RESPIRATORY SYSTEM
LECTURE-6
(GAS TRANSPORT)
Dr. Mohammed Sharique Ahmed Quadri
Assistant Prof. physiology
Al maarefa college
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Gas Transport
• O2 which is taken up by the blood at the lungs
is transported to the tissues for use by the
cells.
• CO2 produced at the cell level is transported to
the lungs for elimination.
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Oxygen Transport
• Most O2 in the blood is transported bound to
hemoglobin.
Method of O2 Transport
• Chemically bound to Hemoglobin – 98.5%
• Physically Dissolved in plasma
– 1.5%
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Oxygen Transport
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Hb is present inside red blood cells [RBC].
Hb is iron bearing protein molecule.
It makes reversible combination with oxygen.
When Hb combines with O2, we call
Oxyhemoglobin [HbO2].
• Hb + O2  HbO2
• When O2 not combined with Hb, we call it
reduced Hemoglobin or Deoxyhemoglobin.
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Oxygen Transport
• Cells consume 250ml of O2 /min at rest.
• During exercise, it can increase 25 times.
Why does Hb combine with O2 in the lungs
and release O2 at the tissues?
• Because of high partial pressure of O2 [PO ] in
the lungs, O2 combines with the Hb.
• When this blood with high po2 reaches the
tissues, PO in the tissues is low, therefore, O is
transferred from blood to tissues.
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Oxygen Transport
Important Points
• O2 combination with Hb [Oxyhemoglobin]
does not contribute to PO .
• PO is only due to dissolved O2 in plasma which
is 1.5 % ( 0.3ml/100ml) .
PO is the primary factor determining the
percent Hb saturation.
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Oxygen Transport
• In Hb, 4 atoms of iron are present in Heme
portion of Hb.
• Each atom can combine with O2 molecule, so
Hb molecule can carry 4 molecules of O2.
• Hb is considered fully saturated when all Hb
present is carrying its maximum O2 load .
• The percent Hb [% Hb] saturation can vary
from 0 to 100%.
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Oxygen Transport
• The most important factor determining the
% Hb saturation is PO of the blood [which
refers to concentration of O2 physically
dissolved in blood ].
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Oxygen-Hemoglobin (O2-Hb) dissociation (saturation) curve
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Oxygen-Hemoglobin (O -Hb)
dissociation (saturation) curve
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• The relationship between blood PO and %Hb
saturation is not linear.
• The relationship is S – shaped for O2-Hb
dissociation.
• At pressures of PO 60 – 100mmHg curve
flattens off or plateaus i.e. within this pressure
range of 60-100mmHg change in PO produces
only small change in extent to which Hb is
binds to O2.
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Oxygen-Hemoglobin (O -Hb)
dissociation (saturation) curve
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• If the PO range is 0 - 60mmHg, it shows small
changes in PO results in large change in
extent to which Hb combines with O2 i.e.
lower part is steep.
IMPORTANT
• Upper part of curve – Plateau or Flat
• Lower part of curve – Steep
• Both parts have physiological significance.
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Oxygen-Hemoglobin (O -Hb)
dissociation (saturation) curve
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 Significance of Plateau Phase of O2-Hb curve
• We can see if PO falls from 100mmHg to
60mmHg, there is little change in O
percentage saturation.
• Same way, if PO increases to 600mmHg [by
breathing pure O ] there will be only little
change in Hb saturation [instead of Hb 97.5%
saturation, it can increase to 100% saturation].
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Oxygen-Hemoglobin (O -Hb)
dissociation (saturation) curve
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• Therefore PO range between 60 – 600mmHg, there is
only little change in amount of O being carried by Hb.
• It provides safety margin in O2 carrying capacity of
blood.
 Clinical Application
• In Pulmonary disease, PO may decrease due to poor
ventilation or gas exchange.
• Physiologically PO may decrease at high altitude.
• In these circumstances, if PO falls up to 60mmHg,
body will be little affected, but if PO falls below
60mmHg body will be affected.
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Oxygen-Hemoglobin (O -Hb)
dissociation (saturation) curve
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 Significance of Steep Portion of O – Hb curve
• Steep portion of O – Hb dissociation curve is
from 0 – 60mmHg i.e. present at systemic
capillaries when O2 is unloaded from the Hb.
• At tissue level, PO falls from 100mmHg to
40mmHg but % of Hb saturation is still 75%
[i.e. 25% of O2 is given to tissues].
• Hb in venous blood is 75% saturated at
40mmHg.
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Oxygen-Hemoglobin (O -Hb)
dissociation (saturation) curve
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• If tissue cells are metabolizing more actively and
need more O2, PO can drop from 40mmHg to
20mmHg in tissues, more O2 can be given by
blood and O percent can drop from 75% to 30%.
• Therefore small drop of PO can give more O2 to
tissues.
• This is important in providing more oxygen when
tissue metabolism increases as in exercising
muscles .
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Factors At The Tissue Level Promote
The Unloading Of O2 From Hb
• Shifting of O dissociation curve to right or
more dissociation [giving] of O2 to tissues or
less affinity of O2 for Hb.
• The factors are
1- Increase CO2
2- Increase Acidity [increase H+ ion]
3- Increase Temperature
4- Increase 2,3-BPG [bisphosphoglycerate]
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BOHR EFFECT
• Increase CO2 and increase H+ ion causes more
release of O2 from Hb [i.e. less affinity of O2
with Hb]. It is known as ‘Bohr Effect’.
• Both CO and H+ combine with Hb reversibly
at sites other than O2 binding sides and cause
release of O2.
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Effect of 2,3 – BPG on O -Hb
Dissociation Curve
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• 2,3 – BPG is factor inside the RBC, which affect O2-Hb
binding. It is produced during RBC metabolism.
• 2,3 – BPG can bind reversibly with Hb and decreases its
affinity for O2 therefore shifts Hb-O dissociation curve to
right.
• BPG increases in RBC whenever Hb in arterial blood is
under saturated
 Examples
- People living at high altitude
- People suffering from respiratory disease
- Anemia
• 2,3 – BPG, by increasing O2 unloading helps to maintain O2
availability to tissues.
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Applied: Hb Has Higher Affinity For
Carbon monoxide [CO] Than O2
• CO and O2 compete for same binding sites on Hb,
but this affinity is 240 times more for CO.
• Combination of CO with Hb is know as
‘Carboxyhemoglobin’ [HbCO].
• Even when Hb and O2 are normal. If CO is there
Hb will not be available for O2 combination.
• CO poisoning occurs in coal burning.
• CO is odorless, colorless, tasteless and nonirritating.
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CO2 Transport
• CO2 is transported in the blood by three ways:
1. Physically dissolved in Plasma – 10%
2. Bound to Hb – 30%
3. As Bicarbonate – 60%
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CO2 Transport
• CO2 combines with Hb to form Carbamino
hemoglobin (HbCO2).
• CO2 combines with globin portion of Hemoglobin
[in contrast to O2 which combines with Heme
portion].
• REDUCED Hb has more affinity for CO2
HALDANE EFFECT
• Removing O2 from Hb, increases the ability of
reduced Hb to pick up CO2 and H+ ion [CO2
generated H+ ion]. This effect is known as
‘Haldane Effect’.
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CO2 Transport
• CO2 combines with Hb to form Carbamino hemoglobin
(HbCO2).
• CO2 combines with globin portion of Hemoglobin [in
contrast to O2 which combines with Heme portion].
• As bicarbonate – it is most important means of CO2 as
60% of CO is converted into bicarbonate by the
chemical reaction.
• CO2 + H2O  H2CO3  H+ + HCO3• This reaction takes place slowly in plasma but quickly
within RBC due to presence of enzyme carbonic
anhydrase.
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CO2 Transport
 Chloride Shift
• As CO2 combines with H2O and HCO3 is formed in
RBC.
• RBC membrane has HCO3 – Cl carrier that
passively facilitates the diffusion of these ions in
opposite direction across the membrane.
• HCO3 is moved out of the cell and in its place Cl is
moved into RBC from plasma to restore electric
neutrality.
• This inward shift of Cl in exchange for HCO3
[generated by CO2] is known as chloride shift.
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‘IMPORTANT’
Remember
• Bohr Effect - Increase CO2 and increase H+ ion
causes more release of O2 from Hb [i.e. less
affinity of O2 with Hb]. It is known as ‘Bohr
Effect’.
• Haldane Effect - Removing O2 from Hb,
increases the ability of reduced Hb to pick up
CO2 and H+ ion [CO2 generated H+ ion]. This
effect is known as ‘Haldane Effect’.
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References
• Human physiology by Lauralee Sherwood,
seventh edition
• Text book physiology by Guyton &Hall,11th
edition
• Text book of physiology by Linda .s
contanzo,third edition
Thank you
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