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Part 4 Oxygen and Carbon Dioxide Transport in Blood

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Two forms of the gases: physical dissolution and chemical combination.

Most of O2 and CO2 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

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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 %

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Oxygen

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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Oxygen Capacity (氧容量） : The maximum quantity of oxygen that will combine chemically with the hemoglobin in a unit volume of blood Normal Value: 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） : The percentage of all the available heme binding sites saturated with oxygen

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 high concentration in red blood cells bec

ause of their content of 2,3-DPG mutase ( 变位酶） .

Diminishes the affinity of hemoglobin for O2

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Importance:

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.

PCO 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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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 %

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Carbon Dioxide

Dissolvedbound to HbHCO3-

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Carbaminohemoglobin （氨基甲酰血红蛋白） 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