Carbon Dioxide Is Transported in the Blood Both in Solution in Plasma and in Chemical Combination

Unlike oxygen, which is bound only to hemoglobin, carbon dioxide (CO₂) is transported in several forms (Figure 48-5). Carbon dioxide is produced in the tissue; therefore tissue Pco₂ is higher than the Pco₂ of the blood arriving in the capillaries.

FIGURE 48-5 Forms of carbon dioxide (CO₂) transport in the blood. All reactions displayed in this diagram can be reversed when the blood reaches the lung and CO₂ diffuses into the alveolus. H₂CO₃, Carbonic acid; HCO₃, bicarbonate.

Approximately 5% of the CO₂ entering the blood is transported in solution. The majority of CO₂ diffuses into the erythrocyte, where it undergoes one of two chem­ical reactions. Most of the CO₂ combines with water and forms carbonic acid (H₂CO₃), which then dissociates into bicarbonate (HCO₃") and hydrogen ion (H⁴), as follows:

H₂O + CO₂ - H₂CO₃ < > H* + HCO₃-

This reaction also occurs in plasma, but in the erythrocyte, the presence of carbonic anhydrase accelerates the hydration of CO₂ several hundred-fold. Ionization of H₂CO₃ occurs rapidly, and H* and HCO₃^- accumulate within the erythrocyte. The reversible reaction is kept moving to the right because H⁺ is buffered by hemoglobin. Most of the HCO₃^- that is produced in the erythrocyte diffuses out along a concentration gradient into the plasma.

The addition of CO₂ to capillary blood is facilitated by the deoxygenation of hemoglobin occurring in the tissues. Deoxyhemoglobin is a weaker acid than oxyhemoglobin and therefore a better buffer. Thus it combines more readily with H⁺ and facilitates the formation of HCO₃^- from CO₂.

The carbamino compounds are the second form in which CO₂ is transported in the blood. Carbamino compounds are formed by coupling of CO₂ to the -NH groups of proteins, particularly hemoglobin. Although carbamino compounds account for only 15% to 20% of the total CO₂ content of the blood, they are responsible for 20% to 30% of the CO₂ exchange between the tissues and the lungs.

When venous blood reaches the lungs, CO₂ diffuses into the alveoli from plasma and erythrocytes, thus causing the reactions shown in Figure 48-5 to move to the left. Simultaneously, the oxygenation of hemoglobin releases H⁺ ions, which combine with HCO₃^- to form H₂CO₃ and thus CO₂.

The blood content of carbon dioxide as a function of Pco₂ is depicted in the CO₂ equilibrium curves shown in Figure 48-6. Curves are shown for oxygenated blood (Po₂ = IOO mm Hg), for partially deoxygenated blood (Po₂ = 50), and for deoxygen­ated blood (Po₂ = O). The curves are almost linear and have no plateau in the physiological range; CO₂ can be added to the blood as long as the buffering capacity is available. The higher CO₂ content of deoxygenated blood resulting from the higher buffering capacity of deoxyhemoglobin is clearly visible.

FIGURE 48-6 Carbon dioxide equilibration curves showing the amount of CO₂ contained in the blood (CO₂ content) as a function of CO₂ tension (Pco₂). Curves are shown for dissolved CO₂ and for total CO₂ content at various oxygen tensions (Po₂).