Dogs are described as smelling far better than people, which is true but tells you very little. The interesting part is what the equipment lets them extract from air.
The anatomy that does the work
The interior of a dog's nose is folded into a large sheet of tissue carrying many times the number of scent receptors a human has. Surface area is the first advantage.
Air entering the nostril splits, with part directed to the lungs and part routed over that sensory tissue. Breathing and smelling are handled on separate paths.
Exhaled air leaves through slits at the side of the nostril rather than back the way it came, so an incoming sample is not diluted by the outgoing one.
Sniffing is not ordinary breathing
A sniff is a rapid series of short inhalations, far quicker than a resting breath. Each one delivers a fresh sample before the previous one has cleared.
This gives the dog a repeated stream of snapshots rather than a single continuous impression. Concentration changes between snapshots carry information about direction.
A dog casting across a lawn is comparing those samples against each other. The rise and fall in strength across a few paces is what identifies which way a trail runs.
Scent as a timeline
Odour molecules left by a passing animal evaporate at different rates. Lighter compounds disperse within minutes while heavier ones persist for hours.
The mixture remaining at any point therefore encodes how long ago the trail was laid. A dog reading a fresh footprint against an older one is comparing ratios, not intensities.
This is why a trained dog can follow a track in the correct direction from only a few steps. The scent gradient tells it which end is newer.
The second nose
Dogs also carry a separate organ in the roof of the mouth, sensitive to the heavy, low-volatility molecules that animals produce as social signals.
This system responds to material that has to be physically contacted rather than inhaled, which explains the licking and nose-pressing that accompanies close investigation.
Information about sex, reproductive state and individual identity travels largely through this channel. It runs in parallel with ordinary smell rather than replacing it.
Why sniffing tires a dog out
Processing odour occupies a large share of a dog's brain, and the rapid sniff cycle is physically demanding on top of that.
A walk that allows unhurried investigation of hedges and posts leaves many dogs noticeably more settled than a brisk walk of the same length. The work is mental rather than muscular.
Owners of dogs with more energy than the household can absorb often get further by lengthening the sniffing than by lengthening the distance.