Which of the Following Statements Are True of Buffer Systems

Which of the following statements are true of buffer systems? (Read carefully and select all of the correct statements.)

A) When body fluids become too acidic,the bicarbonate buffer system forms more sodium bicarbonate from the acid.
B) The phosphate buffer system is used by the kidneys to excrete or retain hydrogen ions.
C) An amino acid acts as an acid when its carboxyl group picks up a hydrogen ion.
D) Proteins as buffers are of greatest importance within cells.
E) An amino acid acts as a base when its amine group gives off a hydrogen ion.
F) A buffer system responds very quickly to a change in the pH of body fluids,but its molecules may be rapidly used up.