In medicine, the partial pressure of carbon dioxide in arterial blood is called  or PaCO. Measurement of  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).  Normal values for humans are in the range 35–45 mmHg.  Values less than this may indicate hyperventilation and (if blood pH is greater than 7.45) respiratory alkalosis.  Values greater than 45 mmHg may indicate hypoventilation, and (if blood pH is less than 7.35) respiratory acidosis.