Gas Transport
Learning Objectives
Covering the the transport of O2 and CO2 in the blood and
tissue fluids.
• Know how O2 and CO2 diffuse in pulmonary capillaries,
systemic capillaries and in tissues.
• Understand and be able to use the O2-hemoglobin
dissociation curve.
• Know the quantity of O2 and CO2 delivered by the
blood.
• Know what causes shifts in the O2-hemoglobin
dissociation curve.
• Know how CO2 is transported in the blood and
understand the Haldane Effect.
Movement of Gases
• Gases move by diffusion, from areas of high
partial pressure to areas of low partial
pressure.
- In the alveoli, O2 moves from the alveoli (high PO2)
to the pulmonary blood (low PO2).
- In the tissues, O2 moves from the blood (high PO2)
to the tissues (low PO2).
• How will CO2 diffuse?
Diffusion of O2 in the Pulmonary Capillaries
• What is the PO2 in the arterial
end?
104 mm Hg – 40 mm Hg = 64 mm Hg.
• What is the net direction of O2
diffusion in the arterial end?
From the alveolar space into the
blood.
• How does the pulmonary
anatomy allow the capillary blood
to reach a PO2 of 104 mm Hg so
quickly in the venous end?
- Surface area (70 cm2 of respiratory
membrane for 60-140 mL blood).
- Thin respiratory membrane.
- PO2.
O2 Diffusion During Exercise
• During exercise, the body may need 20 x more
O2.
• How does the exchange in the pulmonary
capillaries meet this need?
- Increased diffusing capacity (increased surface area and
capillaries and improved VA/Q).
- The blood reaches O2 saturation quickly (see previous slide).
Changes in PO2 in Cardiovascular
System
Diffusion of O2 in the Systemic Capillaries and
Tissues
• The PO2 in the arterial blood is ~ 95 mm Hg and PO2 in the
interstitial fluid is ~ 40 mm Hg. Thus, the PO2 ~ 55 mm Hg for
the diffusion of O2 into the interstitial fluid.
• The PO2 in the interstitial fluid is ~ 40 mm Hg and the PO2 in
the tissues ~ 23 mm Hg. Thus, the PO2 ~ 17 mm Hg for the
diffusion of O2 into the tissues.
Blood Flow and Interstitial PO2
Diffusion of CO2 in the Systemic
Capillaries and Tissues
• Diffuses in the opposite direction as O2,
because CO2 accumulates in the tissues as O2
is consumed.
• Note: CO2 can diffuse ~ 20 x more rapidly than
O2; so less differences in partial pressures are
required.
Diffusion of CO2 in the Pulmonary
Capillaries
• The PCO2 in the tissues ~ 46 mm Hg and the PCO2 in the
interstitial fluid ~ 45 mm Hg. Thus, the PCO2 ~ 1 mm Hg for the
diffusion of O2 into the interstitial fluid.
• The PCO2 in the interstitial fluid ~ 45 mm Hg and the PCO2 in the
arterial capillary blood ~ 40 mm Hg. Thus, the PCO2 ~ 5 mm Hg
for the diffusion of O2 into the blood.
Diffusion of CO2 in the Pulmonary
Capillaries
Blood Flow and the Interstitial PCO2
Transport of O2 by Hemoglobin
• Nearly all the O2 (~ 97%) is
transported in the blood by
hemoglobin.
• One hemoglobin molecule
contains 4 heme prosthetic
groups.
• One hemoglobin molecule
can carry 4 O2 molecules.
O2-Hemoglobin Dissociation Curve
Amount of O2 Carried by Hemoglobin (Volumes)
• In 100 ml of blood,
contains ~ 15 g of
hemoglobin.
• Each gram of hemoglobin
can carry a maximum of
1.34 ml of O2.
• Thus, 100 ml of blood can
carry a maximum of 20 ml
of O2 (15 x 1.34).
Quantity of O2 Released to Systemic Tissues
• At 97% hemoglobin saturation
(arterial), 100 ml of blood
carries ~ 19.4 ml O2.
• At 75% saturation (venous), 100
ml of blood carries ~ 14.4 ml O2.
• Thus, 100 ml of blood delivers ~
5 ml of O2 under normal
circumstances.
• During exercise, 100 ml of
blood can deliver ~ 15 ml O2.
O2 Delivery During Exercise
• During exercise, 100 ml of blood can deliver ~ 15 ml
of O2 (3-fold increase from normal).
• What happens to cardiac output during exercise?
• During exercise, the cardiac output increases 6- to 7fold.
• Thus, by increasing O2 transport and cardiac output,
there can be a 20-fold increase in O2 delivery to
tissues during exercise.
Using the O2-Hemoglobin Dissociation
Curve
• The normal PO2 of the alveoli is
104 mm Hg. This results in a
hemoglobin saturation of 97%.
• What happens to hemoglobin
saturation if the alveolar PO2
drops to 60 mm Hg, while
climbing a mountain?
The hemoglobin saturation only
drops to 89%.
• The venous blood PO2 only needs
to drop to 35 mm Hg (from 40)
for 5 ml of O2 per 100 ml to be
delivered
Shifts in the O2-Hemogobin
Dissociation Curve
• Increases in H+, CO2, and
temperature shift the curve to
the right.
• This enhances the release of
O2 from hemoglobin and is
called the Bohr Effect.
• In tissues, the high CO2
increases the [H+] (recall blood
acid/base reactions involving
bicarbonate). Causing the
extra release of O2.
• In the lungs, the removal of
CO2 decreases the [H+]. This
increases the binding of O2 to
hemoglobin.
• What happens to H+, CO2, and
temperature in exercising
muscle?
Transport of CO2 in the Blood
• Under normal conditions, the blood delivers ~
4 ml of CO2 from the tissues to the lungs in
each 100 ml of blood.
• Why only 4 ml of CO2 per 100 ml if the blood
delivers 5 ml of O2 per 100 ml?
For carbohydrate metabolism, 1 molecule of O2 is
formed for each CO2 consumed.
When fats are metabolized, some of the O2
combines with H+ atoms from the fat to form H2O.
Transport of CO2 in the Blood
• Most of the CO2 in RBCs reacts with H2O, forming
carbonic acid, which dissociates to H+ and
bicarbonate ion.
Transport of Bicarbonate to the Plasma
• Many of the bicarbonate ions are transported out of the
RBC in exchange for Cl- by the bicarbonate-chloride
carrier protein.
CO2 and Hemoglobin
• Some CO2 combines with
hemoglobin forming
carbaminohemoglobin
(CO2HbB).
• In the lungs, the CO2 is
released from CO2HbB.
• Hemoglobin also binds
the H+ released from the
dissociation of carbonic
acid. This helps buffer
the pH of the RBC.
Release of bound CO2 in the Lungs
• How does the CO2 that combined with H2O
and hemoglobin get released in the lungs?
Binding of O2 to hemoglobin tends to displace CO2 from the
blood. This is called the Haldane Effect.
Haldane Effect
• The binding of O2 to carbaminohemoglobin
promotes its conversion to hemoglobin and
CO2.
• The binding of O2 to hemoglobin causes the
release of a H+ ion. The H+ ion can then
combine with bicarbonate ion to form
carbonic acid, which then dissociates to CO2
and H2O.
CO2 and Blood pH
• What happens to the blood pH if CO2 becomes
elevated?
- It drops, because more carbonic acid is formed.
- Normally, the pH of arterial blood is 7.41 and that
of venous blood is 7.37 (0.04 difference).
- During exercise, the pH of venous blood can drop
by 0.5 units.