I) (B2 adrenergic stimulaticn causes powpful relaxation and thus greatly diminishes airway resistance histamne/SRS-A/ prostaglandins/irritant fumes/ channel blockers (eg . verapamil). can cause bronchospasm Bronchial asthma is a uery well known cause of bronchocb striction Bronchoconstriction also occurs in exposure to imiant gases, -and cigarette smoking IN if the denseity of the inspired air is high, the resistance to flow increases, flow volume curve (loop) We have,
previously discussed the lung compliance by means of a pressure volume curve (fig. 4 .2. 2) There we have shown that (i) as the pressure transpulmonary) increases the volume [of lung) rises, and also (ii)the volume of lung itself has an effect on the stretchability of the lung, at high lung volume, the lung refuses to distend easily The compliance test, which is one of the sophisticated lung functoins testis is thus a test for the elasticity of the lung Where the elasticity of the lung is reduced (eg emphysema) the pattern of the hysteresis loop of the compliance changes from the normal pattern There is, altogether, a very different kind of sophisticated lung function test which also gives a loop, but a different kind of loop, and is called a flow volume loop'and gives an indication of airway resistance, below, a flaw volume loop is described When a person is
exhaling (after taking a maximum passible inspiration) with maximum possible effort (as in vital capacity manuever}. it will bv seen that, (i) in the beginning (point 'a; in fig.4.2.5), the flow rate of air which is exiting is very high in a healthy adult, this may be. say, 12 liters per second (ii) afterwards, the flow rate drops sharply Mind thal by this time the lung volume has also been reduced As fig. 4 25 shows, as the volume of the lung reduces. the flow rate drops more and more, despite the fact that the effort fof eapiratoon) remains same Towards the end of the expiration the flow rate of the exiting air is remarkably re duced (In fact, towards the end of an expiratory phase despite the violent effort, the flow rate (the actual volume of air exhaled par second) is remarkably low (may be nil altogether). Similarly, during the inspiration (fig. 4. 25). the flow rate of the entering air is high in the beginning but unllike thal in the expiratory phase it is sustained for some time then declines
although the lungs are still increasing in volume That it, at low lung volumes, flow rate of the entering air is very high but at high lung volume, the flow rate of the entering air is low despite the fact inspiratory effort continues. All these can be graphically shown as in fig. 4.2. 5 Fig. 4 .2.5 Flow volume loop Note(i) at low lung volumes, exit rate of an is very low (may be nil even), (ii) at high volume, air entry is very poor despite the inspiratory effort This type of loop is called flow volume loop because of obvious reasons the interrelationship between the volume (of Fung) and the (rate of) air flow is depicted Explanation. Let us start with the phase of expiration When the lung) art maximally inflated, the elastic recoil is also maximum A intrapleural pressure (Ppl) is sharply ve -+ bronchial tubes are widely open expiratory effort causes high flaw rate As the lung volume falls Pp. is no more so intensely -ve (as the elastrc recoil, by now. has fallen) narrowing of thei bronchi-"increased airway resistance -+ fan of rate of flow A time comes when Ppl is almost atmospheric air flow rate now becomes nil. In the inspiralory phase similarly it can he shown that what is shown in fig. 4.2.5 is what is logically anticipated An additional fact to remember, however, is, skeletal muscles (diaphragm + external mtercoslals) generate graati tension in the initial phases of contraction but as the shortening proceeds, additional force is less and less generated This also contributes to the peculiarly of the shape of fig. 4.2.5 loop. The flow volume loop shows the airway resistance, in particular, the flow volume loop gives us information about the smaller bronchi It is the smaller bronchi that become narrow remarkably at the end of the expiration the larger tubes are not that much affected. Therefore, condition; where the smaller bronchi are prone to become narrow e.g. obstructive lung diseases like asthma and emphysema), the flow volume loop alters I its pattern For example, in a typical case of emphysema, the flow volume loop shows a hollowed out pattern in its descend-ing limb (fig. 4.2.5) which is almost diagnostic of obstruction (- rise of resistance) in the airways In short, rise in airway i resistance (e.g. emphysema) - abnormality of the expiratory part of flow volume loop (hollowed out' pattern) A little reflection will reveal that flow volume loop and compliance loop are not totally divorced phenomenon. Work done in breathing From' what has been stated in This chapter. it is clear, that to move air into [he lungs, the muscles of respiration have to do work to overcome all the different forms of resistances Actually, the following three types of resistances are the causes of work done by the muscles i) Elastic resistance Work has to be done to overcome elastic resistance (also called 'elastic recoil) Earlier in this chapter it has been stated that elastic recoil is mostly due to the presence of elastic fibers in the lung and surface tension in the alveoli Therefore, elastic resistance = force of elastic recoil everted by the lung elastic libers + force of surface' tension ii) Airway resistance. Already discussed it is the resistance offered against the flow of air
previously discussed the lung compliance by means of a pressure volume curve (fig. 4 .2. 2) There we have shown that (i) as the pressure transpulmonary) increases the volume [of lung) rises, and also (ii)the volume of lung itself has an effect on the stretchability of the lung, at high lung volume, the lung refuses to distend easily The compliance test, which is one of the sophisticated lung functoins testis is thus a test for the elasticity of the lung Where the elasticity of the lung is reduced (eg emphysema) the pattern of the hysteresis loop of the compliance changes from the normal pattern There is, altogether, a very different kind of sophisticated lung function test which also gives a loop, but a different kind of loop, and is called a flow volume loop'and gives an indication of airway resistance, below, a flaw volume loop is described When a person is
exhaling (after taking a maximum passible inspiration) with maximum possible effort (as in vital capacity manuever}. it will bv seen that, (i) in the beginning (point 'a; in fig.4.2.5), the flow rate of air which is exiting is very high in a healthy adult, this may be. say, 12 liters per second (ii) afterwards, the flow rate drops sharply Mind thal by this time the lung volume has also been reduced As fig. 4 25 shows, as the volume of the lung reduces. the flow rate drops more and more, despite the fact that the effort fof eapiratoon) remains same Towards the end of the expiration the flow rate of the exiting air is remarkably re duced (In fact, towards the end of an expiratory phase despite the violent effort, the flow rate (the actual volume of air exhaled par second) is remarkably low (may be nil altogether). Similarly, during the inspiration (fig. 4. 25). the flow rate of the entering air is high in the beginning but unllike thal in the expiratory phase it is sustained for some time then declines
although the lungs are still increasing in volume That it, at low lung volumes, flow rate of the entering air is very high but at high lung volume, the flow rate of the entering air is low despite the fact inspiratory effort continues. All these can be graphically shown as in fig. 4.2. 5 Fig. 4 .2.5 Flow volume loop Note(i) at low lung volumes, exit rate of an is very low (may be nil even), (ii) at high volume, air entry is very poor despite the inspiratory effort This type of loop is called flow volume loop because of obvious reasons the interrelationship between the volume (of Fung) and the (rate of) air flow is depicted Explanation. Let us start with the phase of expiration When the lung) art maximally inflated, the elastic recoil is also maximum A intrapleural pressure (Ppl) is sharply ve -+ bronchial tubes are widely open expiratory effort causes high flaw rate As the lung volume falls Pp. is no more so intensely -ve (as the elastrc recoil, by now. has fallen) narrowing of thei bronchi-"increased airway resistance -+ fan of rate of flow A time comes when Ppl is almost atmospheric air flow rate now becomes nil. In the inspiralory phase similarly it can he shown that what is shown in fig. 4.2.5 is what is logically anticipated An additional fact to remember, however, is, skeletal muscles (diaphragm + external mtercoslals) generate graati tension in the initial phases of contraction but as the shortening proceeds, additional force is less and less generated This also contributes to the peculiarly of the shape of fig. 4.2.5 loop. The flow volume loop shows the airway resistance, in particular, the flow volume loop gives us information about the smaller bronchi It is the smaller bronchi that become narrow remarkably at the end of the expiration the larger tubes are not that much affected. Therefore, condition; where the smaller bronchi are prone to become narrow e.g. obstructive lung diseases like asthma and emphysema), the flow volume loop alters I its pattern For example, in a typical case of emphysema, the flow volume loop shows a hollowed out pattern in its descend-ing limb (fig. 4.2.5) which is almost diagnostic of obstruction (- rise of resistance) in the airways In short, rise in airway i resistance (e.g. emphysema) - abnormality of the expiratory part of flow volume loop (hollowed out' pattern) A little reflection will reveal that flow volume loop and compliance loop are not totally divorced phenomenon. Work done in breathing From' what has been stated in This chapter. it is clear, that to move air into [he lungs, the muscles of respiration have to do work to overcome all the different forms of resistances Actually, the following three types of resistances are the causes of work done by the muscles i) Elastic resistance Work has to be done to overcome elastic resistance (also called 'elastic recoil) Earlier in this chapter it has been stated that elastic recoil is mostly due to the presence of elastic fibers in the lung and surface tension in the alveoli Therefore, elastic resistance = force of elastic recoil everted by the lung elastic libers + force of surface' tension ii) Airway resistance. Already discussed it is the resistance offered against the flow of air
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