<P> Thus the second line of defence of the pH of the ECF consists of controlling of the carbonic acid concentration in the ECF . This is achieved by changes in the rate and depth of breathing (i.e. by hyperventilation or hypoventilation), which blows off or retains carbon dioxide (and thus carbonic acid) in the blood plasma . The third line of defence is the renal system, which can add or remove bicarbonate ions to or from the ECF . The bicarbonate is derived from metabolic carbon dioxide which is enzymatically converted to carbonic acid in the renal tubular cells . The carbonic acid spontaneously dissociates into hydrogen ions and bicarbonate ions . When the pH in the ECF tends to fall (i.e. become more acid) the hydrogen ions are excreted into the urine, while the bicarbonate ions are secreted into the blood plasma, causing the plasma pH to rise (correcting the initial fall). The converse happens if the pH in the ECF tends to rise: the bicarbonate ions are then excerted into the urine and the hydrogen ions into the blood plasma . </P> <P> An acid--base imbalance is known as acidemia when the acidity is high, or alkalemia when the acidity is low . </P> <P> The pH of the extracellular fluid, including the blood plasma, is normally tightly regulated between 7.36 and 7.42, by the chemical buffer s, the respiratory system, and the renal system . </P> <P> Aqueous buffer solutions will react with strong acids or strong bases by absorbing excess hydrogen H + ions, or hydroxide OH − ions, replacing the strong acids and bases with weak acids and weak bases . This has the effect of damping the effect of pH changes, or reducing the pH change that would otherwise have occurred . But buffers cannot correct abnormal pH levels in a solution, be that solution in a test tube or in the extracellular fluid . Buffers typically consist of a pair of compounds in solution, one of which is a weak acid and the other a weak base . The most abundant buffer in the ECF consists of a solution of carbonic acid (H CO), and the bicarbonate (HCO − 3) salt of, usually, sodium (Na). Thus, when there is an excess of OH − ions in the solution carbonic acid partially neutralizes them by forming H O and bicarbonate (HCO − 3) ions . An excess of H ions is partially neutralized by the bicarbonate component of the buffer solution to form carbonic acid (H CO), which, because it is a weak acid, remains largely in the undissociated form, releasing very much fewer H ions into the solution than the original strong acid would have done . </P>

What two body systems are involved with ph balance