Hemoglobin is produced in bone marrow bone marrow,soft tissue filling the spongy interiors of animal bones. Red marrow is the principal organ that forms blood cells in mammals, including humans (see blood).In children, the bones contain only red marrow......emoglobin is produced in bone marrow bone marrow,soft tissue filling the spongy interiors of animal bones. Red marrow is the principal organ that forms blood cells in mammals, including humans (see blood). In children, the bones contain only red marrow......