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
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).
See Figure 44a: Before inspiration (