It's basically a one way valve that prevents urine from refluxing backwards from the bladder into the ureters.

基本上如同一个单向瓣膜，阻止尿液从膀胱返流进入输尿管。

Sometimes, the mucus plugs end up acting like one way valves, allowing air to enter, but not escape the bronchioles.

有时，黏液栓会形成一个单向阀，空气能进去却出不来。

See, veins have a bunch of one way valves to make sure blood flows towards the heart without backtracking, but there are a lot of ways these valves can stop working.

看， 静脉有一堆单向阀来确保血液流向心脏而不回流，但是这些阀有很多方法可以停止工作。