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Fisiología Respiratoria
I. Mecánica de la respiración
A. Anatomía
B. Ventilación
1. Músculos respiratorios
2. Flujo del aire
3. Presión Intrapleural
4. Volúmenes pulmonares
5. Trabajo respiratorio
6. Compliance pulmonar
7. Tensión superficial alveolar
8. Resistencia de vías aéreas
9. Compresión dinámica de vías aéreas
10. Espacio muerto
11. Factores determinantes de pCO2 y pO2
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A. Anatomy
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Surface area
2.5 cm2
> 1 x 106 cm2
300 millones de alvéolos
0,3 mm dam.
85 m2! (en 5-6 litros)
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Problemas:
-humo de cigarrillo
-fibrosis quística
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Structure of lung lobule
Each cluster of alveoli is surrounded by
elastic fibers and a network of capillaries.
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B. Ventilation (how we breathe)
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descent of
diaphragm
elevation of
rib cage
V1 V2
Va Vb
V1 < V2
Va < Vb
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Normal Lung at rest
lung collapses to unstretched
size
Pneumothorax
Pleural membranes
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Flow (F) of air
Respirometer
F = k(P1 - P2) = (P1 - P2)/R P = pressure;
k = conductance = 1/R;
R = resistance
P1 P2
F
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Lung Volumes
VT = Tidal volume
ERV = expiratory reserve vol
IRV = inspiratory reserve vol
RV = residual vol
FRC = functional residual capacity
Vital capacity
Total lung capacity
Minute Volume = V = VT x resp. rate
e.g., 0.5 L/breath x 12 breaths/min = 6 L/min
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Functional residual capacity
Vital capacity (sum
total of all except RV)
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Dead Space
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Volumen corriente = 500 ml
Espacio muerto = 150 ml
Llegan al alvéolo = 350 ml
Ventilación pulmonar = volumen corriente x frec.ventilatoria
Ventilación alveolar = (volumen corriente-espacio muerto) x frec.ventilatoria
¿Conviene modificar volumen corriente o frecuencia ventilatoria?
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Minute volume = VE = VT x f
Alveolar ventilation rate = VA = (VT -VD) x f
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Work of Breathing
Compliance Work: force to expand lung against its elastic properties
Force to overcome viscosity of lung & chest wall
Airway Resistance Work: force to move air through airways
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The ability of the lung to stretch is measured as the COMPLIANCE, C
C = ∆V/∆P
where V is lung volume and P is pressure
Vo
lum
e,
lit
ers
3
2
1
0
TLC
MV
RV
FRC
∆P = 6.5 cm H2O
∆V = 1.8 L
∆V/∆P = 1.8 L/6.5 cm H2O
= 0.28 L/cm H2O
Compliance Work: Compliance of lung & chest wall
For comparison:
vein = 0.04 and artery = 0.002 L/cm H2O
lun
g v
olu
me
(%
TL
C)
Translung pressure (cm H2O)
2. Difference between inspiratory &
expiratory curves called hysteresis
1. Curves are not linear
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Like Poiseuille flow in blood vessels, i.e., inversely to r4
Agents that constrict vessels (bronchioles) or accumulate debris
(e.g., mucus) increase resistance (makes airflow difficult).
Remember: ∆P = Raw x Flow
Conductive Airway Resistance.
One might think that because the terminal bronchioles are very narrow they would represent
very high resistance. However, because there are so many (>106) and because they are in
parallel they represent a relatively small portion of the total Raw.
Resistance Work:
Raw = (Palv - Patm)/ Flow
Bronchiolar smooth muscle is under neurohumoral control
Sympathetic stimulation (adrenaline): bronchiole dilation
Parasympathetic stimulation (Ach): bronchiole constriction
Histamine release from mast cells -- allergic/asthmatic response bronchiole constriction
R = 8hl
pr4
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air air air
What is surface tension?
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x x
P
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x x
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x x
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A major component of lung surfactant is dipalmitoylphosphatidylcholine
(DPPC). DPPC has typical phospholipid structure: two fatty acid
residues are water insoluble, hydrophobic; phosphocholine at other end
is charged and water soluble, hydrophilic.
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What is the origin and composition of Lung Surfactant?
Approximate composition of surfactant
Dipalmitoylphosphatidylcholine 62
Other phospholipids 15
Neutral lipids 13
Proteins 8
Carbohydrates 2
Component percent composition
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Importance of Surfactant:
1. Reduces surface tension, therefore increases
compliance
2. Stability of alveoli; LaPlace
3. Helps keep alveoli dry; helps prevent pulmonary
edema
4. Expansion of lungs at birth
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II. PHYSICAL PRINCIPLES OF GAS EXCHANGE
A. Properties of GASES
General Gas Law: PV = nRT
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Accounting for water
Dry atm. air Partial pressure vapor pressure = 47mmHg
% mm Hg mm Hg
O2 20.9 160 149
CO2 0.04 0.3 0.3
N2 & other 79 600 564
total 100 760 713
Partial Pressure = pressure exerted by any one gas in a mixture
Partial Pressure = total pressure x fraction of total represented by
the gas (Dalton’s law), i. e.,
Pgas = Ptotal x fgas
What is the composition of the room air that we breathe?
(in percent & in partial pressure)
(0.21x760)
(0.0004x760)
(0.79x760)
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How do we deal with gases in solution?
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Henry’s Law:
Conc. of gas in solution = partial pressure of gas X solubility coefficient
e.g., [O2] in moles/L: [O2] = PO2 x SO2
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Therefore [Gas] depends on both Pgas and Sgas
SCO2 is 20x higher than SO2
SCO2 = 0.03 mmol/L / mm Hg
SO2 = 1.37 µmol/L / mm Hg
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How fast is DIFFUSION? Diffusion
distance (µm)
Time required
for diffusion
1
10
100
1,000 (1 mm)
10,000 (1 cm)
0.5 msec
50 msec
5 seconds
8.3 minutes
14 hours
start equilibrium
CONCLUSION?
intermediate
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Fick's 1st Law of Diffusion
Rate of diffusion = dm/dt = D · A ·
D = the diffusion coefficient
C = concentration of the substance
A = area available for diffusion
x = the distance for the diffusion
Rate of Diffusion Distance
Area x Concentration
What is the strategy in the evolution of the respiratory apparatus?
available surface area
distance required for diffusion
dC dx
(i.e., thickness)
O2
CO2
P1
P2
thickness
Area
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FACTORES QUE INFLUYEN SOBRE EL TRANSPORTE DE GASES
1. Gradientes de presión parcial de Oxígeno:
105 100 40 40 15 5-2
alvéolos arterias capilares intersticio citosol mitocondrias
2. Superficie de intercambio
3. Distancia de difusión
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Total AREA available for
diffusion of gases is large
in human ~50-100 m2
Diffusion PATH LENGTH is very small, <1 µm
Enfisema!
Edema!
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Characteristics of the Pulmonary Circulation
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“Special” Characteristics of the Pulmonary Circulation
Systemic Circ. Pulmonary Circ.
C.O. (L/min) 6.0 ≈ 5.9
Arterial B.P. (mm Hg) 100 >> 15
Venous B.P. (mm Hg) 2 “≈” 5
Vascular resistance (∆P/flow) 100-2/6=16.3 > 15-5/5.9=1.7
Vascular compliance (∆V/∆P) Csystemic << Cpulm
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Ability to promote a decrease in resistance as blood pressure rises
Special Characteristics of the Pulmonary Circulation: high compliance
R = 8hl
pr4 Remember that resistance to Flow =
viscosity length
radius
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Pulmonary blood vessels are much more compliant than systemic blood vessels.
Also the system has a remarkable ability to promote a decrease in resistance as the
blood pressure rises.
Two reasons are responsible:
Recruitment: opening up of previously closed vessels
Distension: increase in caliber of vessels
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Inspired air:
PO2 = 158 mm Hg
PCO2 = 0.3 mm Hg
Expired air:
PO2 = 116 mm Hg
PCO2 = 32 mm Hg
Arterial blood
PO2 = 95 mm Hg
PCO2 = 41 mm Hg
(physiological shunt)
Gas exchange at alveolar and systemic capillaries
Left Heart Right Heart
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Matching respiration & blood flow:
the Ventilation-Perfusion Ratio
Alveolar ventilation, VA
VA = (VT - VD) x resp. rate
= (0.5 - 0.15) x 12 = 4.2 L/min
Cardiac output = C.O. = Q
Q = stroke vol. x heart rate
= (0.086) x 70 = 6.0 L/min
= ventilation/perfusion ~ 0.8 VA
Q
Ventilation
Perfusion
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VA
Q << 0.8
VA
Q ~ 0.8
Let’s assume that there is a blockage of one
alveolar region
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Special characteristic of blood vessels surrounding alveoli:
hypoxic vasoconstriction
When PO2 within the alveoli decreases there is a decrease in blood
flow to that alveolus
This is called hypoxic vasoconstriction
Thought to be the result of O2-sensitive K+ channels in the smooth muscle
membrane. At low O2 the K+ channels close, the Em rises, and the cell
reaches threshold and depolarizes and contracts.
This phenomenon is just the opposite the
response to hypoxia you get with arteriole smooth
muscle in the systemic circulation, but it is an
important feature of the pulmonary circulation
that helps to match perfusion with ventilation
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Normal Emphysema Asthma Pulm.
Circ.
Exercise Capillary enlargement (e.g., Mitral Stenosis)
Longer paths
for diffusion
Pathological Examples of Altered Respiratory Mechanics
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Carriage of blood gases
How are gases carried by the blood??
all values are in ml of gas/100 ml solution H2O or plasma (pH = 7.4) Whole blood (Hct = 0.45) dissolved combined dissolved combined
O2 (at a PO2 = 100 mm Hg) 0.3 0 0.3 19.5
CO2 (at a PCO2 = 40 mm Hg) 2.6 43.8 2.6 46.4
SCO2 = 30 µmol/L / mm Hg
SO2 = 1.37 µmol/L / mm Hg
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Carriage of blood gases
How are gases carried by the blood??
all values are in ml of gas/100 ml solution H2O or plasma (pH = 7.4) Whole blood (Hct = 0.45) dissolved combined dissolved combined
O2 (at a PO2 = 100 mm Hg) 0.3 0 0.3 19.5
CO2 (at a PCO2 = 40 mm Hg) 2.6 43.8 2.6 46.4
SCO2 = 30.0 µmol/L / mm Hg
SO2 = 1.37 µmol/L / mm Hg
O2: 99% como oxihemoglobina, 1% disuelto CO2: 67% como bicarbonato, 24% como carboxihemoglobina, 9 % disuelto
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HEMOGLOBINA
b
a
b
a
Hemoglobin molecule tetramer, 2a2b
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Oxygenation: Hb (deep red to bluish) + O2 HbO2 (oxyhemoglobin; red)
readily reversible
in fact, since Hb is a tetramer the reaction is really
Hb + 4O2 Hb(O2)4
Oxidation: Hb(Fe2+) Hb(Fe3+) (methemoglobin; brownish)
difficult to reduce
CO reaction: Hb + CO HbCO (carboxyhemoglobin; bright red, pink)
very high affinity (230X greater than for O2)
Spectral characteristics of Hemoglobin:
color changes with reaction of iron heme
(deoxyhemoglobin)
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Myoglobin molecule heme + globin monomer
a
b
a
b
Hemoglobin molecule tetramer, 2a2b
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hemoglobin
myoglobin
Active cell
ml O
2/1
00 m
l b
loo
d
0
5
10
15
20
Tissues
3 ml/100 ml
O2 released
to tissues
17 ml/100 ml
Let’s compare Hemoglobin and Myoglobin
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Why is Hb-O2 association “S-shaped”?
% saturation
100
0 25 50 75 100
tissue PO2 lung PO2
hyperbolic curve
with lowest K
hyperbolic curve
with highest K
PO2, mm Hg
y
100
0 [O2]
1
2 3
4
Conformational change induced by the movement of
the iron atom on oxygenation are transmitted to
parts of the molecule that are far away
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ml O
2/1
00 m
l b
loo
d
0
5
10
15
20
High affinity only Can’t release much
O2 to tissues
Low affinity only Doesn’t hold on to But can’t pick up
much O2 at tissues much O2 at lungs
S-shaped hemoglobin curve Releases much Becomes saturated
O2 at tissues with O2 at lungs
Advantages of “S-shaped” curve for Hb-O2 association
Active cell
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Effect of pH
Effect of PCO2
Effect of
temperature
PCO2 effect is the same as the pH effect
CO2 + H2O H2CO 3 H+ + HCO3-
(Bohr Effect)
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100
0
pH 7.4
pH 7.2
100 mm Hg
% saturation
PO2
R - CH2 - C — NH
HC CH
N
Fe
O2
H+
R - CH2 - C — NH
HC CH
NH+
Fe
O2
H+
Advantages & Mechanistic Basis of the Bohr effect (change in pH or PCO2)
Effect of pH
Protonic association alters O2 affinity
PCO2 effect is the same as the pH effect
CO2 + H2O H2CO 3 H+ + HCO3-
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Bohr Effect: Release of O2 by HbO2 into tissue is enhanced when:
pH is lowered
PCO2 is increased
Adequately oxygenated tissue
Normal pH
PCO2 ~ 46 mm Hg
PO2 ~ 40 mm Hg
adenosine normal
Inadequately oxygenated tissue
Low pH
PCO2 > 46 mm Hg
PO2 < 40 mm Hg
adenosine high
Local regulation (H+, CO2, adenosine, myogenic autoregulation) increases
blood flow to inadequately oxygenated tissue
A second physiologic process, the Bohr effect, simultaneously increases unloading
of O2 by Hb to the poorly oxygenated tissue (right shift in HbO2 curve)
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2,3-Diphosphoglyceric acid has important physiological consequences
2,3 DPG alters O2 affinity
2,3-Diphosphoglycerate
- bind to b chains
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Glucose 6-PO4
3-Phosphogyceraldehyde
1,3-Diphosphoglycerate
Phospho- glycerate kinase
Pyruvic acid
3-Phosphoglyceric acid
2,3-Bisphosphoglycerate (2,3-BPG)
2,3-DPG mutase
2,3-DPG phosphatase
2,3-Bisphosphoglyceric acid:
Where does it come from & what does it do to Hb?
Normal RBC glycolysis
How 2,3 BPG is produced
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Biochemical & functional differences of Fetal Hemoglobin
advantage
Expression of Hb differs during
development
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Arterial blood Venous blood CO2(%)
Total CO2 49 52.7 100
CO2 in solution 2.6 3.0 11
H2CO3 negligible negligible 0
HCO3- 43.8 46.3 67
Carbamino compounds 2.6 3.4 21
How is CO2 carried by the blood??
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
HCO3- ↔ H+ + CO3
2-
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Environment
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Control of Respiration
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A. Location of Respiratory Centers & Control Units 1. Medullary Respiratory Centers
a. Inspiratory Area - DRG - dorsal respiratory group neurons
b. Expiratory Area - VRG - ventral respiratory group neurons
2. Pontine Centers:
a. Pneumotaxic Center - located in upper pons (“off switch”)
b. Apneustic Center - located in lower pons (prevents turn-off)
3. Pulmonary Receptors
a. pulmonary stretch receptors (Hering-Breuer reflex)
b. other receptors??
B. Control of Respiratory Activity
1. Central or Medullary Chemoreceptors
2. Peripheral Chemoreceptors
3. Interaction between Central and Peripheral Chemoreceptors.
4. Cortical Factors
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The basis of rhythmic breathing. During inspiration the activity of inspiratory
neurons increases steadily (ramps up). At the end of inspiration, the activity
shuts off abruptly and expiration occurs by virtue of elastic recoil of lungs.
Recorded from
DRG neurons
Basic rhythmic breathing and Inspiratory Neuronal Activity
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Control neuromotor del diafragma
pCO2 distensión pulmonar
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The Oxygen Sensors (where are they?)
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The Oxygen Sensors (How do they work?)
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Receptores medulares de H+
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A Summary of
Chemoreceptor
Reflexes
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CHANGES IN BLOOD GASES WITH EXERCISE
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Responses of the Respiratory System to Exercise
Influencing factors and theories to account for respiration response to exercise.
1. Cerebral cortical influences.
2. Limb movements - proprioceptors from limbs are believed to play influencing role.
3. Increase body temp. may play modulating role on activities of controlling centers.
4. Small oscillations in arterial PO2 & PCO2 may occur, even though mean values are
constant, and the chemoreceptors may be sensitive to these fluctuations.
5. "Set-point" may be reset so that system is driven to a different control level.
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Hypoxia - O2 deficiency at the tissue level. There could be several causes.
1. Hypoxic-hypoxia (PO2 of arterial blood is reduced). e.g., breathing O2-poor gas or at
reduced atmospheric pressures. When PO2 of inspired air falls to about 60-70 mmHg there is
loss of consciousness (alv. PO2 ~35-40 mm Hg).
At sea level when breathing in an environment with 10% or less O2, you are in trouble.
With altitude, composition of air remains about same but there are pressure changes.
Delayed effects of altitude
Acclimatization
2. Anemic-hypoxia
Essentially low Hb content. Also in CO poisoning, effective Hb content is reduced by
HbCO complexing.
3. Stagnant-hypoxia (ischemic hypoxia)
Hypoxia due to circulation that is so slow that tissue does not receive its necessary
"flow" of O2. Shock, congestive heart failure (or localized restriction) can lead to damage of
important organs.
4. Histotoxic-hypoxia
Inhibition of tissue oxidative processes by poisons. e.g., Cyanide poisoning - combines
with cytochrome oxidase preventing O2 from serving as the ultimate electron acceptor.
RESPONSES TO CHANGE IN RESPIRATORY GASES
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PO2, mm Hg
San Francisco (sea level) 150
Lake Tahoe (6,500) 110
Mt. Whitney (14,500) 80
Mt. Everest (29,500) 30
Altitude, Barometric Pressure and PO2
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ACLIMATACION
1. Aumento de ventilación
2. Aumento de excreción de bicarbonato
3. Cambio en la curva de saturación de la Hb (DPG)
4. Aumento de glóbulos rojos
5. Aumento de Hb
6. Angiogénesis
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Pressure - cm water
Vo
lum
e
- m
l
air
inflation
saline
inflation
Question: Would the compliance of a lung filled with
air be less than one filled with water??
Experiment: