Where are red blood cells primarily formed in the body?

Red blood cells, or erythrocytes, are primarily formed in the bone marrow through a process known as erythropoiesis. The bone marrow is the main site of blood cell production in adults, as it contains stem cells that can differentiate into various blood cells, including red blood cells.

While the spleen does play a role in filtering and recycling old or damaged red blood cells, it is not a primary site for their formation. In infants, the liver can produce blood cells, but this function diminishes after birth as the bone marrow takes over. Therefore, for adults, the correct site of red blood cell formation is predominantly the bone marrow, with the spleen having a more supportive role rather than one of primary production.