Here's why:

* pH Scale: The pH scale measures the acidity or alkalinity of a solution. A lower pH indicates a higher concentration of hydrogen ions, making it more acidic.

* Hydrogen Ion Concentration: The concentration of hydrogen ions (H+) in a solution directly influences its acidity.

* Body Fluid Regulation: The body tightly regulates the pH of its fluids (blood, interstitial fluid, etc.) to maintain a healthy balance. This regulation is crucial for various physiological processes.

In summary, the higher the concentration of hydrogen ions (H+) in body fluids, the more acidic the fluids become.