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Section III Oxygen and Carbon Dioxide Transport in Blood

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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

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I. Transport of Oxygen

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Oxygen Transport

• Method Percentage

• Dissolved in Plasma 1.5 %

• Combined with Hemoglobin 98.5 %

Bound to HgbDissolved

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Hemoglobin StructureProtein made up of 4 subunits

Every subunit contains a heme moiety attached to a polypeptide chain.

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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

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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.

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When saturated with O2 (4 O2 in one hemoglobi

n molecule), it is always written Hb4O8.

The reaction is rapid, requiring less than 0.01 se

cond.

The deoxygenation (reduction) of Hb4O8 is also

very rapid.

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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 bloodOxygen 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:

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The oxygen-hemoglobin dissociation curve:（氧离曲线）

the curve relating percentage saturation of the O2-carry

power of hemoglobin to the PO2.

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The oxygen-hemoglobin dissociation curve

A. Flattened upper portion

B. Steep middle portion

C. Lower portion

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Shifting the Curve

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Factors that Shift the Oxygen-Hemoglobin Dissociation Curve

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1. pH and PCO2: Bohr effect

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2. Temperature

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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 c

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

The affinity of hemoglobin for O2 diminishes as the co

ncentration of 2,3-DPG increase in the red blood cells.

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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.

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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,

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Effect of CO & Anemia on Hb-O2 affinityEffect of CO & Anemia on Hb-O2 affinity

Normal blood with Hb=15 gm/dl, anemia with Hb=7.5 gm/dl, and normal blood with 50% COHb (carboxyhemoglobin).

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5. Fetal Hemoglobin

Advantage

Increased O2 release to the fetal tissues under the hypoxic condition.

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II Carbon Dioxide Transport Method Percentage

• Dissolved in Plasma 7 - 10 %

• Chemically Bound to

Hemoglobin in RBC’s 20 - 30 %

• As Bicarbonate Ion in

Plasma 60 -70 %

Dissolvedbound to HbHCO3-

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Carbaminohemoglobin Formation

• Carbon dioxide molecule reversibly attaches to an amino portion of hemoglobin.

CO2 + Hb HbCO2

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Carbonic Acid Formation

• The carbonic anhydrase stimulates water to combine quickly with carbon dioxide.

CO2 + H2 0 H2 CO3

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Bicarbonate Ion Formation

• Carbonic acid breaks down to release a hydrogen ion and bicarbonate.

H2 CO3 H+ + HCO-3

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CO2 Transport and Cl- Movement

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Carbon Dioxide Dissociation CurveCarbon Dioxide Dissociation Curve

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.

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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

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Interaction Between CO2 and O2 Transportation

1. Bohr effect

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2. Haldane effect