Diagram showing the process of red blood cell production in a person with healthy kidneys and a person with diseased kidneys. On the top half of the diagram, on the left side, a kidney labeled “Healthy kidney” starts the process by producing EPO. Six drops represent “Normal EPO.” An arrow beneath the EPO drops points from the kidney to a cross section of a bone. Several cells labeled “Normal red blood cells” are spilling out of the bone marrow. Above the red blood cells, the label “Normal oxygen,” with three arrows pointing to the head and torso of a smiling man, indicates the man is receiving enough oxygen. On the bottom half of the diagram, on the left side, a kidney labeled “Diseased kidney” produces only two drops, labeled “Reduced EPO.” Under the EPO drops, an arrow points to the cross section of a bone. A small number of cells, labeled “Reduced blood cells,” are spilling out of the bone marrow. Above the red blood cells, the label “Reduced oxygen,” with one arrow pointing to the head and torso of a frowning man, indicates the man is not receiving enough oxygen.