Hydrogen ion

A positively charged hydrogen particle written as H⁺. Its concentration helps determine acidity: when CO₂ falls during over-breathing, H⁺ concentration falls and blood pH rises. The pH scale is a logarithmic measure of hydrogen-ion concentration, so small changes in pH reflect relatively large changes in H⁺. The body keeps blood pH tightly between about 7.35 and 7.45. Chemoreceptors respond to H⁺ changes, which is part of how the brain adjusts breathing.