CO2 Transport Linda Costanzo, Ph.D.
OBJECTIVES:
After studying this lecture, the student should understand:
1. How carbon dioxide is carried in blood, especially as bicarbonate. 2. The effect of oxygen on the carbon dioxide content of blood.
CO2 is carried in the blood in three forms: dissolved CO2, CO2 bound to proteins such as - hemoglobin (called carbaminohemoglobin) and, most importantly, as HCO3 .
I. DISSOLVED CO2 is described by Henry’s law as the partial pressure times the solubility and accounts for 5% of the total CO2 content of blood. The solubility of CO2 in blood is 0.07 ml CO2/100 ml blood/mm Hg (more than twenty times the solubility of O2). Thus, in arterial blood with a PCO2 of 40 mm Hg, dissolved CO2 is:
Dissolved CO2 = PCO2 x solubility
= 40 mm Hg x 0.07 ml CO2/100 ml blood/mm Hg
= 2.8 ml CO2/100 ml blood, or 2.8 vol%
II. CARBAMINOHEMOGLOBIN
CO2 binds to terminal amino groups on hemoglobin and plasma proteins such as albumin, so-called carbamino compounds. Carbamino compounds account for 3% of the total CO2 in blood, 2/3 of which is carbaminohemoglobin.
- III. HCO3
- - 92%, of the CO2 is carried in blood as HCO3 . The reactions that produce HCO3 are as follows:
+ - CO2 + H2O W H2CO3 W H + HCO3 Carbonic anhydrase
In the tissues, CO2 generated from aerobic metabolism is added to venous blood. In the red cells of venous blood, the above reactions occur, generating H+ and - + HCO3 . The H remains inside the red cells, buffered by deoxyhemoglobin. The - - - HCO3 exchanges with Cl across the red cell membrane (Cl– HCO3 exchange) and travels to the lungs in the plasma. In the lungs the reactions occur in reverse, - HCO3 re-enters the red cells, CO2 is regenerated and then is expired.
Figure 1.
The graph below shows the relationship between the CO2 content of blood and PCO2. In contrast to O2, which has a sigmoidal relationship with PO2, CO2 content is linear as PCO2 changes over the physiologic range. Even though the carbamino - portion of the CO2 content is saturable as PO2 increases, the dissolved and HCO3 forms are linear. Note that the lower the PO2, the higher the CO2 content, which is attributed to greater binding of CO2 to hemoglobin in its deoxygenated form, called the Haldane effect. (Makes sense, since CO2 must be carried to the lungs in venous blood where hemoglobin is relatively deoxygenated.)
Figure 2. (Fig.35-9) Blood CO2 equilibrium curves (atrial and venous). Venous blood can transport more CO2 than arterial blood at any given PCO2. Compared with the hemoglobin-oxygen equilibrium curve, the CO2 curves are essentially straight lines between PCO2 of 20 and 80 mm Hg.
IV. PRACTICE QUESTIONS
1. Which of the following statements is correct about CO2 transport in blood?
A. Systemic venous pH is higher than systemic arterial pH. B. Deoxyhemoglobin carries more CO2 than oxyhemoglobin. - C. Most HCO3 is carried inside the red blood cells. D. Binding of H+ to hemoglobin increases the affinity of hemoglobin for O2.
EXPLANATIONS
2. Answer = B. Deoxyhemoglobin carries more CO2 than oxyhemoglobin (Haldane effect). Binding of H+ to hemoglobin decreases the affinity for - O2 (Bohr effect). Most HCO3 is carried in the plasma, not in the red cells. Systemic venous pH is lower than systemic arterial pH because CO2, a weak acid, has been added to it.