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Thread: compliance curve of the lung ? plz help

  1. #1
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    Feb 2009
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    Default compliance curve of the lung ? plz help

    hi , i am studying form : Guyton & Hall: Textbook of Medical Physiology

    and my Question is :

    why the change in volume of the lung in expiration is much higher than the inspiration in the same giving pressure ?

    i can understand that the volume we inhale is the same that we exhale , but why the change in volume is different , that i cant understand , they told me , i should think about it like blowing up balloon , but this make the matter even harder for me ...

    so help

  2. #2
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    Apr 2008
    Mauritius!!! SSRMC
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    Resistance / compliance

    In the absence of respiratory effort, the lung will come to lie at the point of the FRC. To move from this position and generate respiratory movement, two aspects need to be considered which oppose lung expansion and airflow and therefore need to be overcome by respiratory muscle activity. These are the airway resistance and the compliance of the lung and chest wall.

    Resistance of the airways describes the obstruction to airflow provided by the conducting airways, resulting largely from the larger airways (down to division 6-7), plus a contribution from tissue resistance resulting produced by friction as tissues of the lung slide over each other during respiration. An increase in resistance resulting from airway narrowing such as bronchospasm leads to obstructive airways disease.

    Compliance denotes distensibility (stretchiness), and in a clinical setting refers to the lung and chest wall combined, being defined as the volume change per unit pressure change. When compliance is low, the lungs are stiffer and more effort is required to inflate the alveoli. Conditions that worsen compliance, such as pulmonary fibrosis, produce restrictive lung disease.

    Compliance also varies within the lung according to the degree of inflation. Poor compliance is seen at low volumes (because of difficulty with initial lung inflation) and at high volumes (because of the limit of chest wall expansion), with best compliance in the mid-expansion range.

    Work of breathing

    Of the two barriers to respiration, airway resistance and lung compliance, it is only the first of these, which requires actual work to be done to overcome it. Airway resistance to flow is present during both inspiration and expiration and the energy required to overcome it, which represents the actual work of breathing, is dissipated as heat.

    Although energy is required to overcome compliance in expanding the lung, it does not contribute to the actual work of breathing as it is not dissipated but converted to potential energy in the distended elastic tissues. Some of this stored energy is used to do the work of breathing produced by airways resistance during expiration.

    The work of breathing is best displayed on a pressure-volume curve of one respiratory cycle which shows the different pathways for inspiration and expiration, known as hysteresis. The total work of breathing of the cycle is the area contained in the loop.

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