What is the primary site of red blood cell production after birth?

The primary site of red blood cell production after birth is red bone marrow. This specialized tissue is responsible for hematopoiesis, the process of producing blood cells, including red blood cells. After birth, the liver and spleen, which play significant roles in fetal development, largely reduce their involvement in erythropoiesis. The red bone marrow contains stem cells that differentiate into various blood cell types and is where the majority of red blood cells are generated throughout adulthood. The kidneys have a role in regulating erythropoiesis by releasing erythropoietin, a hormone that stimulates red blood cell production in the bone marrow, but they do not serve as the primary production site themselves. This makes red bone marrow the correct answer for the location of red blood cell production postnatally.