system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In medical terms, when they say someone has, "lost a lot of blood," what does "a lot" mean?
gpt
Try to picture 5 litres of blood. That's about 13 or so regular cans of Pepsi, and also roughly (give or take depending on gender/size/etc) the amount of blood in a person. Your average blood donation takes about 8% of that, so pretty much just one can of Pepsis worth. 

However... should you find yourself unlucky enough to lose more than 20% (...About 3 cans of Pepsi) they go into what is known as Hypovolemic shock. Its pretty nasty. 

So yeah, medically speaking, a fair representation of "a lot" would be near or greater than 20% of your blood, or roughly 2-3 cans of Pepsi's worth.