Final answer:
The first urine-draining structure is the ureters, which are muscular tubes that move the urine from the kidneys to the bladder.
Step-by-step explanation:
The first urine-draining structure is the ureters, which are muscular tubes that move the urine from the kidneys to the bladder. The urine is not passively drained but is propelled by waves of peristalsis. The ureters enter the bladder and create a one-way valve that allows urine into the bladder but prevents reflux back into the ureters.