The term for the tissue inside bones that produces red blood cells is bone marrow. In adults, this blood-forming tissue is found mainly in red marrow, located within the spongy interiors of bones such as the pelvis, sternum, ribs, vertebrae, and the ends of some long bones.
Red marrow contains hematopoietic stem cells, which develop into red blood cells, white blood cells, and platelets. Red blood cells carry oxygen using hemoglobin and are continually replaced because they typically circulate for about 120 days.
A common mix-up is confusing bone marrow with the hard outer bone or with cartilage. Marrow is soft tissue inside the bone, while compact bone provides structural strength. Adults also have yellow marrow, which stores fat; during severe blood loss, some yellow marrow can shift toward blood-cell production. The marrow cavity is not literally filled with air—the question’s wording refers broadly to internal spaces within bone.