Chapter 8

Carriage of gases in the blood

and acid/base balance

Liu Baoyi

Respiratory Department

of Qilu Hospital

April, 2013

Oxygen and Carbon

Dioxide Transport in Blood

Two forms of the gases: physical dissolution and

chemical combination.

Most of oxygen and carbon dioxide in the blood is

transported in chemical combination

Only the gas in physical dissolution express PP and

diffuse to a place with low PP.

Dynamic balance between the two forms:

Physical dissolution Chemical combinationPP

PP

Basic Mechanism of the Gases Transportation

I. Transport of Oxygen

Oxygen Transport

• Method Percentage

• Dissolved in Plasma 1.5 %

• Combined with Hemoglobin 98.5 %

Bound to Hgb

Dissolved

Hemoglobin StructureProtein made up of 4 subunits

Every subunit contains a heme moiety attached to a

polypeptide chain.

Oxyhemoglobin Formation

• An oxygen molecule reversibly attaches to the

heme portion of hemoglobin.

• The heme unit contains iron ( +2 ) which

provides the attractive force.

O2 + Hb HbO2

In normal adults, most of the hemoglobin

contains 2α and 2 β chains.

Each of the 4 iron atoms can bind reversibly on

O2 molecule.

The iron stays in the ferrous state, so that the

reaction is an oxygenation, not an oxidation.

When saturated with O2 (4 O2 in one

hemoglobin molecule), it is always written

Hb4O8.

The reaction is rapid, requiring less than 0.01

second.

The deoxygenation (reduction) of Hb4O8 is also

very rapid.

Oxygen Capacity (氧容量）: The maximum quantity

of oxygen that will combine chemically with the

hemoglobin in a unit volume of blood;

normally it amounts to 1.34 ml of O2 per gm of Hb or

20 ml of O2 per 100 ml of blood.

Oxygen Content（氧含量）: how much oxygen is in

the blood

Oxygen Saturation （血氧饱和度）: A measure of

how much oxygen the blood is carrying as a

percentage of the maximum it could carry

Basic Concepts:

The oxygen-hemoglobin dissociation curve:

（氧离曲线）

the curve

relating

percentage

saturation of

the O2-carry

power of

hemoglobin to

the PO2.

The oxygen-hemoglobin dissociation curve

A. Flattened upper portion

B. Steep middle portion

C. Lower portion

Factors that Shift the Oxygen-

Hemoglobin Dissociation

Curve

Shifting the Curve

1. pH and PCO2: Bohr effect

2. Temperature

3. 2,3-diphosphoglycerate, 2,3,-DPG

(2, 3-二磷酸甘油）

A byproduct of anaerobic glycolysis.

Present in especially high concentration in red blood

cells because of their content of 2,3-DPG mutase.

The affinity of hemoglobin for O2 diminishes as the

concentration of 2,3-DPG increase in the red blood

cells.

Importance:

The normal DPG in the blood …

Hypoxic condition that last longer than

a few hours…

Disadvantage:

The excess DPG also makes it more

difficult for the hemoglobin to

combines with O2 in the lungs.

4. Effect of Carbon Monoxide (CO)

CO combines Hb at the same point as does O2,

and can displace O2 from hemoglobin.

CO binds with about 250 times as much tenacity

as O2.

Therefore, a PCO only a little greater than 0.4

mmHg can be lethal.

In the presence of CO (low concentration), the

affinity of hemoglobin for O2 is enhanced,

Normal blood with Hb=15 gm/dl, anemia with Hb=7.5 gm/dl,

and normal blood with 50% COHb (carboxyhemoglobin).

Effect of CO & Anemia on Hb-O2

affinity

5. Fetal Hemoglobin

Advantage

Increased O2

release to the

fetal tissues

under the

hypoxic

condition.

6. Myoglobin

Carbon monoxide poisoning

1. Why CO is poisonous?

2. Symptoms of CO poisoning.

3. The treatment of CO poisoning

II Carbon Dioxide TransportMethod Percentage

• Dissolved in Plasma 7 - 10 %

• Chemically Bound to

Hemoglobin in RBC’s 20 - 30 %

• As Bicarbonate Ion in

Plasma 60 -70 %

Dissolved

bound to Hb

HCO3-

Carbaminohemoglobin

Formation

• Carbon dioxide molecule reversibly attaches to

an amino portion of hemoglobin.

CO2 + Hb HbCO2

Carbonic Acid Formation

• The carbonic anhydrase stimulates water

to combine quickly with carbon dioxide.

CO2 + H2 0 H2 CO3

Bicarbonate Ion Formation

• Carbonic acid breaks down to release a

hydrogen ion and bicarbonate.

H2 CO3

H+ + HCO-3

CO2 Transport and Cl- Movement

Haldane effect

For any given PCO2,

the blood will hold

more CO2 when the

PO2 has been

diminished.

Reflects the

tendency for an

increase in PO2 to

diminish the affinity

of hemoglobin for

CO2.

Carbon Dioxide Dissociation Curve

Mechanism of Haldane effect

Combination of oxygen with hemoglobin in the

lungs cause the hemoglobin to becomes a

stronger acid. Therefore:

1) The more highly acidic hemoglobin has less

tendency to combine with CO2 to form CO2

Hb

2) The increased acidity of the hemoglobin also

causes it to release an excess of hydrogen

irons

Interaction Between CO2 and O2 Transportation

1. Bohr effect

2. Haldane effect

Acid/base balance

34

Interpretation of arterial blood gases