LECTURE 20
Oxygen Transport by Blood
By
Dr. Khaled Khalil
Assistant Professor of Human Physiology
At the end of this session, the student should be able to:
 Describe the forms in which oxygen is transported in the blood.
 Define oxygen partial pressure (tension), oxygen content, oxygen
capacity and percent hemoglobin saturation.
 Illustrate and describe oxyhemoglobin dissociation curve.
 Describe factors affecting shape of oxyhemoglobin dissociation
curve (shift to right and shift to left).
 Describe the diffusion of oxygen from capillary blood to tissues
and different factors affecting it.
GUYTON & HALL Textbook of Medical Physiology, 12th edition,
page: 495-502.
O2 is present in the blood in two forms:
I. Physical solution:
1- Each 100 ml of arterial
blood contains 0.3 ml of O2
dissolved physically in
plasma.
2- Each 100 ml of venous
blood contains 0.13 ml of
O2 dissolved physically in
plasma.
II. Chemical combination:
More than 98% of the O2 is
bound to hemoglobin of the
red blood cells.
I. Physical solution:
Although it is small amount but it has the following significance:
It determines the rate and direction of oxygen flow.
As O2 diffuses from high pressure to low pressure, physically
dissolved O2 acts as a pathway for the supply of O2 to and from
hemoglobin. When the blood reaches the tissues, it is this small
amount that is first transported to the cells and then, it is replaced
rapidly by more oxygen liberated from Hb.
Hemoglobin
- It is an oxygen carrying pigment.
2) Heme
1) Globin
- a protein composed of four
-Four
pigment
polypeptide chains.
containing a single ferrous ion in
- There are many types of
the center of each molecule.
polypeptide chains according
Each ferrous ion can combine
to their amino acid sequence.
with
e.g., -chain, β-chain, γ-chain,
Therefore, One Hb molecule can
and δ-chain.
combine with 4 molecules of O2.
one
molecule
molecules
of
O2.
- According to the type of polypeptide chains forming the globin
portion, Hb can be classified into:
Hb A (formed of 2  + 2 β chains): it represent 98% of Hb in
adults.
Hb A2 (formed of 2  + 2 δ chains): it represents 2% of Hb in
adults.
Hb F (formed of 2  + 2 γ chains): it represents the main Hb
in fetus.
Binding of Oxygen to Hb is characterized by:
 The reaction is reversible and rapid.
 The reaction is oxygenation not oxidation. i.e., the iron in the
heme remains in the reduced form “Fe++ or ferrous iron”.
The combination of Hb and O2 occurs in steps and each step
accelerates the next one.
i.e.
Hb + O2
HbO2
(25 % saturation)
HbO2 + O2
HbO4
(50 % saturation)
HbO4 + O2
HBO6
(75 % saturation)
HbO6 + O2
HBO8
(100 % saturation)
Oxygen content of the blood:
-It is amount of O2 present in chemical combination with Hb in 100
ml of blood.
Oxygen capacity of the blood:
- It is the amount of oxygen present in chemical combination with
Hb in 100 ml of blood when it is fully saturated.
- In normal subjects, Hb content is about 15 gm % and each gram of
Hb can combine with 1.33 ml O2.
- O2 capacity = 15 x 1.33 = 19.95 ml (about 20 ml O2 / 100 ml.)
- Oxygen capacity depends upon Hb content. So, It is decreased in
anaemia.
Percentage (%) saturation of hemoglobin with oxygen:
- This equals = O2 content / O2 capacity x 100
- Under normal condition, the Hb of systemic arterial blood is
only 97% saturated with O2 (not fully saturated).
- This is due to addition of venous blood from bronchial and
coronary veins to the arterial blood which is called physiological
shunt.
Coefficient of O2 utilization:
- Definition: It is the % of the blood that gives its oxygen as its
passes through the tissue capillaries.
It equals =
O2 content in arterial blood – O2 content in venous blood
O2 content in arterial blood
- The total quantity of oxygen bound with Hb in normal arterial blood is
approximately 19.5 ml/100 ml blood. On passing through tissue capillaries, this
amount is reduced to about 14.5 ml /100 ml blood. Thus, during rest about 5 ml
of O2 is utilized by the tissues by each 100 ml of blood. The normal value for
utilization coefficient is approximately 25%.
-In muscular exercise, It is increased to 75%.
O2 Dissociation Curve
 It is the curve which study the factors that affect the %
saturation of Hb with O2 in relation to O2 tension of the blood.
How to obtain the curve?
- Blood samples are placed in special vessels known as tonometer
used for blood equilibration with gases at body temperature, and
each is exposed to a certain O2 tension.
- The O2 content in each tonometer is determined.
- O2 content is divided by the O2 capacity to get the % saturation.
- % saturation is plotted against the O2 tension.
Shape of the curve:
- The oxygen dissociation curve has a Characteristic sigmoid or S
shape.
- This is because the combination of O2 with the haem groups of
the Hb occurs in steps as the affinity of haem to O2 is increased
after the haem group is oxygenated.
Physiological significance of the curve:
The curve has 3 parts:
1) Upper flat part (plateau).
2) Middle curved part (slope).
3) Lower vertical part (steep).
1) The
At
upper flat
mmHg.), % Hb saturation 60 mmHg., causes decrease of % Hb
part
is 100%. (note that inside saturation from 100% to 90% (i.e., only
(plateau):
the body, % saturation 10%).
O2
tension
(100 So, decrease O2 tension from 100 to
will be 97% only due the The functional significance of the flat
physiological shunt).
At
O2
tension
part of the curve is that the arterial O2
(80 saturation does not change much until
mmHg), % Hb saturation PO2 has decreased to about 60 mmHg.
is 93 %.
At
O2
This enables persons living at high
tension
(60 altitudes or with lung disease to get
mmHg), % Hb saturation enough O2 from this blood.
is 90%.
2)
This
Middle
that: At O2 tension (40 mmHg, causes decrease of % Hb saturation
curved
mm Hg), the % Hb from 100% to 70% (i.e., 30% decrease of %
part
saturation is 70%.
(slope):
part
shows So, decrease O2 tension from 100 to 40
O2 saturation which are given to the tissues
during rest).
This satisfies the tissues needs during
rest & maintains an oxygen reserve in the
blood for emergency condition.
Lower
This part shows that:
This enables peripheral tissues to
vertical part With further decrease of withdraw large amount of oxygen for
(steep):
O2 tension (below 40 only a small drop in capillary PO2 as
mmHg), there is marked occurring in muscular exercise.
decrease in the % Hb
saturation (i.e., more O2
supply to the tissues).
Factors affecting O2 dissociation curve:
A number of factors can influence the affinity of HB to oxygen.
These factors may cause shift of the curve either to right or to left.
Shift to right: Hb gives more O2 to tissues even under high O2
tension.
Shift to left: Hb gives less O2 to the tissue even under low O2
tension.
Factors that shift the curve to Factors that shift the curve to
right:
left:
1) Increased H+ concentration.
1) Decreased H+ concentration.
2) Increased Partial pressure of
2) Decreased partial pressure of
CO2.
CO2.
3) Increased temperature.
3) Decreased temperature.
4) Increased 2,3 DPG Function.
4) Decreased 2,3 DPG function.
5) Carbon monoxide poisoning.
2,3diphosphoglycerate (2,3 DPG)
 It is a chemical substance produced by the mature RBCs to
increase the release of O2 from Hb.
 Formation: Mature RBCs lack mitochondria. Therefore, they
obtain energy through the anaerobic glycolysis. During the course of
glycolysis, 1,3 DPG is formed where it can be converted to 2,3 DPG
by a side reaction using 1,3 DPG mutase enzyme.
 Function: 2,3 DPG combine with β-chain of Hb causing release of
O2 to the tissue.
2,3 DPG is increased in :
2,3 DPG is decreased in:
1) All conditions of hypoxia 1) Fetal hemoglobin.
as in: High altitudes.
2) Muscular exercise.
3) Certain hormones such as:
testosterone.
2) Stored blood.