Acid-Base Balance PDF

ACID-BASE BALANCE The normal pH of the blood is slightly alkaline and is maintained in the narrow range of 7.35 to 7.45. That is because body enzymes, can function optimally at a particular pH to drive metabolic reactions. Thus, life can be sustained only if pH is maintained within specified limits...

ACID-BASE BALANCE The normal pH of the blood is slightly alkaline and is maintained in the narrow range of 7.35 to 7.45. That is because body enzymes, can function optimally at a particular pH to drive metabolic reactions. Thus, life can be sustained only if pH is maintained within specified limits. Maintenance of pH is important because during metabolic reactions, acids are produced which include volatile acids like carbonic acid and non-volatile acids like lactic acids, keto acids, sulfuric acid, phosphoric acid. Some amount of bicarbonate is genCOerated from organic acids. These can profoundly alter pH of body fluids. Thus, for proper understanding acid-base balance, knowledge of acids, bases, and pH is essential. According to proton transfer theory of Bronsted and Lowry, acids are proton donors and bases are proton acceptors. Examples: SO 1. HCL↔H+Cl 2. H~2~ CO~3↔~H+HCO~3~ Acid conjugate base 1. NH~3~ +H↔NH~4~ 2. HCO~3~ + H↔H~2~ CO~3~ Strong acids dissociate completely in a solution to give very high H concentration. Example: HCL, HNO~3,~ H~2~ SO~4~ etc. Weak acids do not ionize completely. Hence, H ions are less. Example: H~2~CO~3,,~ CH~3~COOH etc. Conjugate bases of strong acids (HCl) are weak bases (Cl) and conjugate bases of weak acids (H~2~CO~3~) are strong bases (HCO~3~). Dissociation of Water The degree of dissociation increases with temperature. The concentration H ions in pure water will always be 1/100,00,000gram moles/liter at 25 degrees centigrade. This is not only for water, but for all solutions. Any increase in hydrogen ions (H) is accompanied by decrease in OH negative so that the product (Kw) is constant. pH pH is the negative logarithm of hydrogen ion concentration and it is inversely related to hydrogen positive ions concentration. pH scale is a logarithmic scale from 0-14. pH of pure water is and it is neither acidic nor alkaline but neutral.7 **Buffer** Buffer is a system that resists any change in its pH when a small amount of acid or alkali is added to it. It comprises two major components: A weak acid (HA) and its conjugate base (A negative ) and it is prepared by mixing: 1. Weak acid and its salt formed with a strong base. Example: CH~3~ COOH and CH~3~ COONa→acetate (acid) (base) (buffer) 2. Weak acid and its conjugate base. Example: H~2~ CO~3~ and HCO~3~ →bicarbonate (acid) (base) (buffer) 3. Weak base and its salt formed with strong acid. Example: NH~3~ NH~4~ Cl 4. Mixture of 2 salts of polybasic acids. **Factors Affecting pH of a Buffer** 1. The value of pK: lower the value of pK, lower is the pH of the solution. 2. The ratio of salt to acid concentrations: actual concentration of salt and acid in a buffer may be varied widely, with no change in pH, as long as the ration of the concentrations remains the same. **Buffering Capacity** Buffering capacity of a buffer is defined as the ability of the buffer to resist changes in pH balance when an acid or base is added. Buffering capacity is determined by the absolute concentration of the salt and acid. When the ration between salt and acid is 10:1, the pH will be 1 unit higher than the pKa. When the ration between salt and acid is 1:10, the pH will be 1 unit lower than the pKa. Buffer is most effective when: Salt=acid pH=pKa + log 1 =pKa+ O pH=pKa Therefore, buffering capacity is maximum when its pH=pKa. A buffer is effective +1 or -1 unit on either side of pKa. Eg. Phosphate buffer pKa=6.8 It is most effective at pH of 6.8 but can operate in the pH range of 5.8 to 7.8 +-1pKa. Determination of pH in laboratory is by using ion sensitive electrodes. Maintenance of Acid-Base Balance Three mechanisms are involved to maintain pH in the body. i. buffers of thebody ii. respiratory mechanism iii. renal mechanism These mechanisms are minter-related. i. Buffers of the body: a. Extracellular (plasma) buffers b. Intracellular (inside the cells ) buffers a. Extracellular buffers Bicarbonate buffe Phosphate buffer Protein buffer b. Intracellular buffers a. The concentration of its components can be easily controlled by the body. HCO~3~- is regulated by kidneys, hence it is called the metabolic component and H~2~ CO~3~ is regulated by lungs, hence it is called the respiratory component. b. HCO~3-~ level is sufficiently high buffer acid load. **Disadvantages of Bicarbonate Buffer** Efficiency of bicarbonate buffer is less at plasma pH of 7.4, since its pka is 6.1. **Phosphate Buffer** Phosphate buffer comprises of dibasic phosphate (HPO~4~ ^-2^ ) as base and monobasic phosphate (H~2~ PO~4~ -) as acid component. It has pk~a~ of about 6.8 and it is a principal intracellular buffer. Advantages It can operate effectively, since its pk~a~ is closer to blood pH of 7.4. Disadvantages a. Concentration of phosphate buffer in plasma is too low. b. Components of buffer cannot be easily controlled. **Protein Buffers (Pr^-^ /HPr)** Protein buffers are of considerable importance in plasma and intracellular fluids. It is too low in concentration in lymph, interstitial fluids and cerebrospinal fluid (CSF). At pH 7.4, many plasma proteins exist as anions as Pr^\_^ which can accept H^+^ forming conjugate acid HPr. Thus, Pr-/HPr pair serves as a buffer. Buffering capacity of plasma proteins is much less than Hb. Hemoglobin buffer system: Most important buffer in RBC is hemoglobin value bin buffer system. It is the major buffer system of the blood as well as erythrocytes. Hb operates only in erythrocytes. The pK value of imidazole group (7.3) of histidine part of hemoglobin is close to body pH and hence Hb buffer system is effective at body pH. Further, high Hb concentration (14g/dl) makes it the major buffer of blood. Buffers can act quickly but not permanently. They are also unable to replenish the alkali reserve of the body. For the final elimination of acids, the respiratory and renal regulations are very essential. **Respiratory Mechanisms (role of lungs in acid-base balance)** Lungs work in close association with bicarbonate buffer system. They maintain acid-base balance mainly by 2 mechanisms. a. Regulation of H~2~ CO~3~ level of plasma b. Excretion of CO~2~ produced in the body by various metabolisms. **Regulation of pH by regulating H~2~ CO levels** It is mediated by respiratory center of the medulla. When plasma pH falls (acidosis), respiratory center is stimulated and thus respiratory rate is increased (hyperventilation). More carbon dioxide is exhaled and bicarbonate level falls. With this, HCO~3~ -/H~2~ CO~3~ ratio increases and pH is restored to normal. When plasma pH by increases (alkalosis), respiratory center is depressed and thus respiratory rate is lowered. Hence, CO~2~ is accumulated and H~2~ CO~3~ level is raised. HCO~3~-/H~2~ CO~3~ ratio is decreased and pH is restored to normal. Advantages It is very rapid mechanism (2-3 minutes), because respiratory center is highly sensitive to pH changes. Disadvantages 1. It does not ensure complete restoration of pH. 2. It cannot function in respiratory disease. **Excretion of CO~2~ by lungs** Lungs excrete CO~2~ produced by metabolism. CO~2~ in water forms H~2~ CO~3~ which can alter plasma pH. Lungs excrete CO^2^ and thus pH alteration of plasma is prevented.