 Usually, the arterial blood is the relevant context; the symbol for formula_2 in arterial blood is formula_4.Measurement of formula_4 in the systemic circulation indicates the effectiveness of ventilation at the lungs' alveoli, given the diffusing capacity of the gas.It is a good indicator of respiratory function and the closely related factor of acid–base homeostasis, reflecting the amount of acid in the blood (without lactic acid).