Acid–base homeostasis

In humans and many other animals, acid–base homeostasis is maintained by multiple mechanisms involved in three lines of defense: • Chemical: The first lines of defense are immediate, consisting of the various chemical buffers which minimize pH changes that would otherwise occur in their absence. These buffers include the bicarbonate buffer system, the phosphate buffer system, and the protein buffer system. • Respiratory component: The second line of defense is rapid consisting of the control the carbonic acid (H2CO3) concentration in the ECF by changing the rate and depth of breathing by hyperventilation or hypoventilation. This blows off or retains carbon dioxide (and thus carbonic acid) in the blood plasma as required. and mostly depends on the renal system which can add or remove bicarbonate ions () to or from the ECF. Bicarbonate ions are derived from metabolic carbon dioxide which is enzymatically converted to carbonic acid in the renal tubular cells. There, carbonic acid spontaneously dissociates into hydrogen ions and bicarbonate ions. When the pH in the ECF falls, hydrogen ions are excreted into urine, while bicarbonate ions are secreted into blood plasma, causing the plasma pH to rise. The converse happens if the pH in the ECF tends to rise: bicarbonate ions are then excreted into the urine and hydrogen ions into the blood plasma. The second and third lines of defense operate by making changes to the buffers, each of which consists of two components: a weak acid and its conjugate base. It is the ratio concentration of the weak acid to its conjugate base that determines the pH of the solution. Thus, by manipulating firstly the concentration of the weak acid, and secondly that of its conjugate base, the pH of the extracellular fluid (ECF) can be adjusted very accurately to the correct value. The bicarbonate buffer, consisting of a mixture of carbonic acid (H2CO3) and a bicarbonate () salt in solution, is the most abundant buffer in the extracellular fluid, and it is also the buffer whose acid-to-base ratio can be changed very easily and rapidly. ==Acid–base balance==

The pH of the extracellular fluid, including the blood plasma, is normally tightly regulated between 7.32 and 7.42 by the chemical buffers, the respiratory system, and the renal system. Aqueous buffer solutions will react with strong acids or strong bases by absorbing excess ions, or ions, replacing the strong acids and bases with weak acids and weak bases. Similarly an excess of H+ ions is partially neutralized by the bicarbonate component of the buffer solution to form carbonic acid (H2CO3), which, because it is a weak acid, remains largely in the undissociated form, releasing far fewer H+ ions into the solution than the original strong acid would have done. Henderson–Hasselbalch equation The Henderson–Hasselbalch equation, when applied to the carbonic acid-bicarbonate buffer system in the extracellular fluids, states that: These chemoreceptors are sensitive to the levels of carbon dioxide and pH in the cerebrospinal fluid. A rise in the in the arterial blood plasma above reflexly causes an increase in the rate and depth of breathing. Normal breathing is resumed when the partial pressure of carbon dioxide has returned to 5.3 kPa. The converse happens if the partial pressure of carbon dioxide falls below the normal range. Breathing may be temporally halted, or slowed down to allow carbon dioxide to accumulate once more in the lungs and arterial blood. The sensor for the plasma HCO concentration is not known for certain. It is very probable that the renal tubular cells of the distal convoluted tubules are themselves sensitive to the pH of the plasma. The metabolism of these cells produces CO2, which is rapidly converted to H+ and HCO through the action of carbonic anhydrase. When the extracellular fluids tend towards acidity, the renal tubular cells secrete the H+ ions into the tubular fluid from where they exit the body via the urine. The HCO ions are simultaneously secreted into the blood plasma, thus raising the bicarbonate ion concentration in the plasma, lowering the carbonic acid/bicarbonate ion ratio, and consequently raising the pH of the plasma. ==Imbalance==

Acid–base imbalance occurs when a significant insult causes the blood pH to shift out of the normal range (7.32 to 7.42 Similarly, an alkalosis would cause an alkalemia on its own. or metabolic (indicating a change in the Base Excess of the ECF). There are therefore four different acid-base problems: metabolic acidosis, respiratory acidosis, metabolic alkalosis, and respiratory alkalosis. One or a combination of these conditions may occur simultaneously. For instance, a metabolic acidosis (as in uncontrolled diabetes mellitus) is almost always partially compensated by a respiratory alkalosis (hyperventilation). Similarly, a respiratory acidosis can be completely or partially corrected by a metabolic alkalosis. == References ==