The time required for a drop of blood to make the complete circuit of both the systemic and pulmonary systems. Circulation time is determined by injecting a substance into a vein and timing its reappearance in arteries at the injection point. The blood with the contained substance must pass through veins to the heart and through the right atrium and ventricle, through the pulmonary circuit to the lungs, and back through the left atrium and ventricle, and then out through the aorta and arteries to the place of detection. Dyes such as fluorescein and methylene blue and substances such as potassium ferrocyanide and histamine have been used as tracers. Average circulation time is about 1 min.