They then pass through a long tube, called the ductus deferens, or vas deferens, to another storage area, the ampulla. After emerging from the testes, sperm are stored in the epididymis, in which secretions of potassium, sodium, and glycerylphosphorylcholine (an energy source for sperm) are contributed to the sperm cells. As sperm travel through the male reproductive tract, they are bathed in fluids produced and secreted by the various tubules and glands of the reproductive system. In the sexually mature human male, sperm cells are produced by the testes (singular, testis) they constitute only about 2 to 5 percent of the total semen volume.