I have yet to find a person not affected in this way while drinking, and I can't put my finger on why it happens.

Alcohol as many people know inhibits ADH (Anti-Diuretic Hormone) in the Pituitary and makes you need to urinate more. Okay, fine... But why can people last for a few hours (well, a substantial amount of time at least) before urinating for the first time while drinking alcohol. But after urinating for the first time, the urge to go again comes incredibly quickly, hence the common colloquialism: "opening the floodgates".

Can anyone help to explain?