In what part of the body are blood cells formed?

Blood cells are primarily formed in the bone marrow, which is part of the skeletal system. The bones contain the red marrow responsible for hematopoiesis, the process of blood cell production. This includes the formation of red blood cells, white blood cells, and platelets, all of which are critical components of the circulatory and immune systems. While other organs like the liver play roles in metabolism and detoxification of blood and the heart is vital for pumping blood, the actual creation of cells takes place in the bone marrow located in the skeletal system. Therefore, the skeletal system is the correct choice in the context of blood cell formation.