Structure and Function of the Respiratory Tract in Dogs (2023)

The nasal cavity is the air passageway within the facial area of the skull. It consists of right and left halves that are divided by the nasal septum, a thin-walled structure, which is part cartilage and part bone. Deep within the nose are numerous fine, paper-like bony plates that are lined with a mucous membrane called the turbinates. The blood supply to the turbinates is very extensive.

The olfactory region (area responsible for smell) is located in the back of the nasal cavity. The mucous membrane of this region contains special nerves designed for smell.

Rhinitis is an inflammation of the mucosa (lining) of the nasal cavity. It can be caused by infectious agents (bacteria, virus, fungal agents, parasites) or noninfectious disorders, including foreign bodies, allergies, trauma, dental disease, and environmental irritants. Sneezing and nasal discharge are commonly seen. Rhinitis may extend into the adjacent sinuses of the face, resulting in sinusitis.

Neoplasia (tumors) may develop within the nose of dogs. Most nasal tumors are malignant cancers, such as adenocarcinomas, osteosarcoma, lymphosarcoma, and squamous cell carcinoma. Tumors may develop initially on one side of the nose, but with time may affect both nasal passages. Besides sneezing and nasal discharge, swelling of the face or around the eye may also be noted.

Laryngitis is inflammation of the larynx. It most commonly occurs with tracheitis and upper respiratory infections. Clinical signs may include coughing, nasal discharge, difficulty or noisy breathing and a change in voice.

Brachycephalic syndrome is a condition of brachycephalic dogs in which several upper airway abnormalities occur together and seriously decrease the passage of air. Dogs with this syndrome may have congenitally small nostrils (stenotic nares), overly long soft palates that hang down into the nasopharynx, underdeveloped tracheas (tracheal hypoplasia), herniation of the vocal folds into the larynx (everted laryngeal saccules), and laryngeal paralysis. All of these conditions predispose the dog to respiratory difficulties, particularly if they become excited or overactive, are confined to small areas, are obese, or are exposed to heat and humidity. Clinical signs may be mild (noisy breathing, snoring, gagging or retching phlegm, exercise intolerance) to severe (respiratory distress, cyanosis, overheating, collapse, shock) with this condition.

Laryngeal paralysis is an acquired disease of some older, large breed dogs. The cartilages that normally control the opening and closing of the larynx become paralyzed and the larynx does not open well. These dogs exhibit changes in their voice (hoarse, raspy bark), noisy breathing, and exercise intolerance. They may also overheat and collapse in respiratory distress.

Polyps and tumors may also develop within the larynx.

Infectious tracheobronchitis (kennel cough) is a contagious upper respiratory disorder, and is common in dogs that have been exposed to other infected dogs, such as at a boarding facility, dog show, or playgroup. The disease is usually caused by a mixture of viruses (e.g. parainfluenza, adenovirus) and bacteria (especially Bordetella). The cough associated with kennel cough is often harsh, occurs in spasms, and may sound as though the dog is choking. At the end of the cough the dog make gag or retch. The trachea is often very sensitive, and any light manipulation of the neck results in spasms of coughing.

Tracheitis or inflammation of the trachea may also occur with exposure to irritants such as smoke, chemicals, dust, or foreign bodies. Certain parasites may also migrate to the trachea, causing inflammation within the airway. Tracheitis occasionally develops after the use of endotracheal tubes during general anesthesia.

Obstruction of the trachea may develop from the inhalation of foreign material, from the growth of tumors of the trachea, or from tumors or masses impinging on the trachea from surrounding tissues.

Tracheal collapse is a disease seen most often in middle-aged to older toy breed dogs (e.g. miniature poodle, Yorkshire terrier, Pomeranian). In these dogs the soft membrane across the top of the C-cartilage of the trachea becomes weak and stretches out. This allows the C-cartilages to collapse onto themselves, making it difficult for air to pass into the lungs. The cough associated with tracheal collapse has been described as a “goose honk.” It may occur with excitement, exercise, pressure on the neck from collars and leashes, and during eating or drinking. Spasms of coughing, respiratory distress, weakness and fainting are sometimes also seen with this condition.

The most common disorder of the bronchial tree of dogs is bronchitis. Bronchitis is inflammation of the lower airways that may arise with infections (e.g. bacterial, viral, mycoplasmal, parasitic), irritants (e.g. smoke, dust, foreign material), allergies, diseases of the lungs, etc. The most common presenting sign is usually coughing. Bronchitis may either be acute or chronic in nature.

Edema or fluid in the lungs may occur with a variety of conditions, including heart disease, electric cord shock, trauma, snake bite and other exposure to other toxins, smoke inhalation, near drowning, overdosage of intravenous fluids, and numerous other systemic diseases.

Cancer of the lungs may be primary or secondary (metastatic). Primary lung tumors are usually malignant and arise from either cells of the lungs or airways. They include squamous cell carcinomas, the bronchoalveolar carcinomas, and pulmonary adenocarcinomas. Many cancerous tumors in the body can spread to the lungs via the blood vessels. Such tumors include cancers of the kidneys, spleen, bones, skin, mammary glands, and soft tissues of the body.

Pulmonary contusions or bruises may develop when trauma causes hemorrhaging into the lungs. They are common after automobile accidents, dog bite wounds, falling from heights, and other forms of blunt chest trauma.

Pleural effusion is the accumulation of fluid within the chest cavity. Such fluid may be composed of blood (hemothorax), lymph chyle (chylothorax), pus or purulent material (pyothorax), fluid from heart failure, twisting of a lung (lung lobe torsion), other systemic diseases, or cancer.

Pneumothorax is the accumulation of air in the chest cavity. It is often secondary to trauma with rupture or laceration of the lungs, trachea or lower airways, or from penetrating injuries to the chest. Air is also allowed into the chest cavity when the chest is surgically opened or penetrated during various medical procedures.

A diaphragmatic hernia occurs when there is laceration or rupture in the diaphragm. The diaphragm is the muscle that separates the abdomen from the chest cavity. When the diaphragm is torn, abdominal organs may slide into the chest cavity and compromise normal respiration. A diaphragmatic hernia develops most often from blunt trauma to the chest and abdomen, such as being struck by an automobile or from a fall. In some rare instances, animals may be born with a diaphragmatic hernia if the membranes that separate the chest cavity from the abdominal cavity do not form correctly.