That's true, but there is a lot of water in the body aside from the blood.
While most people will feel physical effects after losing 5% of their body weight in water, many can lose up to 8-10% without significant adverse physical or mental impact. Loss over 10% is considered serious dehydration.
World champion marathon runner Haile Gebrselassie famously won the Dubai marathon in 2005 with a 10% body weight loss during the race:
https://www.ncbi.nlm.nih.gov/pubmed/22450589