Answer: hemoglobin

AnswerCrossword Clue
HEMOGLOBINIt makes blood red
hemoglobinA red protein responsible for transporting oxygen in the blood of vertebrates. Its molecule comprises four subunits, each containing an iron atom bound to a heme group
hemoglobinthe iron-containing pigment which is the oxygen-carrying substance present in the red blood cells of vertebrates also HAEMOGLOBIN