how can sharks smell blood?

[Greg_D]

you will often see people in wildlife films chucking blood and whatnot over the side of boats to attract sharks, it is said that they can sense blood over a quarter of a mile away.

My question is thus, how do they sense it so quickly? surely they need a bit of the original blood to sense and it would surely take days for that microscopic bit of blood to make its way over that sort of distance, not minutes or maybe an hour as is often portrayed in the programmes in question. and the chances of a shark being in the exact bit of water as a microscopic bit of blood over 400metres away from source is surely infinitesimally small, yet they are usually on the scene within moments, how come?

Greg