Blood- Gas Exchange
By
Dr. M.B.Bhat.
Introduction
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Atmospheric air  Enter into alveoli 
Dissolved in the alveolar fluid  Diffuse
through the membrane barriers  Enter
into the blood.
And Vice versa
Composition & Partial pressure of gases
at various level of respiratory system is
as follows --
Composition & Partial Pressure of
Atmospheric dry air
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Dalton law of Partial pressure (explain)
Accordingly, at sea level 1 ATP (760mm Hg)
Conc. Of gas
Partial pressure
Oxygen – 20.98%
160 mm Hg
CO2
-- 0.04%
0.3 mm Hg
Nitrogen-- 78.6%
600 mm Hg
Others -- 0.92%
Others are inert gases (Argon & Helium etc)
Presence of water vapor alter this composition
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Blood-gas exchange depends upon –
Diffusion capacity of the gas
Solubility of the gas
Affinity of the gas with Hemoglobin
Diffusion of gas
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Kinetic movement of molecular motion
Law of diffusion- D = 1/√MW
Graham’s law – D = 1/√d (density)
Diffusion coefficient (Dc) = 1/√MW x d
Fick’s Law of diffusion (D) = (A/T) (Dc x ∆P)
A—surface area; T-thickness;
Dc –Diffusion coefficient
∆P—Presence difference
Pulmonary diffusion capacity = D α Dc X ∆P
Solubility of Gas
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Henry’s law -Volume of gas dissolved in a liquid is proportionate to
partial pressure & solubility coefficient of the gas
Partial pressure = Volume of gas dissolved/ solubility
coefficient of the gas
Solubility coefficient depend upon the solubility nature of
the gas
To dissolve given volume of gas -- more the solubility
coefficient, less the partial pressure needed
Solubility coefficient of various gas
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Carbon di oxide –
0.57
Oxygen
-- 0.024
Carbon monoxide -- 0.018
Nitrogen
-- 0.012
Helium
-- 0.008
Solubility coefficient of CO2 is about 24 times more than
O2 &
Diffusion coefficient of O2 is about 1.2 times more than
CO2
Hence Diffusion capacity of CO2 is about 20 times more
than O2.
Diffusion of dissolved gas
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Fick’s law modified to diffusion of
dissolved gas -D = (A/T) (Dc x ∆P x S)
(S- solubility coefficient)
Hence pulmonary diffusion capacity is
D α Dc x ∆P x S
Affinity of gas with Hb
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Affinity of CO with Hb is 210 times more
than oxygen
Affinity of O2 with Hb increase O2 content
in blood 70 times
Affinity of CO2 with Hb increase CO2
content in blood by 17 times
Affinity of Hb with N2, Helium (inert gases)
etc are negligible
Role of Hb affinity in Pulmonary
diffusion of gas
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Pulmonary diffusion is divided into –
Diffusion limited diffusion – for high
affinity gases –Example -- CO. (Gas does
not equilibrate with blood even after
0.75sec)
Perfusion limited diffusion –for low affinity
gases –Example –N2O, He (Gas equilibrate
with blood within < 0.1sec)
O2 falls in between equilibrate with blood
in 0.3sec
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There is an inverse relationship exists
between Hb affinity & partial pressure
Large volume of gas can be transferred
with little change of partial pressure in
high Hb affinity gases.
Pulmonary Diffusion coefficient of
various gases
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“Relative rates (volumes) at which different
gases at the same pressure levels will diffuse are
proportionate to their diffusion coefficient” (at
infinite time)
Assuming O2 as 1; relative coefficient of various
gases are –
DO2 = 1
DCO = 20.4
D He = 0.95
DCO2 = 0.81
DN2 = 0.53
Total Diffusion capacity of lungs
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DL O2 = 15 to 35 ml / min / mm Hg at rest
(average = 21ml / min / mm Hg)
(During exercise it increases 3 to 4 times –
average about 65 ml / min / mm Hg)
Mean partial pressure difference of O2 across
respiratory membrane at rest = 11 mm Hg
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Volume of O2 diffused = DL O2 x ∆P
21 x 11 = 231ml / min (250ml/min)
TDL O2= 250ml/min
Mean ∆P of pulmonary capillaries for
CO2 is < 1mm Hg (0.5mm Hg)
Volume of CO2 diffused = DLCO2 x ∆P
As CO2 diffusion capacity is 20 times of
O2 (which is 21ml; DLCO2 is 21 x 20 =
420 ml)
420 ml x 0.5 = 210ml/min (200ml/min)
TDL CO2 = 200ml/min
Factors affecting Pulmonary
diffusion capacity
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Blood-gas barrier –
(1).surfactant –(2).alveolar fluid –
(3).alveolar epithelium –(4).basal lamina –
(5).Insterstitial fluid –(6).capillary
endothelium –(7).Blood plasma –
(8).Membrane of RBC – (9).Hb
Out of this –alveolar-capillary membrane
includes 3, 4, 5 & 6
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Total thickness 0.5µm –in diseased conditions it
may increase pul. Edema)
Characteristic of alveolar membrane – In
pulmonary fibrosis it decreases (Diseases such as
Sarcoidosis & Beryllium poisoning that cause
pulmonary fibrosis)
The RBC & Hb component
Characteristic of gases (Partial pressure, solubility
Diffusion coefficient & affinity with Hb)
Surface area of alveolar membrane –is about
70m2; & Pul.capillary bed also same;
Imbalance of ventilation & blood flow
(physiological dead space & shunt)
Ventilation-Perfusion ratio
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Two factors that determine the alveolar
PO2 & PCO2 are –
Rate of alveolar ventilation (VA) 4 liters
Rate of alveolar perfusion (Q) CO2=5liters
The ventilation-perfusion ratio = VA/Q =
4/5 = 0.8
Variation in VA/Q ratio
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When VA is normal & Q is normal=VA/Q is
normal
When VA is zero & Q is normal =VA/Q is zero
When VA is normal & Q is zero =VA/Q is
infinity
VA/Q is zero –composition of alveolar PO2 &
PCO2 approaches to that of venous blood
VA/Q is infinity –composition of alveolar PO2
& PCO2 approaches to that of atmospheric
air
Physiological shunt (Qps)
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Volume of blood does not take part in blood-gas
exchange is called (physiological) shunt
When VA/Q is below normal –Physiological shunt
increased
It is calculated by using the formula –
Qps = QT (CiO2 – CaO2)/CiO2 –CVO2)
Qps = Volume of blood in physiological shunt
QT = Cardiac out put
CiO2 = concentration of arterial O2 in ideal VA/Q ratio
CaO2 =concentration of O2 in arterial blood
CVO2 =concentration of O2 in venous blood
Normal Qps = Blood in the bronchial circulation (2% of
CO) & Blood of coronary circulation return to left atrium
through thebesian veins
Physiological dead space volume
(VDs)
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Volume of gas does not takes part in
blood-gas exchange
When VA/Q is above normal – Dsv
increased
It is calculated by using Bohr’s equation –
VDs = VT (PaCO2 –PECO2) / PaCO2
Normal variation in VA/Q ratio
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Normal individual, in upright posture—
VA/Q ratio variation from Top to Bottom of
lungs is –from 3 at top to 0.6 at bottom
Reasons –
Effect of gravity
Variation in intra-pleural pressure
Effect of gravity
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Due to gravitation effect – Flow of gas &
blood increases from apex to base
Increase is relatively more with blood
Hence, VA/Q ratio –decreases from apex
to base
Variation in intra-pleural pressure
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Due to gravitation effect on lung mass
–visceral pleura is more stretched at
the apex
So, the intra pleural pressure is more
negative at apex ( – 5mm Hg) than at
base ( – 2mm Hg)
Hence, apical alveoli is more stretched
During inspiration –as it need more
pressure to stretch (due to stiffness)
apical alveoli than basal alveoli –more
air goes to base
Variation in pulmonary blood flow
due to effect of gravity
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Due to gravitation effect; from the heart
level – to above BP decreases & below BP
increases -- 0.7mm Hg/ cm
Vertical dimension of lungs is 30 cm
amount variation of 21mm Hg
From heart level above to apex by – 14
mm Hg & below to base by –7mm Hg
Accordingly, blood flow to lungs can be
divided into 3 zones
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At apex –Zone 1 –Minimum blood flow
Capillaries pressures are close to intra alveolar
pressure (flow only during systole)
Middle –Zone 2 – Intermittent blood flow
Pul. Arterial capillary pressure is > alveolar
pressure; but Pul.venules pressure is < alveolar
pressure during expiration –Hence during
expiration blood accumulated near venous end
and during inspiration ‘falls’ into vein. (Blood flow
is determined by artery-alveolar pressure
difference rather than artery-venous difference)
Bottom –Zone 3 – continuous blood flow
Both arterial & venous pressure is well above
alveolar pressure through out respiratory cycle
Clinical significance
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Unilateral lung diseases –ask the patient lie on
the side –good lung in dependent position (In
infants it is opposite)
Tuberculosis bacteria attack–at apex of lung –
because of less air flow (less mechanical
disturbance) & relatively high alveolar PO2 –
favorable environment
Chronic obstructive lung diseases –exhibit very
serious physiological shunt & dead space
During exercise, blood-gas flow to lungs improve
everywhere and effectiveness of gas exchange
appears optimum