Fig. 24.8 ). Once CO2 diffuses through the tissue and enters plasma, it quickly dissolves. The reaction of CO2 with H2O to form carbonic acid (H2CO3) is the major mechanism for the generation of HCO3 − in red blood cells:
Equation 24.3
The reaction normally proceeds quite slowly; however, it is catalyzed within red blood cells by the enzyme carbonic anhydrase. The HCO3 − diffuses out of the red blood cell in exchange for Cl−, in a process known as the chloride shift, which helps the cell maintain its osmotic equilibrium. This chemical reaction (see
Fig. 24.8 ) is reversible and can be shifted to the right to generate more HCO3 − in response to more CO2 entering the blood from tissues, or it can be shifted to the left as CO2 is exhaled in the lungs, which reduces HCO3 − levels. The free H+ is quickly buffered within the red blood cell by binding to Hgb. Buffering of H+ is critical for keeping the reaction moving toward the synthesis of HCO3; high levels of free H+ (low pH) cause the reaction to shift to the left.
Regulation of Hydrogen Ion Concentration and Acid-Base Balance
The H+ concentration (pH) has a dramatic effect on many metabolic processes within cells, and regulation of pH is essential for normal homeostasis. In the clinical setting, blood pH is measured to assess the concentration of H+ . The normal pH range for adults is 7.35 to 7.45 and is maintained by the lungs, kidneys, and chemical buffer systems. In the respiratory system, conversion of CO2 to HCO3 − , illustrated as follows, is a major mechanism of buffering and regulating the H+ concentration (pH):
Equation 24.4
As PACO2 changes, so does the concentration of HCO3 − and H2CO3, as well as PaCO2. The Henderson-Hasselbalch equation is used to calculate how changes in CO2 and HCO3 − affect pH:
Equation 24.5 where α= solubility. Thus
Equation 24.6 log[HCO3 − ]pH = 6 1. + 0 03 × PCO2
In these equations, the amount of CO2 is determined from the PCO2 and its solubility (α) in solution. For plasma at 37°C, α has a value of 0.03. Also, pK′ is the negative logarithm of the overall dissociation constant for the reaction and has a logarithmic value of 6.1 for plasma at 37°C.
Acute hyperventilation secondary to exercise or anxiety reduces PCO2 and thereby causes an increase in pH (respiratory alkalosis). Conversely, if PCO2 increases because of hypoventilation secondary to an overdose of a respiratory depressant, the pH decreases (respiratory acidosis). Acid-base imbalances are also caused by metabolic disorders such as metabolic acidosis (e.g., lactic acidosis, ketoacidosis, and renal failure) and metabolic alkalosis (e.g., hypokalemia, hypochloremia, vomiting, high doses of steroids).
In contrast to O2, the dissociation curve for CO2 in blood is linear and directly related to PCO2 (
Fig. 24.9). The degree of Hgb saturation with O2 has a major effect on the CO2 dissociation curve. Although O2 and CO2 bind to Hgb 1.
Gases (nitrous oxide, ether, helium) that have a rapid rate of air-to-blood equilibration are perfusion limited. Gases (CO) that have a slow air-to-blood equilibration rate are diffusion limited. Under normal conditions, O2 and CO2 exchange are perfusion limited but can be diffusion limited in some situations.
2.
The major transport mechanism of O2 in blood is within the red blood cell bound to Hgb, and for CO2, it is within red blood cells in the form of HCO3 − .
3.