Final answer:
A buffer agent maintains plasma pH by absorbing excess hydrogen ions when the fluid becomes too acidic and absorbing hydroxide ions when it becomes too alkaline, rather than changing the pH from one extreme to the other.
Step-by-step explanation:
A buffer agent in the blood is designed to maintain a stable pH by absorbing excess hydrogen and hydroxide ions in the body fluids.
When there is an increase in hydrogen ions, which would lower the pH and make the blood more acidic, the buffer acts as a weak base and binds these excess ions.
Conversely, if there is a decrease in hydrogen ions leading to a higher pH and making the blood more alkaline, the buffer will donate hydrogen ions, functioning as a weak acid, hence preventing a significant shift in pH.
In summary, the correct answer to the question is: D. none of the above.
A buffer agent does not simply convert plasma from alkaline to acid or vice versa, but rather it helps maintain the pH by either absorbing extra hydrogen ions to prevent acidification or absorbing hydroxide ions to prevent alkalinization, depending on the body's needs.