Acid-base balance buffers

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

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 C02 is turned into an acid by hydration to give H2C03. Hydrated C02 is then much like a carboxylic acid. It gives up a proton to a base and makes bicarbonate, HCO 3. 

The pH value is kept constant by buffer systems that cushion minor disturbances in the acid-base balance (C). In the longer term, the decisive aspect is maintaining a balanced equilibrium between H" production and uptake and H" release. If the blood s buffering capacity is not suf cient, or if the acid-base balance is not in equilibrium—e.g., in kidney disease or during hypoventilation or hyperventilation-shifts in the plasma pH value can occur. A reduction by more than 0.03 units is known as acidosis, and an increase is called alkalosis. 

Hasselbalch equation, which is important for understanding buffer action and acid-base balance in the blood and tissues of vertebrates. This equation is simply a useful way of restating the expression for the dissociation constant of an acid. For the dissociation of a weak acid HA into H+ and A-, the Henderson-Hasselbalch equation can be derived as follows ... 

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 pH of human blood must be kept within the narrow range of 7.1 to 7.7. Outside this range, proteins in the body lose their structure and ability to function. Fortunately, a number of buffers maintain the necessary acid/base balance. The carbonic acid/hydrogen carbonate buffer is the most important. 

A description of acid-base balance involves an accounting of the carbonic (H2C03, HCOh COa", and CO2) and noncar-bonic acids and conjugate bases in terms of input (intake plus metabolic production) and output (excretion plus metabolic conversion) over a given time interval. The acid-base status of the body fluids is typically assessed by measurements of total CO2 plasma pH and PCO2, because the bicarbonate/carbonic acid system is the most important buffering system of the plasma. Occasionally, measurement of total titratable acid or base, or other acid and base analytes (e.g., lactate and ammonia [NH3]) is necessary to determine the etiology of an acid-base disorder. 

Much blood chemistry depends on the acid-base balance in blood. The pH of your blood is slightly basic, about 7.4. If you re healthy, the pH of your blood does not vary by more than one-tenth of a pH unit. If you stop and think about aU of the materials that go into and out of your blood, it is amazing that the pH remains so constant. Many of those materials are acidic or basic, which is why the constancy of the pH of blood is fascinating. Blood is an example of an effective buffer. 

I. Acid Base Balance. Trauma from whatever cause may strain control of the acid-base balance by increasing production of acids, especially where there is tissue anoxia, by diminishing power of lungs and kidneys to eliminate them, and by lowering of buffering capacity through anemia and hypoproteinemia [for review, see Walker (W2)]. 

Large amounts of acids and smaller amounts of bases normally enter the blood. Some mechanisms must neutralize or eliminate these substances if the blood pH is to remain constant. In practice, a constant pH is maintained by the interactive operation of three systems buffer, respiratory, and urinary. Only when all three parts of this complex mechanism function properly can the acid-base balance be maintained. 

When blood pH is nonnal and a state of acid-base balance exists, components of the bicarbonate buffer are present in plasma in a ratio of 20 parts of bicarbonate (HCO3 ) to 1 part of carbonic acid (H2CO3). 

Buffer—An acid-base balancing or control reaction in which the pH of a solution is protected from major change when acids or bases are added to it. 

In blood, the main buffer system is bicarbonate at a concentration of [HCOj"] = 0.02-0.03 M (20-30 mEq/ I). Hemoglobin provides a further 10 mEq/l buffer capacity, and phosphate makes a small contribution of 1.5 mEq/l. The 5 liters of blood in an average adult human are thus able to absorb about 0.15 mole before the pH becomes dangerously low. The major buffers of the body are, however, present in other tissues. The total musculature of the body, for example, can neutralize about 5 times as much acid as the blood, and the blood HCO37CO2 system represents only about a tenth of the total buffer capacity of the body. Since all the buffer systems of the body are able to interact and buffer each other, all changes in the acid/ base balance of the body are reflected in the blood. This mutual buffering by the shift of H from one body system to another is known as the isohydric principle. 

Changes in the pH of the body are resisted through varied buffer systems that convert a strong acid or base to a weak one and thus bind H+ ions or leave more ions free. The body has several mechanisms for regulation of the acid-base balance of the body. The first mechanism is respiration. Respiration affects the acid-base balance by influencing the amount of carbon dioxide in the bloodstream. Carbon dioxide mixes with water to form carbonic acid, a weak acid, which breaks down into hydrogen ions (H ) and bicarbonate (HC03 ) 6... 

In the renal buffering process, sodium (Na+) is exchanged for hydrogen ions (H+) and binds with some of the bicarbonate (NaHCOj), which later breaks down again as Na" is actively removed through a Na - K+ mechanism (discussed in more detail in Chapter 5). The H" ions are bound with carbonic anhydrase on the border of the proximal tubules of the kidneys, which convert the H first to H COj and then to H O and CO. Some H+ ions also bind with the ammonia (NHj) produced in the kidneys as a result of amino acid catabolism and an abundant anion found in the glomerular filtrate, chloride (CT), to form ammonium chloride (NH Cl), a weak acid that is excreted in the urine. Thus it is clear that other electrolytes are involved in the acid-base balancing process and can be affected by acid-base imbalances. These impacts will be discussed with each electrolyte. 6... 

Sodium also plays a role in acid-base balance. Sodium binds well to chloride and bicarbonate and thus plays a part in the metabolic buffer system, preventing a strong acid from greatly affecting the pH of the blood by changing it to a weak acid. 

A hormone produced in the adrenal gland, aldosterone, signals the kidneys to excrete or retain potassium based on the body s needs. If potassium levels are high, aldosterone is secreted, causing an increase in potassium excretion into the urine. Serum levels of potassium also are influenced by the levels of other electrolytes and acid-base balance. In alkalosis, for example, potassium may shift out of the cell as hydrogen ions shift into the cell to buffer the excessive acid, and when serum potassium concentration is low, potassium is retained by excreting sodium and chloride. 

An important function of the kidney in regulating the acid-base balance is ammonium secretion. Ammonia probably forms in the collecting tubes. The tubule cells form ammonia by deaminating glutamine. The ammonia reacts with protons to yield ammonium ions. It is not known whether these ammonium ions are formed within the tubular cell or within the tubular fluid. In any event, the trapping of protons by ammonia and the excretion of the ammonium ion into the tubular fluid confer buffering properties to the tubular fluids, reduce the difference between the pH of the tubular and the intracellular fluid, and further facilitate the excretion of hydrogen ions. 

Explain how acid-base balance is maintained in the body. (Section 25.6) Explain how buffers work to control blood pH. (Section 25.7)... 

To find the pH of any 0.100 M solution of any acid, base, or 1 1 buffer, we simply put a straightedge at 45° on this plot passing through pH = pX of the acid-base as in Figure 4-1. For other concentrations and buffer ratios we shall have to plot the required material balance Rg curves. Six cases are shown in Figure 4-3 the three from Figure 4-2 and the three for 0.0100 M acid, base, and buffer. Three equilibrium condition ratio (H/X g) lines are shown... 

Henderson-Hasselbalch equation is important for imderstanding buffer action and acid-base balance in the blood and tissues of the mammalian system. The equation is derived in the following way. Let us denote a weak acid by the general formula HA, and its salt by the general formula BA (B being the metal ion and A being the conjugate base). The sedt dissociates completely, while the weak acid dissociates only partly. We can write the equilibrium reactions for the dissociation of HA and BA in the buffer solution as follows ... 

Endogenous acid production (H" ), and thus control of acid-base balance, is regulated by the kidney through three mechanisms, i.e., the production of NH4 ions and reabsorption of filtered bicarbonate in the proximal tubule and the buffering of hydrogen ion secretion by ammonia in the collecting tubule. 

In a quantitative approach to acid-base balance, it is the charge on buffer base which is of interest. Fifteen hydrogen ions combine with one protein ion... 

Burton, R. F. (1992) The roles of intracellular buffers and bone mineral in the regulation of acid-base balance in mammals. J. Comp. Biochem. Physiol, 102A, 425-32. 

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