The sperm-forming tubes in a testicle discharge into a kind of transit depot. Between six and eight ducts lead to the epididymis. In the epididymis those ducts merge into a single tube. While the sperm-forming tubes in a testicle have a combined length of 250 metres, an epididymis is a duct of approximately 6 metres in length. The epididymis curls in a comma-shape behind the testicle. One can distinguish a head (caput), a body (corpus) and a tail (cauda). On a level with the head of the epididymis the network of drainage tubes in the testicle connects to the narrow epididymal duct, and the tail then connects to the seminal duct, the ductus deferens.
The blood supply comes both from the testicular artery and from its own epididymal artery. Drainage of blood takes place through the previously mentioned plexus pampiniformis. During a passage of several days through the epididymis the still infertile sperm cells mature into fertile cells. One of the most striking changes is an increase in the percentage of moving spermatozoa and their swimming speed. Biochemical changes in their surface increase the ability of sperm cells to attach themselves to the ovum.
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The epididymis is highly dependent on testosterone, and the head is exposed to high concentrations through the influx from the testicles. Further down stream in the epididymis, the concentration of testosterone is much lower. Besides the testosterone supply via the rete testis (testicular network of tubes), the epididymis is also supplied with testosterone via the bloodstream. The exposure of different sections of
the epididymis to different concentrations of testosterone make it a unique organ.
At orgasm the sperm cell stored in the tail of the epididymis are forced into the seminal duct. The impetus is provided by the contraction of the smooth muscles in the wall of this long, tubular organ. If a man does not ejaculate for two weeks, sperm cells will appear spontaneously in his urine.