Which vessel contains blood with the greatest concentration of reduced hemoglobin?

The vessel that contains blood with the greatest concentration of reduced hemoglobin is the venous blood, specifically in the systemic veins. This is because venous blood has delivered its oxygen to the tissues, resulting in a higher proportion of deoxygenated (reduced) hemoglobin compared to arterial blood, which is rich in oxygen. As blood returns to the heart and lungs through the veins, it carries more reduced hemoglobin due to the oxygen being released to the tissues.