The Bicarbonate Buffer

The C02-bicarbonate buffer is a little different from buffers using the usual kind of acids and bases, but it is extremely important in maintaining the acid-base balance of the blood. The acid form of the bicarbonate buffer is actually a gas dissolved in water. Dissolved CO2 is turned into an acid by hydration to give H2CO3. Hydrated CO2 is then much like a carboxylic acid. It gives up a proton to a base and makes bicarbonate, HCO3. 

When CO2 is dissolved in water, there is never very much H2CO3, so we can ignore it and count CO2 as the acid and HCO3 as the base. 

There are two ways of dealing with the bicarbonate buffer system. The first uses the Henderson-Hasselbalch equation and an effective pA a of 6.1. If there is more base (HCO 3) than acid (CO2), the pH will always be bigger than the pAT. This is usually the case physiologically (pH = 7.4 pATa = 6.1) so that on a molar basis there is always more than 10-fold more HCO 3 than CO2. 

You might be wondering why the bicarbonate buffer can buffer effectively at pH 7.4 when its pAT is 6.1. The answer is that it doesn t buffer all that well. What makes it unique and the major buffer system of the blood is that CO2, being a gas, can be exhaled by the lungs. Exhaling CO2 is equivalent to exhaling protons. 

It s not that a proton is exhaled it s just left behind and turned into water. This gives the body control over the concentration of the CO2 by controlling the breathing rate. 

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X 10 M), and an equivalent amount of OH (its usual concentration in plasma) would swamp the buffer system, causing a dangerous rise in the plasma pH. How, then, can this bicarbonate system function effectively The bicarbonate buffer system works well because the critical concentration of H2CO3 is maintained relatively constant through equilibrium with dissolved CO2 produced in the tissues and available as a gaseous CO2 reservoir in the lungs. ... 

Under the conditions of temperature and ionic strength prevailing in mammalian body fluids, the equilibrium for this reaction lies far to the left, such that about 500 CO2 molecules are present in solution for every molecule of H2CO3. Because dissolved CO2 and H2CO3 are in equilibrium, the proper expression for H2CO3 availability is [C02(d)] + [H2CO3], the so-called total carbonic acid pool, consisting primarily of C02(d). The overall equilibrium for the bicarbonate buffer system then is... 

Dissolve TNBS in 0.1M sodium bicarbonate, pH 8.5, at a concentration of 0.01 percent (w/v). Prepare fresh. Note TNBS may be prepared as a stock solution in ethanol at a concentration of 1.5 percent. This solution is stable to long-term storage and may be diluted as needed in the bicarbonate buffer to the required concentration. 

This reaction is essential in maintaining a constant pH in blood by the bicarbonate buffer system. Carbonic anhydrase, which contains a single zinc atom in its structure, has a molecular weight of about 30,000. In this structure, zinc is surrounded tetrahedrally by three histidine molecules and one water molecule. The exact role of the catalyst is not known, but it is believed to involve hydrolysis that can be represented as... 

Henderson-Hasselbalch Equation Titration Curves p/—Isoelectric Point The Bicarbonate Buffer Imbalance in Blood pH Acidosis and Alkalosis... 

Blood, Lungs, and Buffer The Bicarbonate Buffer System... 

The numbers listed were derived from the inhibition index values given by Ukita et al. (454)- The substrate was cy tidine 2 3 -phosphate, 8.8 mM. The Michaelis constant was reported to be 24 mM in the bicarbonate buffer 0.03 M, pH 7.6, 37°, that was used in all of the measurements. The inhibition constants were derived on the assumption of competitive inhibition in all cases. This type of inhibition was specifically shown for those compounds with a second number in parentheses. This latter number was obtained from multipoint double reciprocal plots in the usual manner. The agreement of the two... 

It is not yet clear which estimates of the ratio between the levels of protein and of carbohydrate metabolism during hypoxia should be regarded as reliable. It seems likely that the increase in respiratory quotient in freshwater fish to values of 2.5-2.8, as found by Mohamed and Kutty (1983a, 1986), indicates a predominance of protein expenditure over that of carbohydrate. A hypoxic environment shifts the acid-base balance of the fish towards acidosis (Kotsar, 1976), thereby inducing the redistribution of electrolytes, alteration of ion exchange and the activity of Na+-K+-Mg2+-ATPases and alkaline phosphatases. It also leads to an increased level of C02 in the blood, which enhances the bicarbonate buffer system (Kotsar, 1976). In section 2.1, we... 

The foregoing latexes prepared using persulfate initiator and bicarbonate buffer contained surface sulfate groups and, in some cases, surface hydroxyl groups. None of these latexes contained carboxyl groups. These latexes had final pH values of 7-8. If the bicarbonate buffer was omitted, the final pH of the latex was 2-3 because of the bisulfate ion generated by the persulfate decomposition. 

The omission of the bicarbonate buffer gave latexes of lower pH with weak-acid groups. The formation of weak-acid groups in these latexes is not merely an effect of pH during the polymerization potassium dihydrogenphosphate, sodium borate, and sodium... 

Thus the bicarbonate buffer in the liver with its components CO2 and HC03 is also used for maintaining pH homoeostasis. The various cellular and subcellular compartments can only keep their own specific pH values constant provided the pH value of the extracellular space is not subject to major fluctuations. A constant balance between the formation and excretion of CO2 and HCOj must therefore be guaranteed. Bicarbonate neutralization takes place, energy-driven and irreversibly, in the urea cycle. In this context, HC03 is considered to be a relatively strong base, whereas NH4+ is regarded as a weak acid. 

The bicarbonate buffer system of blood plasma works well because ... 

The buffer value (p) is defined as the amount of base required to cause a change in pH of one unit. The buffer value of the bicarbonate buffer in plasma is 55.6 mmol/L. Derivation of this value is obtained by taking partial differentials of the Henderson-Hasselbalch equation, which is presented in detail in the second edition of this textbook. ... 

The Henderson-Hasselbalch equation was developed independently by the Ameriean biological chemist L. J. Henderson and the Swedish physiologist K. A. Hasselbaleh, for relating the pH to the bicarbonate buffer system of the blood (see below). In its general form, the Henderson-Hasselbalch equation is a useful expression for buffer caleulations. It can be derived from the equilibrium constant expression for a dissociation reaction of the general weak acid (HA) in Equation (1.3) ... 

The punching of the paper discs may also be done after the covalent coupling of the protein (Lehtonen and Viljanen, 1980b). The use of PBS in the coupling was claimed to be as effective as the bicarbonate buffer. 

The body s buffering system can be divided into three components bicarbonate/carbonic acid, proteins, and phosphates. The bicarbonate buffer is the most important of the body s buffers, because (1) there is more bicarbonate present in the extracellular fluid (ECF) than any other buffer component (2) the supply of carbon dioxide is unlimited and (3) the acidity of ECF can be regulated by controlling either the bicarbonate concentration or the PCO2. 

Carbonic acid represents the respiratory component of the buffer pair because its concentration is directly proportional to the PCO2, which is determined by ventilation. Bicarbonate represents the metabolic component because the kidney may alter its concentration by reabsorption, generating new bicarbonate, or altering elimination. The bicarbonate buffer system easily adapts to changes in acid-base status by alterations in ventilatory elimination of acid (PCO2) and/or renal elimination of base (HCO3). 

The three most important buffers in the body are the bicarbonate buffer, the phosphate buffer, and the protein buffer. Each is adapted to solve specific physiological problems in the body. 

Recall that buffering capacity is greatest at or near the pKd of the acid-con-jugate base pair. Bicarbonate buffer is clearly unusual in that its pK,d is 6.37. It would appear at first glance that bicarbonate buffer is ill-suited to buffer blood. The ratio of HC03 to C02 required to maintain a pH of 7.4 is approximately 11 to 1. In other words, the bicarbonate buffer operates in blood near the limit of its buffering power. In addition, the concentrations of COz and HC03 are... 

One of the principal effects of diarrhea is the excretion of large quantities of sodium bicarbonate. In which direction does the bicarbonate buffer system shift under this circumstance What is the resulting condition called ... 

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