Blood Buffer System

Buffers Blood

Bicarbonate/Carbon Dioxide Buffer

The pH of any buffer system is determined by the concentration ratio of the buffer pairs and the pKa of the system p. 378). The pH of a bicarbonate solution is the concentration ratio of bicarbonate and dissolved carbon dioxide ([HCO3-]/[CO2]), as defined in the Henderson-Hasselbalch equation (^ A1). Given [HCO3] = 24mmol/L and [CO2] = 1.2mmol/l, [HCO3-]/ [CO2] = 24/1.2 = 20. Given log20 = 1.3 and pKa = 6.1, a pH of 7.4 is derived when these values are set into the equation (^ A2). If [HCO 3 ] drops to 10 and [CO2] decreases to 0.5 mmol/L, the ratio of the two variables will not change, and the pH will remain constant.

When added to a buffered solution, H+ ions combine with the buffer base (HCO3- in this case), resulting in the formation of buffer acid (HCO3- + H+ ^ CO2 + H2O). In a closed system from which CO2 cannot escape (^ A3), the amount of buffer acid formed (CO2) equals the amount of buffer base consumed (HCO3 ). The inverse holds true for the addition of hydroxide ions (OH- + CO2 ^ HCO3 ). After addition of 2 mmol/L of H+, the aforementioned baseline ratio [HCO3-]/[CO2] of 24/1.2 (^ A2) changes to 22/3.2, making the pH fall to 6.93 (^ A3). Thus, the buffer capacity of the HCO3-/ CO2 buffer at pH 7.4 is very low in a closed system, for which the pKa of 6.1 is too far from the target pH of 7.4 (^ pp. 138,378 ff).

If, however, the additionally produced CO2 is eliminated from the system (open system; ^ A4), only the [HCO3 ] will change when the same amount of H+ is added (2 mmol/L). The corresponding decrease in the [HCO3-]/[CO2] ratio (22/1.2) and pH (7.36) is much less than in a closed system. In the body, bicarbonate buffering occurs in an open system in which the partial pressure (PCO2) and hence the concentration of carbon dioxide in plasma ([CO2] = a •PCO2; ^ p. 126) are regulated by respiration (^ B). The lungs normally eliminate as much CO2 as produced by metabolism (1500020 000 mmol/day), while the alveolar PCO2 remains constant (^ p. 120ff.). Since the plasma PCO2 adapts to the alveolar PCO2 during each respiratory cycle, the arterial Pco2 (Paco2) also remains constant. An increased supply of H+ in the periphery leads to an increase in the Pco2 of venous blood (H+ + HCO3- ^ CO2 + H2O) B1 ). The lungs eliminate the additional CO2 so quickly that the arterial PCO2 remains practically unchanged despite the addition of H+ (open system!).

The following example demonstrates the quantitatively small impact of increased pulmonary CO2 elimination. A two-fold increase in the amount of H+ ions produced within the body on a given day (normally 60 mmol/day) will result in the added production of 60 mmol more of CO2 per day (disregarding non-bicarbonate buffers). This corresponds to only about 0.3% of the normal daily CO2 elimination rate.

An increased supply of OH- ions in the periphery has basically similar effects. Since OH- + CO2 ^ HCO3-, [HCO3-] increases and the venous PCO2 becomes smaller than normal. Because the rate of CO2 elimination is also reduced, the arterial PCO2 also does not change in the illustrated example (^ B2).

At a pH of 7.4, the open HCO3/CO2 buffer system makes up about two-thirds of the buffer capacity of the blood when the PCO2 remains constant at 5.33 kPa (^ p. 138). Mainly intracellular non-bicarbonate buffers provide the remaining buffer capacity.

Since non-bicarbonate buffers (NBBs) function in closed systems, their total concentration ([NBB base] + [NBB acid]) remains constant, even after buffering. The total concentration changes in response to changes in the hemoglobin concentration, however, since hemoglobin is the main constituent of NBBs (^ pp. 138,146). NBBs supplement the HCO3-/ CO2 buffer in non-respiratory (metabolic) acid-base disturbances (^ p. 142), but are the only effective buffers in respiratory acid-base disturbances (^ p. 144).

I— A. Bicarbonate buffers in closed and open systems

Delicious Diabetic Recipes

Delicious Diabetic Recipes

This brilliant guide will teach you how to cook all those delicious recipes for people who have diabetes.

Get My Free Ebook


  • mareta
    What is an open buffer system?
    7 years ago

Post a comment