Blue sharks were once the most common shark species, but they are now an endangered species, due to over-fishing. As the shark moves, water flows through two forward facing nostrils, positioned along the sides of the snout. The water enters the nasal passage and moves past folds of skin covered with sensory cells. In some sharks, these sensitive cells can detect even the slightest traces of blood in the water. A great white shark, for example, would be able to detect a single drop of blood in an Olympic-size pool.
Most sharks can detect blood and animal odors from many miles away.
Another amazing thing about a shark's sense of smell is that it's directional. In this way, a shark can figure out where a smell is coming from and head in that direction. Sharks also have a very acute sense of hearing. Research suggests they can hear low pitch sounds well below the range of human hearing. Sharks may track sounds over many miles, listening specifically for distress sounds from wounded prey.