According to the American Society of Hematology (ASH), the average life span of a red blood cell is 120 days. New cells form in bone marrow and take about seven days before they mature and enter the bloodstream. Red blood cells make up about 40 percent of blood's volume.
ASH explains that one of the functions of red blood cells is to carry oxygen from lungs to body tissues. There, they pick up carbon dioxide, a waste produce, and transport it to the lungs where it is exhaled. The protein hemoglobin is primarily responsible for these jobs. Hemoglobin gives red blood cells their color, and because of the large number of red blood cells, blood is also red.