The amazing dogs’ nose
All mammals possess five senses TOUCH, HEARING, TASTE, SIGHT and SMELL. The majority of humans use sight as their main sense to assess the world around them. The smell sense is subsidiary and the human nose is unlikely to be able to smell cancers,
In the dog the sense of smelling, the olfactory system, is the main special sense and it is extremely sensitive and efficient.
Possibly 30% of their brain is dedicated to analysing odour. It is estimated that the percentage of a dog’s brain devoted to analysing odours is 40 times larger than that of a human.
“We might notice if our coffee’s been sweetened with a teaspoon of sugar; a dog can detect a teaspoon of sugar in a million gallons of water: two olympic sized pools full.”
How is the dog’s nose structured to enable it to be so efficient?
The structure of the Dog’s nose is important. The nose is formed by bones, muscles and soft tissue and includes a blood supply of arteries and veins plus nerves, which are connected to specialised areas in the brain.
The combined functioning of these tissues is shown in this short home video of a labrador showing the constant movement involved in sniffing. Note how the dog wriggles its nostrils, which it can do separately, and opens and closes the slit at the side of the nostril. The need to lick the nose is explained later.
Anatomy of the dog’s nose
The dog has two nostrils (nares) divided by a cartilaginous and bony septum. The tip of the dog’s nose – rhinarium – is typically moist and cool to touch. When a dog flares its nostrils to sniff, the shape of the nostril openings change thus allowing redirection of air into the upper part of the snout and more direct airflow to the olfactory area. The slit at the side of the nares is used in the expiration of breath during sniffing as explained later.
The Muzzle of the dog is Important
THE HUMAN HAS UP TO 5 MILLION OLFACTORY RECEPTORS, WHEREAS THE DOG HAS UP TO 300 MILLION. [1&3]
This is possible because the long nose contains a labyrinth of thin bones, called turbinates, which are all lined by an epithelium. This provides a very large surface area for the air breathed to pass over. In the anterior part of the nose the bones are known as maxillo-turbinates, and they are lined by a respiratory type epithelium producing mucus.
It is in the posterior part of the nose containing ethmoid turbinates that the lining epithelium is olfactory in type and this contains the 300 million olfactory receptors, or neurones, needed to recognise the odour molecules. The neurones need maximum contact with the air containing the odour molecules and therefore have hair like projections – or cilia. In mammals it is thought that each olfactory receptor cell only expresses one single odorant receptor gene. 
Once the odour is recognised, a pathway of tiny nerves relay signals to the brain area dedicated to olfaction – the olfactory lobe – which then interprets the odours in combination with other specialised areas of the brain. As part of the olfactory anatomy there is a basal plate of bone known as the Lamina Transversa (not seen in humans) which creates an olfactory shelf or recess separated from the respiratory section. This allows odours to accumulate and be held for recognition even while air is exhaled.
Cross section views
The complexity of the labyrinth of thin turbinate bones in the nose is best seen in cross section. Below are two views of the anterior opening of the nostril in a dog’s skull showing the maxillo-turbinates, which would be covered by respiratory type epithelium.
Did you know for social contact the dog has a different site for a second separate sense of smell area?
It consists of two elongated fluid filled sacs above the roof of the mouth. It is named the VOMERONASAL ORGAN or JACOBSON’S ORGAN – and it detects body scents (pheromones – hormone like substances) allowing sexual and social assessment of other dogs.
The importance of Mucus
Why does the dog lick its nose? Because mucus is needed. The moisture caused by a covering of mucus assists in the collection of odour molecules. Odour molecules dissolve in the mucus and are transported in the air breathed in up to the olfactory receptors in the top of the dog’s nose. If there is not enough mucus the dog licks its nose. The average dog produces about a pint of mucus a day.
MUCUS is a viscous, slippery substance that consists chiefly of mucin, water, cells, and inorganic salts and is secreted as a protective lubricant coating by cells and glands of the mucous membranes. MUCIN is a nitrogenous substance found in mucous secretions; a lubricant that protects body surfaces.
Dogs use sniffing to maximise the detection of odours. Sniffing consists of a series of rapid inhalations and expirations, normally 3 to 10, but possibly up to 30, during which the normal breathing mechanism is disrupted. Each nostril sniffs air from separate areas so that during sniffing there is a bilateral scent intake. NOTE: the lateral slit at the side of the nostril
To maximise the efficiency during sniffing the dog needs each sniff intake to be unobstructed. Consequently, the expired air is passed out through the slits at the side of each nostril creating an air turbulence and allowing new odours to be inhaled directly into the centre of each nostril.
To enable this to happen, a structure just inside the nostrils called the alar fold (see above), opens allowing air to flow through the upper area of the nasal passages. When the dog exhales, the alar fold closes off the upper part and pushes air down and out through the lateral slits and the tiny wind currents created stir up even morescent particles. Watch for small puffs of dust rising up when the dog is sniffing close to the ground.
Importance of airflow during sniffing
A scientific article entitled ‘The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia by Brent A Craven, Eric C Paterson and Gary G Settles’  explains in detail the mechanics of the airflow to the olfactory system during sniffing. In their diagram below the dog’s nostril is coloured grey on the left. During sniffing the inhaled air in the dog separates into two distinct pathways. The upper flow path, approximately 12% of each breath and shown in red, passes straight to the olfactory region, which is connected to the olfactory bulb portion of the brain shown on the right. The rest of the air in the lower pathway, coloured blue, flows down the pharynx into the lung.
Interesting fact: Long ears on dogs and wrinkles along the nose, such as in a Bloodhound, may help entrap odours during sniffing.
What odours indicate disease
Dogs detect odours both direct from the source and as residual odours that persist in an area long after the source has left. Odour molecules are chemicals that can be dissolved in water, and they need to be small enough to be volatile so that they can vaporise and having reached the nose can then be dissolved in mucus
The Medical Detection Dogs are trained to discriminate between different “Volatile Organic Compounds” (VOCs)
In the human the normal metabolic processes are altered in disease and this may result in the production of volatile organic compounds (VOCs) and these diffuse into the blood stream and are then excreted in breath or in urine . Dogs can detect these VOCs at an incredible, tiny concentration of 0.001 parts per million. . However it should be noted that training a dog to detect the “odour fingerprint” for a particular type of disease among the thousands of odours normally found in a sample of breath, urine, blood or faeces without recourse to the “pure source” is extremely challenging. . In addition the dog has to learn to ignore the vast number of background odours. Thus, training a Medical Detection Dog is more complex than training a dog to detect explosives or drugs.
References: Tyson P. Dogs’ Dazzling Sense of Smell. Nova Science Now. posted 10.04 www.pbs.org/wgbh/nova/nature/dogs-sense-of-smell.htm  Alexandra Horowitz. Book entitled “INSIDE OF A DOG” publ by Simon and Schuster UK Ltd ,2012. 2nd Ed. p72  Coren Stanley Phd, Hodgson Sarah from “Understanding your Dog for Dummies. www.dummies.com/how-to/content/ understanding-a-dogs-sense-of-smell  “Press Release: The 2004 Nobel Prize in Physiology or Medicine to Richard Axel and Linda B. Buck”. Nobelprize.org. Nobel Media AB 2014. Web. 25 Jul 2014. <http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/press.html> Craven B A, Paterson EC and Settles G G.‘The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia’. J.R.Soc.Interface 6th June 2010 vol 7, no 47,p 933-943. Haick H, Broza Y Y, Mochalski P, Ruzsanyi V, Amann A. ‘Assessment, origin and implementation of breath volatile markers’. Chem. Soc. Rev. 2014;43: 1423-1449 D0I :10 1039/c3cs60329f. Review article Waggoner L P, Jones W, Williams M, Johnston J M, Edge C, Petrousky J A. Effects of extraneous odours on canine detection. In:DePersia AT Pennella JJ (eds) Enforcements and Securities Technologies: Proc.SPIE, vol. 3575,1998:355-62 Willis CM, Church SM, Guest CM, Cook WA, McCarthy N, Bransbury AJ, Church MRT, Church JCT Olfactory detection of human bladder cancer by dogs:proof of principle study. BMJ 2004;329:712. Guest CM. The Science Behind Medical Detection Dogs. The Sniff . Spring/Summer 2014