# BIOL 2P93 Lecture Notes - Alveolar Pressure, Chronic Obstructive Pulmonary Disease, Asthma

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4 Feb 2013
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Mechanics I—Static Lung Mechanics
I. Properties of Elastic Structures
a. Terms
i. Static Mechanics—relationship between pressure & volume in lung
ii. Dynamic Mechanics—relationship between pressure & flow in lung
b. Elastic Structures
i. Volume is proportional to pressure
ii. Transmural pressure—difference in pressure across the wall of the lung
= internal surface pressure – external surface pressure = distending P
1. ↑ V => ↑ Tm
2. PTm = Pin – Pout
iii. Elastic Recoil Pressure—the equal & opposite tendency of a structure
which resists the transmural pressure
iv. Transmural Pressure-Volume Curve—graphs relationship between
volume (y) & transmural pressure (x)
1. compliance—slope of P-V curve; compliance = ΔV/ΔP
a. high compliance = easily distensible
b. low compliance = stiff / non-distensible
2. elastance—opposite of compliance; elastance = ΔP/ΔV
v. Terms
1. unstressed volume = Vo = volume when PTm = 0
2. Limiting Volume = Elastic limit = max volume; ↑ P ≠ ↑ Volume
3. Elastic instability—when elastic structure either ruptures or
collapses at high/ low PTm’s
vi. Soap Bubble—has constant tension in its wall => PTm falls as Volume ↑
II. Calculating PTm in Respiratory System
a. PALV = alveolar pressure
b. PPL = pulmonary pressure
c. PL = transpulmonary pressure = PALV - PPL
d. PBS = body surface pressure (can be considered 0 b/c us. = atmospheric P)
e. PCW = pressure of chest wall = PPL – PBS
f. PRS = Pressure of respiratory system = PALV - PBS
III. Pressure-Volume Curve of Lung
a. Compliance of normal lung = 200 ml/ cm H2O
i. => need 2.5 cm H2O of pleural pressure to overcome elastic recoil of
lungs during tidal volume breath
ii. => need 30 cm H2O of PPL to achieve TLC (further increases => rupture)
b. Normal Values
i. Unstressed lung volume ~= residual volume
ii. Unstressed chest wall volume ~= 2/3 TLC (large)
iii. At low volumes, chest wall => stiff b/c diaphragm reaches elastic limit
* negative pleural pressure => relaxed chest wall gets smaller
* mm. activation => negative pl. pressure => thorax enlarges
iv. At high volumes, lung becomes stiff b/c elastic recoil increases
c. Pressure-Volume curve of Lung
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