Your dog’s sense of smell is approximately 1000–10,000x more sensitive than yours. One of the reasons a dog has a much better smelling ability than we do has to do with the number of scent receptors: For every scent receptor a human has, a dog has about 50. 

Dogs can detect smells in parts per trillion, for example they could detect one rotten apple in two million barrels (which is about 752,000,000 apples)

Top pic is air flow.

Bottom pic is all of the vessels in the dogs nose.

13 fun fact on dog's sense of smell: click here

Here’s 5 facts about those cold, wet biosensors you may not know:

The percentage of a dog’s brain devoted to analysing odours is 40 times larger than ours

Dogs have 300 million smell receptors compared to our 5 million

For social contact, dogs have a second, separate sense of smell area

Dog’s lick their noses to produce mucus which helps them collect odour molecules. The average dog produces a pint of mucus a day

Sniffing is a series of up to 30 rapid inhalations