Transport of
Oxygen & Carbon Dioxide
Tanveer Raza MD MS
razajju2@yahoo.com
Hemoglobin (Hb) increases total blood
oxygen capacity (30-100 times more)
CO2 transport increases 15- to 20- fold
in combination with chemical
substances in blood
Diffusion
Alveoli
Blood vessels
Tissue
Diffusion of Oxygen from Alveoli to
Pulmonary Capillary Blood
In alveolus PO2
104 mm Hg
PO2 of blood at its arterial end
40 mm Hg
Initial pressure difference causing O2 to diffuse into
pulmonary capillary is (104-40)
64 mm Hg
Diffusion of Oxygen from Alveoli to
Pulmonary Capillary Blood
PO2 rises almost to that
of alveolar air by the
time blood has moved a
third of the distance
through the capillary,
(PO2= 104 mm Hg)
Diffusion of Oxygen
Shunt flow
Blood entering Left Atrium
• About 98% comes from lungs [PO2 = 104
mm Hg]
• 2% comes from bronchial circulation [PO2
= 40 mm Hg]
This blood is not exposed to lung air. It is
called "shunt flow" meaning blood is shunted
past gas exchange areas. PO2 is that of normal
systemic venous blood
Shunt flow
Blood combines with the
blood from alveolar
capillaries (the venous
admixture of blood )
causing PO2 of blood
entering the left heart
and pumped into the
aorta to fall to about 95
mm Hg
Diffusion of Oxygen from Peripheral Capillaries
to the Tissue Fluid and Tissue Cells
Capillaries entering peripheral tissues
In interstitial fluid
PO2 = 95 mmHg
PO2 = 40 mmHg
Rapid diffusion of O2 from capillary blood into the tissues
In capillaries leaving peripheral tissues
PO2 = 40 mmHg
Diffusion of Oxygen from Peripheral Capillaries to
the Tissue Fluid and Tissue Cells
Normal intracellular, PO2=5 – 40 mmHg, average 23 mm Hg
O2 pressure required for chemical processes in cell is 1-3
mmHg
Intracellular PO2 of 23 mmHg is more than adequate and
provides a large safety factor
Effect of Rate of Blood Flow on
Interstitial Fluid PO2
Effect of Rate of Tissue Metabolism
on Interstitial Fluid PO2
Rate of Blood Flow on Interstitial Fluid
PO2
– Increased blood flow will increase tissue
PO2
Rate of Tissue Metabolism on Interstitial
Fluid PO2
– If cells use more O2 the interstitial fluid
PO2 will decrease
Diffusion of CO2

Almost all O2 converts to CO2 in tissues

CO2 diffuses 20 times rapidly than O2

CO2 diffusion is exactly opposite to O2
Diffusion of CO2 from Tissue Fluid and
Tissue Cells to Peripheral Capillaries
Intracellular
 Interstitial
 Pressure difference

PCO2 = 46 mmHg
PCO2= 45mmHg
= 1mmHg
Diffusion of CO2 from Tissue Fluid and
Tissue Cells tp Peripheral Capillaries
 Interstitial
PCO2=45mmHg
 Arterial end capillary PCO2=40mmHg
 Venous end capillary PCO2=45mmHg
Diffusion of CO2 from Pulmonary
Capillary Blood to Alveoli
Arterial end
 Alveolar air
 Pressure difference

PCO2=45mmHg
PCO2=40mmHg
=5mmHg
Diffusion of CO2 from Pulmonary
Capillary Blood to Alveoli
PCO2 equals
that of alveolar
air [PCO2
=40mmHg] by
the time the
blood has
moved a third of
the distance
Effect of Rate of Blood Flow on
Interstitial Fluid PCO2
Effect of Rate of Tissue Metabolism on
Interstitial Fluid PCO2
Role of Hb in O2 Transport
Under normal conditions, O2 is transported to
the tissues almost entirely in chemical
combination with Hb
Hb
Dissolved in water of plasma and blood cells
97%
3%
Combination of O2 with Hb

Loosely and reversibly with heme portion
of Hb

When PO2 is high in pulmonary capillaries
O2 binds with Hb

When PO2 is low in tissue capillaries O2 is
released from Hb
Oxygen-Hemoglobin Dissociation
Curve
Oxygen-Hemoglobin Dissociation
Curve

Demonstrates progressive increase in the
percentage of Hb bound with O2 as blood PO2
increases, which is called the per cent
saturation of hemoglobin
– In systemic arteries the saturation averages 97%
[PO2=95 mm Hg]
– In normal venous blood saturation of averages 75%
[PO2=40 mm Hg]
Oxygen-Hemoglobin Dissociation
Curve
Oxygen-Hemoglobin Dissociation
Curve

Maximum O2 that can combine with Hb in blood
– Hb in normal person 15 gm Hb/100ml blood
– 1 gm of Hb binds with maximum 1.34 ml O2
(1.39 ml when Hb is chemically pure, but impurities
such as methemoglobin reduce this)
– 100% Hb combines with
15 x 1.34 ml O2
approx. 20 ml of O2
– Usually expressed as 20 volumes per cent
Oxygen-Hemoglobin Dissociation
Curve
• Can also be expressed as volume per cent of O2,
instead of per cent saturation of Hb
Thank You