In all those nature documentaries they give facts about animals that smell blood or track other scents for miles. How do they do it?
Also from the way they describe it the smell is instantly transmitted over vast distances. Explain that. 
Animals can smell better than people. It has to do with larger smelly-organs (technical term, that is) and lots of practice.
The smell is not instantly transmitted over vast distances. In short, they are presenting the info in a misleading way. For example, many people have been caused to believe that Bush is an idiot. If he is so much more foolish than the average liberal, then why are they not President instead?
Because his father is…
OK, aside from the pointless political sniping, smell is carried by molecules that spread through the air. It’s not transmitted instantly. The smell molecule has to travel from the source to the nose, where it docks with a molecular receptor, which in turn sends a message to the brain, which interprets it as, “hey! Blood!” The animal can then simply figure out which direction the scent gets stronger, and thus follow it to its source.
As to why they can do it and we can’t, well, many animals that use scent have a much much larger percentage of their brain devoted to smell. They typically also have much better noses.
Phage, you’re relatively new here, so you may be unaware of just how strict a line we take against political shots in GQ. You’re aware now. You will not repeat this behaviour.
General information on the olfactory sense.
Specific information on dogs’ ability to smell contrasted to that of humans.
And, as noted above, animals cannot smell anything before the air or water with the odor gets to them (although sharks use hearing and other “tricks” to seek out food, long before they can smell the blood).