The medullary cavity is the site in a human long bone containing red or yellow bone marrow, depending on the age of the person. At birth, the marrow is red. As the individual grows older, the red marrow is replaced with yellow marrow.
Once yellow marrow replaces red marrow, red blood cell production ceases. Red blood cell production still takes place in older individuals, but it is manufactured at the spongy ends of the long bone. The adult long bone consists of three major structures, the proximal end, the distal end and the shaft, where the medullary cavity is located.