Review of Medical Physiology - William F. Ganong 2002

Respiration
Gas transport between the lungs and tissues
Carbon dioxide transport

Buffer Systems

To understand buffering processes in the body from the moment CO₂ is generated from carbonic acid in the Blood, it is necessary to examine how CO₂ transport occurs. Buffer systems are introduced in Chapter 1 and discussed in detail in Chapter 39.

Transport of Carbon Dioxide in the Blood

The solubility of CO₂ in blood is 20 times greater than that of O₂; therefore, at identical partial pressures, there is considerably more CO₂ in simple solution than O₂. Upon diffusion into erythrocytes, CO₂ is rapidly hydrated to H₂CO₃ in the presence of Carbonic anhydrase. Dissociation of H₂CO₃ yields H⁺ and HCO₃. The H⁺ ion is buffered primarily by Hemoglobin, whereas HCO₃ enters the plasma. A fraction of the CO₂ within erythrocytes reacts with protein amino groups—chiefly those of hemoglobin—to form carbamino compounds.

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Fig. 35-5. Hemoglobin and Myoglobin dissociation curves.

As soon as deoxygenated hemoglobin binds more H⁺ than oxyhemoglobin does, the rapid formation of carbamino compounds is initiated. This promotes O₂ binding to hemoglobin while decreasing its affinity for CO₂ (the Haldane effect). Venous blood carries more CO₂ than arterial blood; thus, CO₂ uptake is facilitated in the Tissues, and CO₂ is released in the Lungs. Over 11% of CO₂ enters the systemic capillary blood and is transported to the lungs as carbamino-CO₂.

In plasma, CO₂ reacts with Plasma Proteins to form small amounts of carbamino compounds, while a minor fraction undergoes Hydration. The hydration reaction proceeds slowly in the absence of carbonic anhydrase.

Chloride Shift

Due to the increased intracellular concentration of HCO₃ in erythrocytes relative to plasma as blood passes through the capillaries, over 70% of this HCO₃ diffuses into the plasma. The movement of excess HCO₃ out of erythrocytes in exchange for Cl^- (Fig. 35-6) is mediated by Band 3, the principal membrane transport protein. This process is known as the chloride shift, as it results in a significantly higher Cl^- content in the erythrocytes of venous blood compared to arterial blood. The chloride shift occurs rapidly, reaching completion within 1 second.

Note that every CO₂ molecule entering an erythrocyte increases the number of osmotically active particles within The Cell by one—either HCO₃ or Cl (see Fig. 35-6). Consequently, erythrocytes take up Water and increase in volume. For this reason, and considering that the small amount of fluid in arterial blood returns via Lymphatic vessels faster than via Veins, the hematocrit of venous blood is normally about 3% greater than that of arterial blood. In the lungs, Cl is driven out of the Cells, causing them to shrink.

Fig. 35-6. Changes occurring in erythrocytes upon The addition of CO₂ to the blood. Note that every CO₂ molecule entering an erythrocyte results in an additional HCO_3- or Cl^- ion inside the cell.

Summary

Given the diverse Functions of CO₂ in plasma and erythrocytes, they are summarized in Table 35-2. As shown in Fig. 35-7, the increase in the CO₂-carrying capacity of blood is defined by the difference between the dissolved CO₂ and total CO₂ lines on the CO₂ dissociation curve.

Each deciliter of arterial blood contains approximately 49 mL of CO₂ (see Table 35-1): 2.6 mL dissolved, 2.6 mL bound as carbamino compounds, and 43.8 mL present as HCO_3-. In the tissues, each deciliter of blood receives 3.7 mL of CO₂, of which 0.4 mL remains in solution, 0.8 mL forms carbamino compounds, and 2.5 mL forms CHO₃. The blood pH drops from 7.4 to 7.36. The reverse process occurs in the lungs, where 3.7 mL of CO₂ is eliminated via the alveoli. Thus, at rest, 200 mL of CO₂ per minute—and significantly more during exercise—is transported from the tissues to the lungs and excreted. This is a remarkable capacity, as this amount of CO₂ is equivalent to buffering 12,500 mEq of H⁺ per 24 hours.

Table 35-2. CO₂ in the blood

Fig. 35-7. CO₂ dissociation curve. Arterial (a) and venous (v) points represent the total CO₂ content in normal human arterial and venous blood at rest (modified and reproduced with permission from Schmidt RF, Thews G [editors]: Human Physiology. Springer, 1983).