Download Lecture 10: The Physics of Breathing

Lecture 10: The Physics of Breathing
ref: J.B. West: Respiratory Physiology 6th ed., Wiliams & Wilkins (2000)
• The Lungs Exchange Gas
• Ventilation
– The exchange of gas with the atmosphere
• Perfusion
– The exchange of gas with the blood
The Airways
• A series of branching tubes from
Generation
– The TRACHEA
1
DP~ few mm
F ~ 1/sec
2
3
4
Conduction Zone
17
18
19
20
21
22
23
Terminal Bronchus
Respiratory Bronchioles
– To the ALVEOLI
Alveolar Ducts
Alveolar Sacs
0.3 mm diameter
(300 Million)
Gas Flows
Main Bronchi (RL)
Respiratory Zone
Gas Diffuses
0
Diffusion: x=1/2D√ t
Lung Volumes and Flows
(Typical)
Tidal Volume
1000 ml
Total Ventilation
15 l/min
Anatomic Dead Space
300 ml
Alveolar Ventilation
10.6 l/min
Alveolar Space
6000 ml
Pulmonary
Capillary Blood
140 ml
Total Lung Capacity
Vital Capacity (max in - max out)
Residual Volume
Normal Lung Volume (inhaled)
Tidal Volume (normal breath)
Functional Residual Capacity
Pulmonary
Blood Flow
10 l/min
7 liters
6 liters
1 liter
4 liters
1 liter
3 liters
Pressure vs Volume
• Intrapulmonary pressure: - 1.5 mm (inhaling)
• Itrathoracic Pressure:
– Chest expands/Lungs recoil
Begin Expiration
3
V (l)
Exhale
Inhale
Begin Ispiration
2
DV
DP
Heavy Breathing
-5
0
+5 IT pressure (mm Hg)
0
-5
-10 IP pressure (mm Hg)
• Compliance: DV/DP (liters/cm H2O) 1 mm Hg = 1.36 cm H2O
(Infant Lungs: Low Compliance/Fibrotic Lung: High Compliance/Flabby Lung)
Airway Resistance
• DP = R * (DV/Dt)
Pressure
R has units of Pa/l/s
Volume Flow
• R = 330 Pa/l/s = 3. 3 cm H2O/l/s = 2.5 mm Hg/l/s
• R ~ hL/A2 for airway
A (cm2)
300
200
Cond. Zone
100
0
Resp. Zone
Diffusion: x=1/2D√ t
0
5 10 15 20
Airway Generation
Other Lung Features
• Surface Tension in alveoli -> collapses alveolus
• Inhaling pressure (200 Pa) -> expands alveolus
• Stability requires 2g/R < 200 Pa (1.5 mm)
– Surfactant stabilizes alveoli
• Foreign Objects: dust etc.
– Cilia move bronchial mucous upward
Liquid Surface Tension
• Fluids are 3 dimensional
Surface Molecule
• T=gL
• P=F/A=2g/R
Bulk Molecule Pulled Evenly
r
(per surface of sphere)
g
cosqc > 0 for H2O-glass
cosqc < 0 for Hg-glass
Water
+0.072 N/m
Hg
0.44 N/m
Lung Surfactant 0
(deflated)
0.02 N/m (inflated)
Stability requires g/2R < 200 Pa (1.5 mm)
–Surfactant stabilizes alveoli
h
g2prcosqc = rpr2h
h = 2gcosqc/rr
qc is the contact angle
The Blood-Gas Interface
Capillary Connects Artery to Vein
Aorta
Vein
Right Heart
Pulmonary
Artery
Left Heart
Pulmonary
Vein
Pulmonary Capillary Bed
O2
CO2
Alveolar Space
The Blood-Gas Interface
• O2 CO2 and other gases FREELY DIFUSE
• Concentration Gradient Drives DIFFUSION
More Dense
Less Dense
NET FLOW
Diffusion: x=1/2D√ t
• DIFFUSION grows with SURFACE AREA
Alveolar Sacs Fill Lung Volume with Surface
Gas Partial Pressure
• Partial Pressure: PaV=naRT Total P=S Pa
N2
O2
CO2
Inhaled
78%
21%
<1%
Exhaled
78%
17%
4%
• Pgas => Pblood as blood moves through capillary ~1/2
s)
• Cblood = lBGCgas (lBG is the solubility or partition coef.)
Blood Plasma lO2 = 2% (42 ml O2/lblood)
lCO2 = 48%