How can sharks do this?

Some of the attributes given to sharks are exaggerations, but remember that even people (with our poor sense of smell) can detect scents when they're in the parts-per-million concentration. Shark sense of smell is 10,000 times better than our. That being said, sharks are supreme predators which have not had to change in millions of years due to the perfect suitability of their form. You are right to be skeptical about a single drop of blood miles away. That's too dilute in the ocean to reach the threshold needed for them to sense it. That said, they are still remarkable. As we are able to detect the direction of sound and tell that it's louder as we get closer, sharks can smell in the equivalent of stereo. That lets them hone in on the source. Here's some more information (also see link).

"Statement No. 1: "Sharks smell blood many kilometers away."

The target of this meaningless statement which can be found in many different versions in the media, is clear. The author wishes to express the notion that sharks have a well-developed sense of smell. This in effect is correct, sharks are capable of recognizing even very small concentrations of scents in their immediate environment. However, the fact that sharks may smell blood at long distances says virtually nothing about their sense of smell, the reason being that the author sets the shark's sense of smell in relation to the distance from its source. Nothing is mentioned about the concentration of molecules necessary for the shark to notice the smell. But this is exactly the decisive point. The actual distance to the source is irrelevant. For example, humans have a significantly worse sense of smell than sharks, and yet we may still identify the smell of blood from significant distances if the amount of blood is large enough for the molecules to reach our mucous membranes (the nose) in adequate concentrations. Personally, I at least once smelled the smoke of a forest fire burning about 100 km away! But not because my sense of smell is as phantastic as that of sharks, but rather because the source of the smell (the fire) was big enough to allow a sufficient concentration of smoke to reach my nose from a distance of 100 km.

The significance lies in the concentration of molecules and the decisive question can be posed accordingly: Does the available concentration of odorous substances exceed the perception limit of a shark (or a human)? This is the only way to properly discuss the perception limits of a species. All other statements on distance have nothing to do with the question and are irrelevant. So what are the facts? How effective are shark noses in reality? Sharks can recognize most amino acids, the basic component of proteins, even in such small concentrations of 10-10 molar (a "one molar" solution is equivalent to the molecular weight of a substance in grams dissolved in one liter of water). Permit me to express this astonishing sensory perception in a more understandable manner. First, it means that sharks smell about 10,000 times better than humans. Second, to reach this concentration about 30 mg (measuring half a pinch, editor's comment) of an amino acid would have to be dissolved in a large swimming pool filled with saltwater! Third, such a swimming pool would have to have a volume of 2,600 cubic meters and this amount of saltwater would contain 3.5% of cooking salt and other ions, in other words, an approximate total of about 92 tons of salt. It is very hard to imagine that a shark can still perceive these few milligrams of amino acid when mixed with tons of salt. Fourth, sharks can even sense the presence of certain amino acids better than others. For example, they smell the amino acid Serin in concentrations of only 10-14 molar! Fifth and finally, it must also be said that although such sensory perception levels may be quite astronishing to humans, for other animals they are fairly run-of-the-mill. Ordinary animals such as lobsters or catfish have a comparable if not better sense of smell than sharks.

However, all comparisons with chemoreception performance such as the sense of smell must be made in relation to the concentration of the odorous substance and not the distance to their source."

sharks noses are extremely sensitive, thats why they can pick up the scent of blood. Your forgetting that sharks are completely different to human, even other animals. Just like a cheetah can run so many miles per hour. A shark can pick up a trace of blood from miles away. We are all equipped with amazing abilities.

Through "Brownian motion" and water currents, molecules in blood that has been spilled in water diffuse in all directions at a faster rate than you might expect, although detecting "a drop of blood from miles away" is mostly hyperbole...

http://www.ehow.com/how-does_4579345_shark-smell-b...

You are not understanding the vast amount of things that are in 1 drop of blood. Study Physiology.