Question

Identify and describe three methods of acid-basis homeostasis. Indicate the type of defense and the associated mechanisms of action.

First line of defense

Second line of defense

Third line of defense

Answer 1

Acid-base homeostasis operates to maintain extracellular arterial pH between 7.35 and 7.45, and intracellular pH between 7.0 and 7.3 in order to provide an optimal milieu for enzymatic and metabolic processes. Since pH is equal to –log [H+], with the concentration of hydrogen ions in Eq/L (or mol/L), this normal extracellular pH correlates with a hydrogen ion concentration of 35–45 nEq/L, or 35 × 10−6 to 45 × 10−6 mEq/L

In humans and many other animals, acid–base homeostasis is maintained by multiple mechanisms involved in three lines of defence[5][6]:

• The first line of defence are the various chemical buffers which minimize pH changes that would otherwise occur in their absence. They do not correct pH deviations, but only serve to reduce the extent of the change that would otherwise occur. These buffers include the bicarbonate buffer system, the phosphate buffer system, and the protein buffer system.[citation needed]

• The second line of defense of the extracellular fluid pH consists in controlling 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.[5][7]

• The third line of defense is the renal system, which can add or remove bicarbonate ions to or from the ECF.[5] The bicarbonate is derived from metabolic carbon dioxide which is enzymatically converted to the carbonic acid in the renal tubular cells.[5][8][9] The carbonic acid spontaneously dissociates into hydrogen ions and bicarbonate ions.[5] When the pH in the ECF tends to fall (i.e. become more acidic) 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).[10] The converse happens if the pH in the ECF tends to rise: the bicarbonate ions are then excreted into the urine and the hydrogen ions into the blood plasma.

Physiological corrective measures make up the second and third lines of defense. This is because they operate by making changes to the buffers, each of which consists of two components: a weak acid and its conjugate base.[5][11] It is the ratio concentration of the weak acid to its conjugate base that determines the pH of the solution.[12] 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 (HCO−
3) 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.[13]

An acid–base imbalance is known as acidaemia when the acidity is high, or alkalaemia when the acidity is low.