I worked this out a couple of months ago and I found it pretty interesting. I hope you guys do too.
In the Amazon River, on average 135,000m3 of water passes every second, this is equivalent to 135,000,000 litres of water a second. There are 6,706,993,152 (July 2008 est.) people in the world. An adult human body contains on average 5.6 litres of blood. 5.6 x 6,706,993,152 = 37,559,161,651.2 litres of blood in the entire human population. 37,559,161,651.2 / 135,000,000 = 278.2160. This means that if all the blood was drained from every human being in the world there would only be enough to keep the Amazon river flowing for a mere 4 minutes 36 seconds.