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Section II Respiratory Gases
Exchange
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I Physical Principles of Gas Exchange
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• Partial pressure
– The pressure exerted by each type of gas in a
mixture
• Diffusion of gases through liquids
– Concentration of a gas in a liquid is determined
by its partial pressure and its solubility
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Partial Pressures of Gases
Basic Composition of Air
• 79% Nitrogen
• 21 % Oxygen
• ~ 0% Carbon Dioxide
Pb
In a mixture of gases, each gas exerts a partial
pressure proportional to its mole fraction.
Total Pressure = sum of the partial pressures of each gas
Total Pressure (at sea level)
Pbarometric = 760 mm Hg
Pgas = Pb x Fgas
PN = 760 x 0.79 = 600.4 mm Hg
P02 = 760 x 0.21 = 159.6 mm Hg
760 mm
Hg
Pb
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Partial Pressure of Gases in Fluids
Each gas has a specific solubility
O2 Solubility coefficient = 0.003 ml/100 ml Blood
C02 = 0.06 ml/100 ml Blood (x 20 of 02)
Gases dissolve in fluids by moving down a
Partial Pressure gradient rather than a concentration gradient
Consider a container of fluid in a vacuum
That is opened to the air
Molecules of gas begin to
enter the fluid
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Partial Pressure of Gases in Fluids
After a short time,
the number of molecules the number of molecules
ENTERING
=
LEAVING
At equilibrium, if the gas phase has a PO2 = 100 mm Hg,
the liquid phase also has a PO2 = 100 mm Hg
An easy way to talk
about gases in fluids.
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• Transport of gases between the alveoli and
(pulmonary) capillaries and eventually from
the capillaries to the tissues
• diffusion dependent on perfusion and the
partial pressure (pp) exerted by each gas
• gases diffuse from area of  conc. (pp) to 
conc. (pp)
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Diffusion


concentration   pp of gas  diffusion
CO2 more soluble than O2, therefore it
diffuses faster
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Diffusion: Blood Transit time in the Alveolus
Alveolus
Blood capillary
Time for exchange
PO2
mm Hg 100
Saturated very quickly
Reserve diffusive Capacity of the
lung
P
45
40
CO2
0
Time
0.75 sec
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II Gas exchange in the lung and in
the tissue
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Oxygen and Carbon Dioxide
Diffusion Gradients
• Oxygen
– Moves from alveoli into
blood.
– Blood is almost completely
saturated with oxygen
when it leaves the capillary
– P02 in blood decreases
because of mixing with
deoxygenated blood
– Oxygen moves from tissue
capillaries into the tissues
• Carbon dioxide
– Moves from tissues into
tissue capillaries
– Moves from pulmonary
capillaries into the
alveoli
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Diffusion Gradients of Respiratory Gases
at Sea Level
Partial pressure (mmHg)
Gas
% in
dry air
Dry
air
Alveolar
air
Venous
blood
Diffusion
gradient
Total
100.00
760.0
760
760
0
H2O
0.00
0.0
47
47
0
20.93
159.1
105
40
65
0.03
0.2
40
46
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79.04
600.7
569
573
0
O2
CO2
N2
NB. CO2 is ~20x more soluble than O2 in blood => large amounts move
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into & out of the blood down a relatively small diffusion gradient.
PO2 and PCO2 in Blood
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III. A-a gradient, the efficiency of
the gas exchange in alveoli
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What is an A - a gradient ?
The DIFFERENCE between:
Oxygen Content in
Alveolus Gas
(measured during exhalation)
Oxygen
Content in
arterial blood
(equivalent to
that leaving
lungs)
In a healthy person, what would you expect the A - a to be?
No difference, greater than 0, or less than 0
Normal: A – a, up to ~ 10 mm Hg, varies with age
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Factors contributing to A - a Gradient
1. Blood Shunts
2. Matching
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SIMPLE CONCEPT OF A SHUNT
AIR FLOW
Alveolar
CO2
arterial vessel
SPACE
O2
BLOOD FLOW
Blood
Mixing
No Gas Exchange = SHUNT
Lowered O2/l00 ml
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NEXT NEW
CONCEPT
Matching What?
Blood to Air Flow
Total Ventilation
Oxygen
Exchange
Total Perfusion, Q
If the volumes used for exchange are aligned
– We might consider the system to be
“ideally matched”
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Dead Air Space (Airways)
Alveolar Ventilation (VA)
Oxygen
Exchange
Arterial Perfusion (Qc)
Slide or Misalign the distribution volumes
Some Volumes are wasted,
Matching Ratio = VA/Qc = 0.8
Shunt (Qs)
(Bronchial
Artery)
Normal Case; Small Shunt, low volume Dead Space
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Matching ventilation & perfusion
Ventilation and perfusion
(blood flow) are both
better at the bottom (base)
of the lung than that at the
top (apex).
But the change in blood
flow is more steep than in
ventilation.
Therefore the
ventilation/perfusion ratio
rises sharply from the base
to the apex.
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Matching ventilation & perfusion
(cont)
Result:
V/Q is greater or less
than 0.8 in different
regions
If V/Q <0.8 = shunt
like, If V/Q > 0.8 little
benefit, Increases A a gradient
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Dead Air Space
Alveolar Ventilation
VA
Oxygen
Exchange
Arterial Perfusion Q
Shunt
= Lung Disease with a Large A – a gradient
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IV Factors Affecting the Gas
Diffusion in the Lung
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1. The Properties of the Gas
1) Molecular weight. Diffusion rate is inversely proportional
to the square root of the molecular weight
2) Temperature
3) Solubility in water
Each gas has a specific solubility
O2 Solubility coefficient = 0.003 ml 02/100 ml Blood
C02 = 0.06 ml/100 ml Blood (x 20 of 02)
PO2
mm HG 100
Saturated very quickly
Reserve diffusive Capacity of the
lung
P
45
40
CO2
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0
Time
0.75 sec
2. Partial Pressure of the Gases
1) Alveoli ventilation
2) Blood perfusion in the lung capillary
3) Speed of the chemical reaction
The slow speed of the chemical reaction HCO3- + H+
----- H2CO3 ---H2O + CO2 reduces the CO2
exchange in the lung.
So, during the gas exchange in the external
respiration, the exchange of CO2 is a little lower
than that of O2.
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3. Properties of the Lung
1) Area of the respiratory membrane
2) Distance of the diffusion
3) VA/Qc
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V Pulmonary Diffusion Capacity
Concept:
The ability of the respiratory membrane to exchange a gas
between the alveoli and the pulmonary blood
defined as the volume of a gas that diffuses through the
membrane each minute for a pressure of 1 mmHg.
DL = V/(PA – PC)
V is a gas that diffuses through the membrane each minute,
PA is the average partial pressure of a gas in the air of alveoli,
PC is the average partial pressure of a gas in the blood of
pulmonary capillary.
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Factors Affecting the DL
1. Body posture
2. Body height and weight
3. Exercise
4. Pulmonary diseases
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VI Internal Respiration
• All cells require oxygen for metabolism
• All cells require means to remove carbon
dioxide
• Gas exchange at cellular level
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Concept: Gas exchange between the capillary and the
tissues throughout the body
Process:
Factors affecting the internal respiration:
1. Distance between the cells and the capillary
2. Rate of metabolic rate
3. Speed of the blood flow in capillary
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EXTERNAL AND INTERNAL
RESPIRATION
ATMOSPHERE
SYSTEMIC
CIRCULATION
HEART
PULMONARY
CIRULATION
LUNGS
TISSUE
CELL
O2 + FOOD
CO2 + H2O
+ ATP
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