Final answer:
A buffer is a substance that keeps the pH in cells within the 6.5 to 7.5 range by neutralizing small amounts of acids or bases.
Step-by-step explanation:
A substance that keeps the pH in cells within the 6.5 to 7.5 pH range is called a buffer.
Buffers are solutions of weak acids and their conjugate bases. They can neutralize small amounts of acids or bases in body fluids, helping to maintain pH balance.
For example, if there is a slight decrease in the pH of a bodily fluid below 7.35, a buffer acting as a weak base will bind excess hydrogen ions. Similarly, if pH rises above 7.45, the buffer will act as a weak acid and contribute hydrogen ions.