ACID BASE

REGULATIONSubmitted by: Pesiar ilman

BICARBONATE BUFFER SYSTEM

Bicarbonate buffer system is consisted by two ingredients: a weak acid H2CO3 and a bicarbonate salt.

CO2+H2Ocarbonic anhydrase⇔

H 2CO3

H2CO3 is formed by the reaction of water and CO2.

QUANTITATIVE DYNAMICS OF BICARBONATE BUFFER SYSTEM

the concentration of H2CO3 cannot be measured because it rapidly dissociate to CO2 and H20 or to H+ and HCO3-. However the proportion of CO2 dissolved in the blood is directly proportional to undissolved H2CO3. Therefore the

equation can be written K is about 1/400 K’ because the proportionality ratio between H2CO3 and CO2 is 1:400.

However in most clinical laboratory measure CO2 is measured by CO2 tension (PCO2). The solubility of CO2 in the blood at physiologic condition is 0.03 mmol/mm Hg at body temperature. So the equation can be written

… Henderson-Hasselbach Equation

PHOSPHATE BUFFER SYSTEM

The phosphate buffer system is not important in extracellular buffer system but play important roles in intracellular and renal tubular fluid.

The phosphate buffer system has a pK of 6.8, which not far from body fluids normal pH 7.4 . It allows the system to work near its maximum power.

The phosphate buffer system is especially important in tubular fluids of the kidneys for two reasons: (1) phosphate usually concentrated in tubule, therefore increasing the buffering power and (2) tubular fluid usually has a lower pH ha extracellular fluid does. Bringing the operating range of buffer closer to pK of the system.

PROTEIN ARE IMPORTANT INTRACELLULAR BUFFERS

Proteins are among the most plentiful buffers in the body because of their high concentrations, especially within the cells.

The pH of the cells, although slightly lower than in the extracellular fluid, nevertheless changes approximately in proportion to extracellular fluid pH changes. There is a slight diffusion of H+ and HCO3- through the cell membrane, although these ions require several hours to come to equilibrium

with the extracellular fluid, except for rapid equilibrium that occurs in the red blood cells. CO2, however, can rapidly diffuse through all the cell membranes.

RESPIRATORY REGULATION OF ACID BASE BALANCE

The higher the alveolar ventilation, the lower the PCO2; conversely, the lower the alveolar ventilation rate, the higher the PCO2. As discussed previously, when CO2 concentration increases, the H2CO3

concentration and H+ concentration also increase, thereby lowering extracellular fluid pH

RENAL CONTROL OF ACID BASE BALANCE

The kidneys control acid-base balance by excreting either acidic or basic urine. Excreting acidic urine reduces the amount of acid in extracellular fluid, whereas excreting basic urine removes base from the extracellular fluid.

The overall mechanism by which the kidneys excrete acidic or basic urine is as follows: Large numbers of HCO3- are filtered continuously into the tubules, and if they are excreted into the urine this removes base from the blood. Large numbers of H+ are also secreted into the tubular lumen by the tubular epithelial cells, thus removing acid from the blood. If more H+ is secreted than HCO3-is filtered, there will be a net loss of acid from the extracellular fluid. Conversely, if more HCO3-is filtered than H+ is secreted, there will be a net loss of base.

Each day the body produces about 80 mEq of nonvolatile acids, mainly from the metabolism of proteins. These acids are called nonvolatile because they are not H2CO3 and, therefore, cannot be excreted by the lungs. The primary mechanism for removal of these acids from the body is renal excretion. The kidneys must also prevent the loss of bicarbonate in the urine, a task that is quantitatively more important than the excretion

of nonvolatile acids. Each day the kidneys filter about 4320 mEq of bicarbonate (180 L/day × 24 mEq/L); under normal conditions, almost all this is reabsorbed from the tubules, thereby conserving the primary buffer system of the extracellular fluid.

Both the reabsorption of bicarbonate and the excretion of H+ are accomplished through the process of H+ secretion Bay The tubules. Because The HCO3- must react with a secreted H+ to form H2CO3 before it can be reabsorbed, 4320 mEq of H+

must be secreted each day just to reabsorb the filtered bicarbonate. Then an additional 80 mEq of H+ must be secreted to rid the body of the nonvolatile acids produced each day, for a total of 4400 mEq of H+ secreted into the tubular fluid each day.