Final answer:
When the unionised form of a weak acid enters tissues, the relative proportion of its conjugate base A- in the blood increases to maintain the equilibrium, similar to the bicarbonate buffering system and the chloride shift mechanism.
Step-by-step explanation:
As a weak acid (A) circulates in the plasma, it will be present in both its ionised and unionised forms, represented by the equilibrium A- + H+ ↔ AH. When the unionised form (AH) crosses cell membranes to enter tissues, it leaves behind its conjugate base A- in the blood. Since some of the unionised acid is being removed from the equilibrium, according to Le Chatelier's principle, the equilibrium will shift to the right to replace the unionised acid that has left, causing the relative proportion of A- in the blood to increase. This is to maintain equilibrium and is similar to the bicarbonate buffering system present in the blood, where carbon dioxide reacts with water to form bicarbonate and hydrogen ions, and the chloride shift helps to maintain the electrical neutrality of the erythrocytes and blood plasma.