Typically, that internal sphincter muscle opens up when the bladder is about half full.

一般当膀胱充盈量达到一半时，内括约肌会开放。

The first door is the internal sphincter muscle, which is made of smooth muscle and is under involuntary control, meaning that it opens and closes automatically.

一个阀门是内括约肌，由平滑肌组成，不受意识支配，意味着它可以自行开合。