, the diaphragm normally descends about 1.5 cm, with as much as a 10-cm descent during a maximal inspiration. With the downward movement of the diaphragm, the volume of the thoracic cavity and the lungs increase. Normally about two thirds of the tidal volume is directly attributable to diaphragmatic contraction. Motor innervation to the diaphragm originates from the spinal cord at C3 through C5. These nerves join to comprise the phrenic nerve.
Explain the external intercostals? how do they pull? why is this important? their paralysis leads to?
In human, the 12 ribs articulate with the thoracic vertebrae, allowing them to rotate. The external intercostal muscles are attached between adjacent ribs and oriented such that their contraction elevates the anterior end of each rib, pulling it upward towards a horizontal plane. This action increases the antero-posterior diameter of the rib or thoracic cage. From side view, it resembles the action of a “pump handle”. While the importance of external intercostal muscles is overshadowed by the diaphragm, their contraction prevents the rib cage from being pulled downward and inward as the diaphragm descends during inspiration. Intercostal contraction also tenses or strengthens the intercostal spaces so they will not be drawn in by descent of the diaphragm. While the external intercostals can maintain a considerable level of breathing on their own, their paralysis does not prevent ventilation by the diaphragm alone. Innervation of the external intercostals is from the intercostal nerves that arise from the spinal cord at T1 through T12.
Explain the accessory muscles? when are they active? what do they do?
The accessory muscles of inspiration include the scalenes and sternocleidomastoids. These muscles are minimally active during eupnea. Contraction of the scalene muscles elevates the first ribs, whereas the sternocleidomastoids raise the sternum. The accessory muscles are usually inactive until ventilation reaches a fairly high rate of 50 to 100 L/min in an adult. Nevertheless, they are active during ventilation associated with heavy exercise. Additional accessory muscles of the back may also facilitate inspiration at high ventilatory rates.
What are the passive forces of expiration?
During quiet or eupneic breathing, expiration is usually passive. With contraction of the inspiratory muscles to bring about inspiration, elastic components of the lung are stretched. The potential energy stored in the elastic features of the lung is released with relaxation of inspiratory muscles. Elastic recoil of the lung is normally sufficient to bring about expiration without contraction of expiratory muscles. However, with high ventilatory rates, the muscles of expiration comprising the abdominal group and the internal intercostalsare important.
Explain the muscle groups associated with expiration? what are they innervated by? when are they used? The diaphragm does what in the beginning of expiration?
The principal abdominal muscles of the expiratory group include the external and internal oblique and the rectus and transverse abdominus. Contractions of these muscles increase intra-abdominal pressure to force the diaphragm upward. In normal adults, the abdominal muscles may not become active until minute volume is increased several folds from rest (i.e., >40 L/min). The abdominal muscles are innervated by nerve fibers emerging from T-1 through T-12. These muscle groups are inactivated early in anesthesia. However, the abdominal muscles are the principal muscles responsible for coughing and for forced lung expiratory volume measurements, like the vital capacity.
The other muscles of expiration are the internal intercostal. They are attached between adjacent ribs and act antagonistically to the external intercostals. Contraction of the internal intercostals compresses the rib cage to decrease the antero-posterior diameter of the thoracic cage. During coughing their contraction prevents bulging of the intercostal spaces.
The diaphragm is also a participant in expiration. Electrical recordings from the phrenic nerve indicate that the diaphragm continues to contract during the early part of expiration (see bottom figure). Continued diaphragmatic contraction during early expiration opposes some of the lung recoil and results in a slowing of expiration. It also ensures a smooth transition from inspiration to expiration.
explain pulmonary elasticity? elasticity opposes? compliance is what?
The lung is an elastic structure with an anatomical organization that promotes its collapse to essentially zero volume, much like an inflated balloon. The term elastic means a material deformed by a force tends to return to its initial shape or configuration when the force is removed. While the elastic properties of the lung are important to bring about expiration, they also oppose lung inflation. As a result, lung inflation depends upon contraction of the inspiratory muscles. How easily a lung inflates will relate to the compliance of the lung. Compliance is the preferred term to describe the elastic properties of the lung. Compliance is a measure of the ease of deformation (inflation).
explain the relationship of compliance to elasticity?
The lung is an elastic structure with an anatomical organization that promotes its collapse to essentially zero volume, much like an inflated balloon. The term elastic means a material deformed by a force tends to return to its initial shape or configuration when the force is removed. While the elastic properties of the lung are important to bring about expiration, they also oppose lung inflation. As a result, lung inflation depends upon contraction of the inspiratory muscles. How easily a lung inflates will relate to the compliance of the lung. Compliance is the preferred term to describe the elastic properties of the lung. Compliance is a measure of the ease of deformation (inflation).
Compliance = ‘ ‘V/’ ‘P
Low Compliance = Low Distensibility
Low Compliance = High Elastance
HIGH COMPLIANCE = EASY TO INFLATE
LOW COMPLIANCE = HARD TO INFLATE
NEW BALLOON = LOW COMPLIANCE = HARD TO INFLATE
OLD BALLOON = HIGH COMPLIANCE = EASY TO INFLATE
OLD LUNG = HIGH COMPLIANCE = EASY TO INFLATE
YOUNG LUNG = LOW COMPLIANCE= HARD TO INFLATE
EMPHYSEMA = HIGH COMPLIANCE = EASY TO INFLATE
PULMONARY FIBROSIS = LOW COMPLIANCE = HARD TO INFLATE
Explain the lung compliance curve?
If the lung is removed from the thoracic cage, it closely resembles a collapsed balloon. When pressure inside the lung equals outside pressure, or transmural pressure is 0, lung volume is close to zero. Compliance of the lung can be obtained by plotting lung relaxation or recoil pressure (x axis) as a function of lung volume (y axis). Starting from essentially zero lung volume, a measured volume of air is put into the lung and the recoil or relaxation pressure associated with the addition of that air volume is recorded. When repeated in several steps by the sequential addition of measured air volumes and recording of the corresponding recoil or relaxation pressures, a compliance curve for the lung can be constructed. The slope of this plot is lung compliance. Normally, lung compliance is measured under static conditions, meaning no airflow is present at the time the relaxation (recoil) pressure is measured.
Explain the chest compliance curve?
The chest cage also demonstrates compliance characteristics. However, the structure of the chest cage is quite different from the lung. The chest cage would be more analogous to a punctured tennis ball than a balloon. The bony, moderately inflexible rib cage dictates that the chest cavity has considerable air volume when inside and outside pressure are equal, or transmural pressure is 0. Starting at this initial or equilibrium volume of the thoracic cage, in the absence of the lung, it is theoretically possible, though totally impractical, to construct a compliance curve for the chest cage by the incremental addition or removal of measured air volumes. With the sequential addition of air to the chest cavity from its equilibrium volume, a positive (above atmospheric pressure) recoil pressure would result with each air volume addition. On the contrary, if the chest cage is returned to its equilibrium volume and a measured volume of air is removed, a “negative” (below atmospheric pressure) recoil or relaxation pressure would result. A “negative” recoil pressure reflects the tendency of the chest cage to re-expand or return to its equilibrium volume, which would be a larger volume. As more air is incrementally removed from the chest cage, the bony structure of the rib cage soon limits further chest cage compression. At this point, volume changes become minimal, but recoil pressures become increasingly “negative”, as indicated on the left side of the chest wall relaxation curve. As shown, both positive and “negative” recoil or relaxation pressures are plotted on the x axis.
The combined lung and chest compliance curves give?
The relaxation curve:
With the lungs placed insidethe chest cavity and their respective pleural surfaces held together by cohesive forces, the volume of the lung is higher than its equilibrium volume, whereas chest cavity volume is less than it’s equilibrium volume. Thus, the equilibrium volume of combined lung-chest cage represents a point where the tendency of the lung to deflate (to zero volume) is balanced by the tendency of the chest cage to expand. In other words, at the equilibrium volume of the combined lung-chest cage, the lung is expanded above and the chest cage is compressed below their respective equilibrium volumes. The equilibrium volume of the combined lung chest cage corresponds to resting end expiration, or the position (volume) the combined lungchest cage would assume when the respiratory muscles are relaxed. At resting end expiration, the recoil pressure of the lung tending to deflate is opposed to an equal but opposite recoil pressure of the chest wall tending to expand. These equal but opposite recoil forces are reflected by an intrapleural pressure that is typically below atmospheric pressure. Moreover, as a person inhales to expand the chest cage and lung, intrapleural pressure becomes increasingly subatmospheric, reflecting the force tending to separate the lung and chest wall, but this is prevented by the cohesive forces.
What are several important features about the relaxation curve?
The figure below shows separate relaxation curves for the lung (right) and the chest cage (left), along with the combined lung-chest cage relaxation curve (middle). Note that the combined lung-chest cage curve is the algebraic sum of the separate lung and chest cage curves. The slope of each relaxation curve corresponds to the compliance for the structure(s). At end expiration (point A), recoil or relaxation pressure for the lung (right) and chest cage (left) alone are equal but opposite. At this point, lung volume corresponds to functional residual capacity (FRC). As additional air volume is inhaled into the lung, the lung is stretched further and exhibits a greater recoil pressure. At the same time, the chest cage is less compressed, so its negative recoil pressure diminishes as it approaches its equilibrium volume. When a slightly larger air volume is inhaled, the chest cage reaches its equilibrium volume (0 relaxation pressure; point B) and the lung and lung-chest relaxation curves intersect (point C). Thereby, at this lung volume, all measured relaxation for the lungchest cage system is from the lung because the chest cage is at its equilibrium volume (point B), or the volume it would assume if the lung were not present. If an even greater air volume is inhaled (point D), both the lung and chest cage are stretched beyond their equilibrium volumes. Note that the compliance curve for the combined lung-chest cage becomes more flattened (less compliant) at this point because the lung and chest cage are both tending to recoil towards smaller equilibrium volumes. If the total lung-chest cage system is returned to resting end expiration (point A) and air is expelled, a negative relaxation pressure results for both the chest cage and the combined lung-chest cage (point E). At this point, the chest cage is compressed as more and more air is expelled, with the negative recoil pressure reflecting its tendency to expand towards its equilibrium volume (point B). At the same time, the lung contributes little positive relaxation pressure because it is close to its equilibrium volume (i.e., 0 volume) because it is stretched very little.
how do we compute total pulmonary compliance?
During eupneic breathing, the normal tidal volume range is between points A and B. During inspiration, the individual moves up the lung-chest cage compliance curve from Point A towards Point C and in the opposite direction during expiration. Note that over the normal tidal volume range, total pulmonary compliance (combined lung-chest cage) is less than the compliance of either the lung or chest cage alone. This occurs because the lung and chest cage are physically arranged in a serieswith one another. Compliances arranged in series are added as reciprocals to compute total compliance (lung-chest cage compliance), as shown in the equation below the figure.
How do we measure lung compliance in people?
Many pulmonary disorders can alter compliance of the lung, chest cage, or both. Thus, a measurement of compliance can be a useful clinical assessment of a patient’s respiratory system. To determine compliance of the respiratory system, changes in transmural pressures (Pin-Pout) immediately across the lung or chest cage (or both) are measured simultaneously with changes in lung or thoracic cavity volume. Changes in lung or thoracic cage volume are determined using a spirometer with transmural pressures measured by pressure transducers. For the lung alone, transmural pressure is calculated as the difference between alveolar (PA; inside) and intrapleural pressure (Ppl; outside). To calculate chest cage compliance, transmural pressure is Ppl (inside) minus atmospheric pressure (PB; outside). For the combined lung-chest cage, transmural pressure or transpulmonary pressure is computed as PA-PB. PA pressure is determined by having the subject deeply inhale a measured volume of air from a spirometer. The subject then exhales measured air volumes in several steps. After each expired air volume, the subject seals his or her mouth around a manometer and completely relaxes the respiratory muscles with the glottis open. The pressure recorded at the mouth is called relaxation or recoil pressure because the respiratory muscles are relaxed. In addition, the pressure measured at the mouth is the same as alveolar pressure (PA) because the airway is open to the alveoli and no air flow is present (static conditions). Intrapleural pressure is approximated by convincing the subject to swallow a balloon, attached to a pressure transducer, into the esophagus. Esophageal pressure closely reflects average intrapleural or intrathoracic pressure. Thus, compliance curves can be constructed for the lung, chest wall, and total pulmonary system, using the equations presented in the figure.
what are the equations for compliance in people?
What are some factors that affect chest wall compliance?
Chest wall compliance varies from one individual to another, depending on the diameter and shape of the chest and height of the individual. People with deformed chest or musculoskeletal disorders such as kyphoscoliosis often exhibit decreased rib cage mobility, which is manifested as a decrease in chest cage compliance. Likewise, obese people often have reduced chest cage compliance because abdominal fat impedes normal descent of the diaphragm and upward movement of the rib cage during inspiration. While body position does not alter chest wall compliance per se, it may result in a shift in the chest cage compliance curve due to the effect of gravity. This is likely to be encountered as one goes from the upright to the supine position.
What are some factors contributing to lung elasticity?
The lung is an organ that exhibits elastic characteristics. It is one of the few organs of the body whose size can and does change dramatically. When the lung is inflated by contraction of the inspiratory muscles, it is stretched and recoils back to the pre-stretched position upon relaxation of the inspiratory muscles. It is important to consider the factors that impart elasticity to the lung because several clinical disorders, such as pulmonary fibrosis and emphysema, are known to alter the elastic properties of the lung.
Where does lung elasticity come from?
It was thought for a long time that the elastic properties of the lung were accounted for by the stretching and recoil of individual elastin and collagen fibers that comprise much of the lung parenchyma. However, in isolation, lung elastin and collagen fibers exhibit only modest elasticity. The elastic properties of the lung appear to be related more to the weave of these fibers than to the stretching of the individual fibers. Thus, the lung is more like a knitted fabric, where elasticity is imparted to the fabric more by the geometric pattern of the weave or knit of the fibers than by the stretching of the individual fibers. However, experiments conducted by Von Neergaard revealed that the elastic properties of the lung are only partially accounted for by these elastin and collagen fibers.
Explain the Von Neergaard experiments?
In 1929, Von Neergaard performed some simple but ingenious experiments on lungs obtained from cats. The isolated cat lung was nearly maximally inflated with air, then relaxation or recoil pressure was recorded as measured volumes of air were sequentially removed from the lung. After constructing a relaxation curve for the air-filled lung, he degassed the lung and reinflated it to near maximum volume with a physiological saline solution. A second relaxation curve was constructed as measured volumes of saline solution were sequentially withdrawn. The results indicated that the recoil pressure at a particular lung volume (125 ml) was nearly twice as large in the air-inflated lungs than in the fluid-inflated lung. However, if the measured relaxation pressure was solely a result of stretching the elastic and collagen fibers, then recoil pressures at equivalent lung volumes should be the same regardless of whether the lung was inflated with air or a physiological fluid. Inflating the lung with a physiological fluid should not alter the recoil properties of the elastin or collagen fibers. Since this was clearly not the case, Von Neergaard surmised that the several hundred million alveoli comprising the lung are normally lined with a thin fluid film that separated alveolar air from alveolar cells (Type I cell). This air-liquid interface tends to retract or recoil much like a soap bubble because of the surface tension of the fluid film lining alveoli. The sum of all these recoil pressures resulting from surface tension of the fluid lining millions of alveoli contributes as much recoil pressure as the elastin and collagen fibers. However, when alveoli are filled with a physiological fluid, the air-liquid interface is replaced by a liquid-liquid interface that has essentially no surface tension. Thus, fluid-filled lungs only reflect the recoil forces attributable to stretching of elastic fibers, whereas air-filled lungs reflect recoil forces arising from both the elastic fibers and surface tension of the fluid film that lines the alveoli.
What is surface tension?
Surface tension is defined as a manifestation of attracting forces between atoms or molecules. It is measured in units of force/unit length such as dynes/cm. In a beaker of water, consider how attracting forces acting on water molecule A differ from those acting on water molecule B (see figure). The intermolecular attracting forces on molecule A are equal in all directions, whereas molecule B at the air-liquid interface exhibits an imbalance in attracting forces because few water molecules are present in the air phase above the beaker. This imbalance of attracting forces acting on molecule B result in it being pulled downward and sidewise more than upward. This imbalance of forces acts to shrink the liquid-air interface to the smallest possible area. Thus, the force of surface tension acts in the plane of the air-liquid boundary to shrink or minimize the liquid-air interface (see figure). However, unlike the flat interface present in the beaker, alveoli are nearly spherical in shape, so surface tension forces of the fluid film lining alveoli must be considered for a sphere.
Explain surface tension in a sphere? (LaPlace)
The fluid film lining alveoli assumes a spherical shape because of the nearly spherical nature of alveoli. In many regards, the fluid film lining alveoli resembles a soap bubble. In a soap bubble, the relationship between pressure and surface tension is related in an equation derived by LaPlace for spherical objects. The LaPlace equation states that the distending (inflating) or collapsing (recoil) pressure for a soap bubble is directly related to surface tension of the air-fluid interface and inversely related to the radius of the sphere.
explain how to use the law of laplace to see alveolar instability?
If alveolar surface tension is assumed to be constant, say 50 dynes/cm, then the recoil (collapsing pressure) could be calculated for alveoli of different radii using the LaPlace equation (see top figure). Using arbitrary units for radius and pressure, the calculations indicate that alveolus A, with a radius of 1 unit, has a higher pressure than alveolus B, with a radius of 2 units. Since all lung alveoli are interconnected by the airways, the higher pressure in alveolus A would cause it to discharge its air volume into alveolus B. According to the LaPlace equation, as the radius of alveolus A becomes progressively smaller as air moves to alveolus B, pressure in alveolus A steadily increases as the radius decreases with deflation. At the same time, pressure in alveolus B declines further because its radius increases as it fills with air. The implications for the lung as a whole are that alveoli with smaller radii (high pressure) would empty their air volume into larger alveoli and eventually collapse. In a short time, most of the air in the lung would be contained in a few large-radius alveoli, with most remaining alveoli collapsed. Such alveoli instability would result in a marked reduction in surface area for gas exchange.
