urethra
From the collecting ducts of the kidneys, urine enters the ureters , two muscular tubes that move the urine by peristalsis to the bladder see Figure above . The bladder is a hollow, sac-like organ that stores urine. When the bladder is about half full, it sends a nerve impulse to a sphincter to relax and let urine flow out of the bladder and into the urethra. The urethra is a muscular tube that carries urine out of the body. Urine leaves the body through another sphincter in the process of urination . This sphincter and the process of urination are normally under conscious control.