Answer:
Hydrogen ion(H+)
Explanation
The human blood has carbonic acid and bicarbonate anion as buffer to maintain the blood pH between 7.35 and 7.45. A virtual value higher than 7.8 and 6.8 can lead to death.
Taking coffee which has a low pH will lower the pH of the blood. In order to raise the pH back to normal and lower the level of acid present in the system. The human buffering system will to take up Hydrogen ion.