The nostrils of great white sharks can detect minute quantities of blood due to highly sensitive nasal sacs.
"The first of a great white shark's senses to come into play when seeking prey is smell. The nasal sacs in a shark's nostrils can detect minute quantities of blood -- as low as 1 part in 1,000,000,000 -- seeping from an injured animal." (Shuker 2001:40)
Learn more about this functional adaptation.