Urethra. This is a tube that allows urine to flow outside the body. It is also the channel for semen to pass during ejaculation. The brain signals the bladder muscles to tighten. This squeezes urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax to let urine exit the bladder through the urethra. When all the signals happen in the correct order, normal urination happens.
