[Water and electrolyte content of human intervertebral disks under varying load]

The human intervertebral disc acts as an osmotic system. Water, salt and other low-molecular substances penetrate the cartilage plates and anulus fibrosus. The content of water, sodium, potassium and ashes in different regions of 69 human lumbar intervertebral discs was examined before and after loading them with certain weights. Under load the disc looses water - anulus 11%, nucleus 8% - and gains sodium and potassium. The higher concentration of electrolytes in the disc after a long period of weight-bearing enlarge its osmotic absorptive forces and enable the disc to hold the rest-water also against a great amount of pressure. After reducing the pressure water is quickly reabsorbed and the disc gains height and volume. The pumping mechanism keeps up the nutrition and biomechanical function of the intervertebral disc.