Fact 1: Blood pH may become unbalanced if the rate of breathing is too fast or too slow.

Fact 2: Breathing is regulated to keep as normal as possible arterial blood pH and PO2.

Given the two facts above, answer the question "What does breathing help balance acidity levels?" with the following options: 
- blood
 - muscles
 - water
 - air
 - CO 2
 - oxygen
 - Gases
 - protein


 

 blood        