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Respiration is the body’s way of taking in oxygen and expelling carbon dioxide. The dog’s respiratory system can be divided into the upper and lower respiratory tract.

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Upper respiratory tract

© Diffomédia/Royal Canin

The upper respiratory tract consists of the nasal cavities, the nasopharynx, larynx and trachea. The dog’s nasal cavities are located in the bridge of the nose and the forehead, opening to the exterior via the nostrils, which are cartilaginous structures with wide holes to allow air in.

• The nasal cavities are made up of scrolled turbinate bones and nasal sinuses, separated by a bony septum. They are lined by a very extensive mucosa, which is highly vascularised, enabling it to amplify its function of warming and saturating the air with water vapour.

The nasal glands secrete mucus which traps aggressive particles in the air (dust, microbes). Dogs access their sense of smell using another part of the olfactory mucosa. After passing through the nasal cavities and the posterior nares towards the nasopharynx at the back of the mouth, the air has been purified and warmed almost to body temperature.

• The air continues its journey to the lungs through the larynx and trachea. The larynx is made up of four different cartilage structures (cricoid, thyroid, arytenoid and epiglottis), which are attached to the bones of the skull by the hyoid bones. A group of muscles controls the way these cartilage structures move in relation to each other. Although the larynx is open during respiration, when it controls the flow of air by constricting and expanding, it closes during swallowing to ensure that food does not enter the trachea. It also houses the vocal cords, which vibrate when air passes over them, producing such familiar canine sounds as growling and barking.

• The trachea is a long tube, made up of around forty rings of cartilage which are closed by the tracheal muscle. The trachea pushes the air from the larynx in the mouth to the bronchi in the thorax. The contraction of the tracheal muscle reduces the diameter of the trachea, regulating the flow of air. This muscle also prevents excessive dilatation during coughing.

Lower respiratory tract

The lower respiratory tract includes the bronchi, bronchioles and alveoli contained within the lungs and located inside the thoracic cavity, from which they are separated by the pleurae. The thorax is defined by the ribs at the sides and the diaphragm at the back. The lungs are separated from the chest wall by the pleurae, which maintain the pleural cavity, so they are always filled with air. The dog’s lungs have seven pulmonary lobes: three on the left (cranial, middle and caudal) and four on the right (cranial, middle, caudal and accessory).

The bronchi , of which there are seven, one to each pulmonary lobe, branch into bronchioles, alveolar ducts and alveolar sacs where gas exchange takes place across the very thin walls of the sacs into and out of the bloodstream. The lungs are highly vascularised, facilitating the exchange of oxygen and carbon dioxide (oxygenation) over a large surface area.

Respiratory phenomena

Respiration itself is a complex process involving both muscles and blood circulation.

• The exchange of gases between the air in the alveoli and the blood is dependent on the oxygen and carbon dioxide pressure on each side of the capillary wall. Gases move from areas of higher pressure to areas of lower pressure, which means that carbon dioxide moves from the capillaries to the lungs, while oxygen moves in the opposite direction. To ensure constant oxygenation, the air and the blood need to be constantly renewed, which is why the blood is pumped around the body by the heart.

• Pulmonary ventilation renews the air in the alveoli. This is a two-step process. “New” air is inhaled into the lungs, flushing out waste air. Inhalation is essentially caused by the contraction of the diaphragm and the intercostal muscles together with the relaxation of the abdominal muscles. These contractions increase the volume of the thoracic cavity, drawing air into the lungs, which inflate like a balloon. During exhalation, these muscles gradually relax and the thorax decreases in volume due to its elasticity.

© Diffomédia/Royal Canin

A dog’s normal respiratory rate is between ten and thirty breaths per minute. This varies with size (it is higher in small dogs), body condition and level of excitement.

• The respiratory process is regulated by the nervous system, and therefore mostly unconsciously. In exceptional circumstances it is a conscious process (for example, when the normal respiratory rate is increased). Dogs can modify their respiratory rate or volume depending on the physiological conditions, such as strenuous muscle effort, when the dog pants, increasing its respiratory rate and volume by breathing through the mouth. Muscle action consumes more oxygen and heats up the body, leading dogs to speed up their breathing to increase the rate at which oxygen is supplied to the cells (because the heart beats faster, too). Dogs barely sweat (and then only though their footpads), so the loss of water vapour through the lungs is their way of cooling down. By breathing through the mouth, dogs bring cool air from the trachea into contact with the warm blood vessels, which cools them down.


A dog’s normal respiratory rate is between ten and thirty breaths per minute.


Based on the quality of inhaled air (the partial pressure of oxygen in the air decreases with altitude), the partial pressure of oxygen and carbon dioxide in the blood are also regulated, depending on the blood pH, which affects the partial pressure of carbon dioxide in the blood.

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