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Lecture 13

PHGY 210- Lecture 13- Dr. Lauzon.docx

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McGill University
PHGY 210
Ann Wechsler

Lecture Thirteen- Monday, February 1 , 2010st review of Compliance D. Dynamics of a breath From a mechanical point of view, the respiratory system may be regarded as a pump with elastic, flow-resistive and inertial properties (figure 40). At rest, the lungs are at FRC and Ppl is negative due to the opposite forces acting on the lungs and chest wall. During inspiration, the diaphragm contracts and the chest wall is pulled open. This creates a more negative Ppl that causes expansion of the lungs (figure 41). see pictures in slide Flow = F = [P(alv)-P(atm)]/R As the lungs are pulled further away from their resting position (which is below RV), Ppl becomes even more subatmospheric (figure 42). As the volume of the lungs is increased, gas in the lungs is decompressed. The pressure in the alveoli (Palv) drops below atmospheric pressure. The created negative pressure gradient between the alveoli and atmosphere generates air flow to the lungs. As inspiration proceeds, the lungs are filling up with air, and the pressure gradient and the air flow gradually decrease. At the end of inspiration air flow stops because Palv is equal to atmospheric pressure (no pressure gradient). At the onset of expiration, the diaphragm relaxes, elastic recoil of the respiratory system compresses the gas in the lungs, and Palv increases. The positive pressure gradient between the atmosphere and the lungs is reversed and air from the lungs is pushed out to the atmosphere. As lung volume decreases, Ppl slowly returns to its resting level. At the end of expiration, i.e. at FRC, air flow=0 ml/s and Palv=0 cmH2O, and Ppl is about -5 cmH20 (figure 42). see picture in slide •The time course of changes in pleural pressure during inspiration and expiration depends on contraction of the diaphragm and airway resistance. •The dashed area in the graph shows the amount of pleural pressure necessary to overcome airway (and tissue) resistance. E. Airway Resistance In order to have gas flow through the airways, the pressure at the airway opening (Pao) must be different to that in the alveoli (Palv). The resistance of the airways to gas flow (Raw) is the ratio of this pressure difference and the flow. Raw = (Palv – Pao)/Flow where flow is equal to a change in volume per unit of time. A large diameter airway can carry a large flow for a given pressure difference and so has a smaller resistance than a small diameter airway. Airway resistance is therefore related to airway caliber and is an important determinant of lung function. In certain diseases (such as asthma) airway resistance can become very high making breathing difficult. Dynamic compression of airways See Figure 43: When a subject inspires to TLC and exhales to RV, during expiration, flow rises very rapidly to a high value and then declines over the rest of expiration. The descending portion of the flow-volume curve is independent of effort because of the compression of the airways by intrathoracic pressure (see figure 44a). (Reproduced from West: Respiratory Physiology- the essentials). Forced expiration See Figure 44a: Before inspiration (
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