BLOOD GAS TRANSPORT: Contents

CARBON DIOXIDE TRANSPORT

A. Mechanisms of Transport

1. Physical solution

a. proportional to P_CO2
b. about 25 x more soluble than O₂

2. Combined with proteins in the plasma

a. termed carbamino-CO₂
b. combines mainly with Hb

CO₂ + Hb = Hb-CO₂

c. when CO₂ combines with Hb, it tends to reduce Hb affinity for O₂ (noncompetitive; Bohr effect)
d. when O₂ combines with Hb, it tends to reduce Hb affinity for CO₂(noncompetitive; Haldane effect)

3. Combined with water to form carbonic acid

a. CO₂ + H₂O = H₂CO₃

b. the enzyme carbonic anhydrase is required for this reaction to go to completion during passage through the capillary bed

Note: carbonic anhydrase in the blood is mainly within red cells

4. Bicarbonate ion, from dissociation of carbonic acid

a. H₂CO₃ = H⁺ + HCO₃^-

b. spontaneous reaction, catalyst not required
c. equilibrium at normal blood pH is very much toward formation of HCO₃^-

Systemic Tissue:

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5. Distribution of CO2 in blood

ml CO₂ / liter Fraction of total blood CO₂

Dissolved CO₂ 30-50 5-10%

Carbamino-CO₂ 30-50 5-10%

H₂CO₃ small < 1%

HCO₃^- 400-500 80-90%

B. CO₂ Dissociation Curve

C. Blood Carbon Dioxide Content

1. Systemic arterial blood (normal)

a. P_a-CO2 = 40 mmHg ( equilibrate with alveolar P_CO2 )

b. C_a-CO2 = 490 ml CO₂ / liter blood (from dissociation curve)

2. Uptake into systemic capillaries

Determined from Uptake = CO x ( C_v-CO2 - C_a-CO2 )

At rest and with normal cardiac output: Uptake about 40 ml CO₂ / liter blood

3. Systemic venous blood (normal, at rest)

C_v-CO2 = C_a-CO2 + Capillary Uptake

= 530 ml CO2 / dl blood

so

P_v-CO2 = 46 mmHg (from dissociation curve)

	ml CO₂ / liter	Fraction of total blood CO₂
Dissolved CO₂	30-50	5-10%
Carbamino-CO₂	30-50	5-10%
H₂CO₃	small	< 1%
HCO₃^-	400-500	80-90%