Final answer:
Dogs have a keen sense of smell because they possess a high number of olfactory receptors and a larger olfactory epithelium, especially breeds like the bloodhound which were specifically bred for their tracking abilities.
Step-by-step explanation:
Dogs have a remarkably developed sense of smell, which is significantly more sensitive than that of humans. This is due to the fact that they have a greater number of functional genes for olfactory receptors (between 800 and 1200), whereas humans and other primates have fewer than 400. The bloodhound, a breed specifically developed for its tracking ability, has even more impressive olfactory capabilities. Bloodhounds were bred to have more olfactory receptors and a larger olfactory epithelium, which allows them to track scents over great distances and even detect subtle changes in scent. Some breeds of dogs that are bred for their sense of smell may have up to four times more olfactory cells compared to other dogs.