Blood is produced in the bone marrow, a tissue in the central cavity inside almost all of the bones in the body.
In infants, the marrow in most of the bones is actively involved in blood cell formation.
By later adult life, active blood cell formation gradually ceases in the bones of the arms and legs and concentrates in the skull, spine, ribs, and pelvis.
Red blood cells, white blood cells, and platelets grow from a single precursor cell, known as a hematopoietic stem cell. Remarkably, experiments have suggested that as few as 10 stem cells can, in four weeks, multiply into 30 trillion red blood cells, 30 billion white blood cells, and 1.2 trillion platelets—enough to replace every blood cell in the body.
Red blood cells have the longest average life span of any of the cellular elements of blood.
A red blood cell lives 100 to 120 days after being released from the marrow into the blood.
Over that period of time, red blood cells gradually age.

Spent cells are removed by the spleen and, to a lesser extent, by the liver. The spleen and the liver also remove any red blood cells that become damaged, regardless of their age.
The body efficiently recycles many components of the damaged cells, including parts of the hemoglobin molecule, especially the iron contained within it.