Transport of Oxygen and Carbon Dioxide in the Blood | Biochemistry

In this article we will discuss about the Transport of Oxygen and Carbon Dioxide in the Blood:- 1. Function of Hemoglobin 2. Dissociation of Oxy-hemoglobin 3. Clinical Signs of Variation in Hemoglobin Saturation 4. Transport of CO₂ in Blood 5. Effect of CO₂ on Blood pH.

Contents:

1. Function of Hemoglobin
2. Dissociation of Oxy-hemoglobin
3. Clinical Signs of Variation in Hemoglobin Saturation
4. Transport of CO₂ in Blood
5. Effect of CO₂ on Blood pH

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1. Function of Hemoglobin:

a. The transport of oxygen from the lungs to the tissues by the blood is due to the abil­ity of hemoglobin to combine reversibly with oxygen:

Hb + O₂ = HbO₂

(Hb=Reduced (deoxygenated) hemoglo­bin; HbO₂ = Oxy-hemoglobin).

b. At a tension of 100 mm Hg or more, hemoglobin is completely saturated. The oxygen carrying power of the blood is absolutely a function of the hemoglobin (red cell) concentration.

2. Dissociation of Oxy-hemoglobin:

a. The vital relationship between the satura­tion of hemoglobin and the oxygen ten­sion is shown below by the dissociation curve of oxy-hemoglobin in which the per cent saturation is plotted against the oxy­gen tension.

b. The curve drawn with CO₂ at a tension of 40 mm Hg is considered as representative of the normal physiologic condition.

c. The hemoglobin is 95-98% saturated when the oxygen tension is 100 mm Hg in arte­rial blood. There is a slight effect on the saturation of hemoglobin with further in­crease in oxygen tension.

d. The saturation of hemoglobin declines slowly with the fall in oxygen tension and a rapid evolution of oxygen takes place at the oxygen tension of 50 mm Hg. This is the “unloading tension” of hemoglobin.

e. In the tissues, the oxygen tension is about 40 mm Hg and the oxy-hemoglobin disso­ciates and oxygen is readily available to the cells.

f. During the passage of blood through the tissues the oxygen content of the blood falls from 20 to 15 vol%. This gives a con­siderable reserve of oxygenated blood in the event of inadequate oxygenation at the lung.

Factors affecting Dissociation of Oxy-hemoglobin:

a. Temperature:

(i) A rise in temperature decreases hemo­globin saturation.

(ii) At 25°C, hemoglobin is 88% saturated but at 37°C, it is only 56% saturated. There­fore, hemoglobin gives up oxygen more readily while passing from high to low oxygen tension (as from lungs to tissues) in warm-blooded animals than in cold­blooded animals.

b. Electrolytes:

At low oxygen tensions, oxy-hemoglobin gives up oxygen more readily in the presence of electrolytes.

c. Effect of CO₂:

(i) The influence of CO₂ on the shape of the dissociation curve is actually the effect of carbonic acid formation with the lower­ing of the pH of the environment.

(ii) The increase in acidity facilitates the des­iccation of oxy-hemoglobin.

(iii) The ability of CO₂ to shift the slope of the oxy-hemoglobin dissociation curve to the right is known as the Bohr effect. This ef­fect is often described as causing a shift of the P-50 to the right. P-50 is the partial pressure (mm Hg) at which hemoglobin is 50% saturated. 2, 3-biphosphoglycerate, a compound formed during glycolysis in the red cell, also causes a significant shift of the P-50 to the right.

Carboxyhemoglobin:

i. Hemoglobin combines with carbon mon­oxide more readily than with oxygen (210 times as fast) to form cherry-red carboxyhemoglobin.

ii. This reduces the amount of hemoglobin to carry oxygen.

iii. When the carbon monoxide in the inspired air is 0.02%, headache and nausea occur.

iv. In case the carbon monoxide concentra­tion is only 1/120 that of oxygen in the air (about 0.1% carbon monoxide), uncon­sciousness occurs in 1 hour and death in 4 hours.

3. Clinical Signs of Variation in Hemoglobin Saturation:

A decrease in normal oxygenation of blood gives a characteristic bluish appear­ance to the skin. This is said to be cyano­sis. It is characteristic of cyanide poison­ing where respiration is also impaired.

In severe anemia, the concentration of hemoglobin is too low and cyanosis does not take place although the oxygen con­tent of the blood is reduced.

In CO poisoning, the formation of cherry- red carboxyhemoglobin often produces a ruddy appearance in the lips.

4. Transport of CO₂ in Blood:

CO₂ is carried in the cells and the plasma by the blood. It exists in three forms.

The three main frac­tions are:

a. A small amount of carbonic acid.

b. The “carbamino-bound” CO₂ which is transported in combination with proteins (mainly hemoglobin).

c. That carried as bicarbonate in combina­tion with cations of sodium or potassium.

The carbamino-bound CO₂ is important in the exchange of this gas because of the high rate of reaction:

The amount of CO₂ dissolved in the blood is not high, but it is important because any change in its concentration causes the following equilibrium to shift:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺+ HCO₃^–

The above reaction is catalysed by the en­zyme carbonic anhydrase.

5. Effect of CO₂ on Blood pH:

a. CO₂ evolved from the tissues forms car­bonic acid. Most of the carbonic acid formed is promptly converted to bicarbo­nate as shown in the equation below (B⁺ represents, principally, Na⁺ or K⁺).

H₂CO₃ ⇌ H⁺ + HCO₃^– + B⁺ ⇌ BHCO₃^– + H⁺

b. At the pH of blood (7.4), a ratio of 20: 1 must exist between the bicarbonate and carbonic acid. This ratio is calculated from the Henderson-Hasselbalch equation. Any change in H⁺ activity is met by an adjust­ment in the reaction. Any alteration in the ratio disturbs the acid-base balance of the blood in the direction of acidemia or alkalemia.