concept

Henderson-Hasselbalch equation

The equation that pins blood acidity to a single ratio: how much bicarbonate the plasma holds, divided by how much carbon dioxide is dissolved in it. Only the ratio matters, not either number alone - so a body facing three times the carbon dioxide can hold the same pH by carrying three times the bicarbonate. In a resting human, twenty-four units of bicarbonate against forty millimetres of mercury of carbon dioxide gives pH 7.39. It is the tool for reading any body that lives in CO₂-rich air: either raise the numerator at a real cost, or move the pH set-point itself.