 The spermatic veins are provided with valves.The left spermatic vein passes behind the iliac colon and is thus exposed to pressure from the contents of that part of the bowel.Since the left testicular vein goes all the way up to the left renal vein before it empties, this results in a higher tendency for the left testicle to develop varicocele because of the gravity working on the column of blood in this vein.