Why do I need to pee more, nearer the loo?

This week we're flushing out the answer to this question from Charlie: "Maybe this is just me, but it dawned on me that whenever I have to hold in a pee, the need to go increases exponentially when I know that relief is close. Why is this?" So to relieve Charlie of his question, I asked physiologist Bill Colledge from Cambridge University to expel the answer...