Blood has been called the river of life, transporting various substances that must be carried to one part of the body or another. Red blood cells are an important element of blood. Their job is to transport oxygen to the body’s tissues in exchange for carbon dioxide, which they carry to the lungs to be expelled. Red blood cells are formed in the red bone marrow of bones. Stem cells in the red bone marrow are called hemocytoblasts. They give rise to all of the formed elements in blood.

If a stem cell commits to becoming a cell called a proerythroblast, it will develop into a new red blood cell. The formation of a red blood cell takes about 2 days. Blood is made up of both cellular and liquid components. If a sample of blood is spun in a centrifuge, the formed elements and fluid matrix of blood can be separated from each other.

The average life cycle of a human RBC is approximately 120 days. Generally, by this point, the cell is worn out and damaged. RBCs pass through both the spleen and liver, where specialized immune cells called macrophages are found. Macrophages recognize when an RBC is spent, and undergo a process called phagocytosis where they digest the cell. In this process, the iron in hemoglobin is recycled for use in new blood cells and the hem molecule is degraded, conjugated to bilirubin, and eliminated from the body. All the other cellular proteins are either recycled or eliminated. Historically, this process was thought to occur exclusively in the spleen, but recent studies have shown that it occurs in the bone marrow.