2_ Mechanics of breathing

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Allied Science Physiology 09-10. Respiratory System. Lecture 2
Allied Science Physiology. Respiratory System. Lecture 2
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• 
760 mm Hg at sea level
• 
Decreases as altitude increases
• 
Other lung pressures given relative to atmospheric (set Patm = 0
mm Hg)
• Pressure of air in alveoli (not closed system, opened))
• Given relative to atmospheric pressure
• Varies with phase of respiration
– During inspiration = negative
(less than atmospheric)
– During expiration = positive
(more than atmospheric)
• Difference between Palv and Patm
drives ventilation
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•  Pressure inside pleural sac (closed system)
–  Always negative under normal conditions
–  Always less than Palv
•  Varies with phase of respiration
–  At rest, -4 mm Hg
•  Negative pressure due to elasticity in lungs
and chest wall
–  Lungs recoil inward
–  Chest wall recoils outward
–  Opposing pulls on intrapleural space
–  Surface tension of intrapleural fluid hold wall
and lungs together
•  Transpulmonary pressure = Palv – Pip
•  Distending pressure across the lung wall
•  Increase in transpulmonary pressure:
–  Increase distending pressure across lungs
–  Lungs (alveoli) expand, increasing volume
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FRC = Functional Residual Capacity = volume of air in lungs between
breaths (defined as rest); Palv = Patm
Figure 16.8
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Flow =
Patm – Palv
R
• 
• 
• 
• 
• 
Atmospheric pressure constant (during
breathing cycle)
Therefore, changes in alveolar pressure
creates/changes gradients
Boyle’s Law: pressure is inversely
related
to volume in an airtight container (closed
system)
Thus – can change alveolar pressure
by changing its volume
R = resistance to air flow
–  Resistance related to radius of
airways and mucus
Boyle’s Law
Factors determining intra-alveolar pressure:
– 
Quantity of air in alveoli
– 
Volume of alveoli
Inspiration: lungs expand – alveolar
volume increases
Palv decreases
Pressure gradient: air into lungs
Quantity of air in alveoli rises
Palv increases
Expiration: lungs recoil – alveolar volume
decreases
Palv increases
Pressure gradient: air out of lungs
Quantity of air in alveoli decreases
Palv decreases
Figure 16.10
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Figure 16.11a
Figure 16.11b
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(opened system)
(closed system)
Figure 16.13a, b
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Figure 16.15
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Figure 16.16
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•  Associated with increased airway resistance
•  RV increases (harder to expire)
•  FRC and TLC increase
Forced Expiratory Volume (FEV)
•  Percentage of FVC that can be exhaled
within certain time frame
• 
FEV1 = percent of FVC that can be exhaled within 1
second
•  FEV1 ≥ 80% = Normal
•  FEV1 < 80% = Obstructive Pulmonary Disease
Examples:
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Total volume of air entering and leaving
respiratory system each minute
•  Minute ventilation = VT x RR
•  Normal respiration rate = 12 breaths/min
•  Normal VT = 500 mL
•  Normal minute ventilation =
–  500 mL x 12 breaths/min = 6000 mL/min
• 
Air in conducting zone does not participate in gas exchange
• 
Thus, conducting zone = anatomical dead space (~150 ml)
Expiration:
- 500ml expired to atmosp:
- - 350ml old air
- -150ml fresh air
- -150ml old air in
conducting zone
Inspiration:
- -150ml fresh air in
conducting zone
- 500ml enter alveoli:
- - 350ml fresh air
- -150ml old air
Figure 16.17
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Volume of air reaching gas exchange
areas per minute
Alveolar Ventilation = (VT x RR) – (DSV x RR)
Normal Alveolar Ventilation =
(500 mL/br x 12 br/min) – (150 mL/br X 12 br/min) =
4200 mL/min
Can increase alveolar ventilation by increase VT or RR.
Increasing VT is more effective.
Table 16.1
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2_ Mechanics of breathing

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