A blood treatment air purging system and method includes a level detector positioned and arranged to detect a low fluid level in a blood circuit indicating a high amount of air in the blood circuit; a blood pump operable with the blood circuit; a venous patient line of the blood circuit; and at least one blood circuit air vent valve, wherein the system and method are configured or programmed to (i) stop the blood pump, (ii) close the venous patient line, and (iii) open the at least one blood circuit air vent valve to atmosphere or a container when the low fluid level is detected, and (iv) run the blood pump to meter air through the at least one blood circuit air vent valve to atmosphere or the container.