As a shark swims along, water flows into its nostrils and enters the nasal sacs. The nasal sacs are sensitive scent organs, and sharks can detect very weak odors. A shark can smell a drop of blood in 25 gallons of seawater. Small pits covering large areas of skin near the mouth and nose below the shark's snout. They are natural electrical sensors which, at close range, can detect weak electrical fields generated by all animals.
Most sharks have a well-developed sense of vision.
They see better at night than most humans, and research suggests that some species can see color. Sharks have upper and lower eyelids, but these lids don't move. Some sharks have a membrane, the nictitating eyelid, that can be closed over the eye to protect it from damage. As the shark grows, the denticles are shed and replaced by slightly larger ones. The layer of dermal denticles creates a rough surface that is sometimes compared to sandpaper.