Human noses aren’t built to take in much from that pungent world. When we’re trying our damnedest to smell something, we inhale deeply. But as soon as we exhale, we lose the scent. The airflow patterns of the dog nose are strikingly different: They have one air passage for breathing and another for smelling. …
The dog’s nostrils are in gray on the left; the rest shows the inside of the nose. The brownish area on the right is where all of the smelling, or olfaction, happens — all those receptors sensing different types of odor molecules. The red lines represent the airflow paths when the dog is smelling, whereas the blue lines show the paths during breathing.
You can see that because of the nose anatomy, a dog can quickly move odor molecules to the back, letting them linger atop olfactory receptors. The front area, meanwhile, stays clear for continued breathing. Our olfactory receptors, in contrast, sit at the top of our nasal cavity, easily perturbed as we breathe out.
The biology explains all the dog snorting too, which " helps capture new scents below it and pushes them backwards into the nostril."