What prevents backflow of urine into the ureters?

The prevention of backflow of urine into the ureters is primarily achieved by a structure known as a flap of mucous membrane, specifically the ureterovesical junction. This junction is located where the ureters enter the bladder and is critical in maintaining one-way flow of urine from the kidneys to the bladder.

When the bladder fills with urine, the increased pressure at this junction causes the flap of mucous membrane to close. This action prevents urine from flowing backwards into the ureters, ensuring that the urine moves only in a downward direction towards the bladder. The flap mechanism is particularly beneficial during bladder contractions when urine is expelled; it helps maintain proper urinary function and avoids potential complications, such as hydronephrosis, which can occur if backflow happens.

Other potential structures mentioned, such as muscle sphincters or valves, do play roles in general urinary control but are not specifically responsible for preventing backflow at the ureteral entrance. Meanwhile, connective tissue does provide structural support, but it does not function actively in the prevention of backflow like the flap of mucous membrane does.