The nucleus (disc material), which forms the center of the disc, is also composed of primarily of water. It is gel like and is contained by inner concentric layers of annular fibers. When pressure is applied to the intervertebral disc, its nucleus deforms not unlike a water balloon with h pressure placed upon it. When the pressure is released the normal nucleus returns to its original shape because it is contained by the strong annular fibers. The healthy intervertebral disc is actually much stronger than the adjacent bone of the vertebrae. The healthy intervertebral disc has high percentage water. Until the third decade of life, the gel of the inner nucleus pulposus is composed of approximately 90% water; however, the water content gradually diminishes over the next 4 decades to approximately 65%. In the babies the intervertebral disc is comprised of approximately 90% water whereas in a 70-year-old individual the disc is approximately 70% or less water.  Repetitive movement of vertebrae relative to one another facilitates the movement of fluid into and out of the intervertebral disc.