Structure and Function of the Respiratory Tract in Dogs

Below is information about the structure and function of the canine respiratory system. We will tell you about the general structure of respiratory system, how lungs work in dogs, common diseases that affect the respiratory system and common diagnostic tests performed to evaluate the lungs and airways of dogs.

What Is the Respiratory System?

The respiratory system in humans, dogs, and other pets is a series of tracts and organs responsible for respiration, without which life would not be possible. Respiration is the term used to describe breathing. It involves the inhalation of air and the intake of oxygen, as well as the exhalation of waste gases such as carbon dioxide from the lungs.

Besides breathing, the respiratory tract serves other important roles, such as the humidification and warming of air before it enters the body, the trapping and expelling of foreign substances, facilitation of the sense of smell, and the production of vocal sounds (e.g. barking, growling). The respiratory system consists of the nasal passages, the back of the mouth (nasopharynx), the voice box (larynx), the windpipe (trachea), the lower airway passages, and the lungs.

Where Is the Respiratory Tract Located in Dogs?

The respiratory tract is a large, contiguous system comprised of several structures. The respiratory system begins at the nostrils, involves several structures of the head, continues down the neck and ends at the lungs that lie in the chest cavity.

• The nose is positioned in the center of the face. The structure and length of the nose varies greatly in dogs. In dolichocephalic breeds of dogs (e.g. collie, Doberman pinscher, German shepherd dog), the nose is quite long and prominent. In brachycephalic breeds of dogs (e.g. pug, Pekingese, Lhasa apso, bulldog), the nose is quite short and flattened.
• The nasal passages lie within the nose between the nostrils and the back of throat. There are two passages, one on each side of the nose. They are separated by a bony plate or septum until they end at the nasopharynx.
• The nasal cavity is surrounded by sinuses. The sinuses are air filled spaces within the bones of the skull. The major sinuses lie just below and above both eyes.
• The pharynx is the structure that lies at the back of the mouth and throat. It is the cavity behind the tongue and nasal passage through which both food and air are transported to deeper structures. The portion of the pharynx that is part of the respiratory tract is referred to as the nasopharynx, and it connects the back of the nasal cavity to the larynx (voice box).
• The larynx is located directly behind the base of the tongue and soft palate, and lies between the pharynx and the trachea (windpipe). The larynx covers the trachea during swallowing so that food does not enter into the windpipe.
• The trachea is a cylindrical tube that runs from the base of the larynx to the beginning of the airways in the lungs. At its termination in the chest it splits into two branches, with one branch for each set of lungs (right and left). Within the chest it lies just above the base of the heart, and just next to and below the esophagus.
• Once the trachea splits into two branches, the airway passages are called bronchi. The bronchi spread out into lung tissue and continue to divide into smaller and smaller hollow channels as they go further into the lungs. The airways eventually terminate in tiny air pockets within the lungs called alveoli.
• There are two sets of lungs on either side of the chest cavity. They surround the heart and fill most of the chest between the base of the neck and the diaphragm. The diaphragm is the muscle that separates the chest cavity from the abdomen.

What Is the General Structure of the Canine Respiratory Tract?

The respiratory tract is a very structurally diverse system.

• The most forward portion of the respiratory tract is the nose. The external, visible portion of the nose consists of a fixed bony case and a moveable cartilage framework. The front portion of the nose is flattened and devoid of hair and is called the planum nasale, which includes the nares or nostrils. The nostrils are the entrance openings of the nasal cavity and are supported by cartilage.
• The pharynx consists of two parts, including the nasopharynx (associated with the respiratory tract), and oropharynx (associated with the digestive tract). It is a fairly large round cavity that is lined by soft membranes of the back of the throat.
• The larynx is a round structure that is composed of muscles, several cartilages, and soft tissues. The cartilages at the beginning of the larynx are designed to open and close during breathing or swallowing. When the larynx is open, air passes from the nose to the trachea. During swallowing, the laryngeal opening is closed so that food does not fall into the trachea.
• The trachea is a semi-rigid, flexible tube that connects the larynx to the bronchi of the lungs. It is made up of many C-shaped cartilages that are strung together, each alternating with an elastic ligament and muscle. The C-cartilages lie with the open area at the top. A soft membrane covers this open area in the cartilages.
• The bronchial tree within the lungs begins at the bifurcation of the trachea with the formation of a right and a left mainstem bronchus. Each mainstem bronchus divides into lobar bronchi that supply the various lobes of the lung. Within the lobe of each lung, the lobar bronchi divide into smaller segmental bronchi. This process of branching continues until the respiratory bronchioles are formed. The bronchi are cylindrical tubes that are kept from being flattened by overlapping, curved cartilages. The bronchioles eventually give rise to alveoli, which are tiny saclike structures with very thin membranes that allow gases to pass to and from the lungs into the airways.

What Are the Functions of the Respiratory Tract?

• The nose (along with the mouth) is responsible for taking air into the body. Both the fine hairs (cilia) that line the nasal cavity and the mucus that is produced by the cells of the nasal cavity work to filter debris and foreign material from the air before it enters the body. The nasal cavity also warms and moistens the air before it enters the trachea. The blood supply to this area is extensive and contributes to warming the inspired air. Moisture is added to the air by evaporation of mucosal secretions. As air passes over the back portion of the nose, the sense of smell is activated.
• The nasopharynx functions as the passageway between the nasal cavity and the larynx. Air transported through this area passes very near the tonsils. The tonsils are a part of the immunologic system, and are capable of activating certain defense mechanisms of the body when foreign material and infectious agents are detected.
• The larynx guards the entrance to the trachea and regulates both the inspiration and expiration of air. The valvular function of the larynx, which is created by the epiglottis and arytenoid cartilages, is vital to protecting the airway and to preventing the aspiration of food. The larynx also contains the vocal folds, which are necessary for vocalization, such as barking, whining and growling.
• The trachea or windpipe serves to conduct air downward into the lungs. It is also lined by tiny hairs called cilia and mucus producing cells that trap debris and foreign substances. The trachea returns those substances to the mouth through the act of coughing.
• The bronchi bring air from the trachea into the lungs. Like the trachea they are also lined with cilia and mucus producing cells.
• The main function of the lungs is provide a huge surface area over which gases are exchanged between the body’s circulation and the outside air. Oxygen is taken in from the atmosphere and carbon dioxide is exhaled from the blood. The physical act of breathing involves well-coordinated interactions between the lungs, the central nervous system, the diaphragm and the circulatory system.

What Are Some Common Diseases of the Respiratory Tract in Dogs?

There are many primary disorders that affect the respiratory tract. Generally speaking, coughing, sneezing, and/or difficult breathing are the most common signs seen with respiratory disease.

• Diseases of the nasal cavity often cause nasal discharge, sneezing, and/or sterterous breathing (snoring or snorting sounds). Common diseases of the nasal cavity include:
• Pharyngeal disorders often present with stertor, gagging, or retching. Rarely, dogs may develop inflammatory polyps in this area. Polyps are usually benign soft tissue growths that grow from the lining of the back of the nasal passage, the pharynx, or the auditory (Eustachian) tube. They may obstruct the flow of air through the nasopharynx and cause problems within the middle ear.
• Diseases of the larynx often cause respiratory distress and/or stridor (a high pitched audible wheezing sound). They may involve only the larynx or other parts of the upper airway as well.

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.

• Diseases of the trachea are most often associated with cough, respiratory distress, exercise intolerance, weakness, cyanosis (blue color to the normally pink tongue and gums) or fainting (syncope). Common disorders of the trachea include the following:
• Pulmonary or lung disorders often produce coughing, increased respiratory rates, respiratory distress, and exercise intolerance. When the functional capacity of the lungs is severely compromised, the animal is incapable of taking in enough oxygen. In such cases the dog may collapse and stop breathing. Disorders of the lungs include the following:
  Pneumonia is an infection within the lungs. It is most often caused by bacteria, but may also arise with viral, fungal, protozoal, and parasitic infections. Pneumonia may also occur if food, fluid, medications, or vomitus is aspirated into the lungs.

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.

• Diseases of the pleural cavity (the potential space between the lungs and chest wall) can also affect the function of the respiratory system. Normally there is only a tiny amount of fluid in this space, and the fluid serves to lubricate the lungs and to decrease friction between the lungs, the rib cage, and diaphragm. When fluid accumulates in the space, the lungs are compressed and cannot expand to their full amount, thereby decreasing the oxygenation capacity of the lungs. Diseases of the pleural cavity include the following:

What Types of Diagnostic Tests Are Used to Evaluate the Respiratory Tract?

Depending on the history and clinical signs, the following tests may be indicated to help make a diagnosis and design a proper treatment protocol.

• Auscultation of the chest, which is listening to the heart and lungs through a stethoscope, is performed in the exam room. Diseases of the lower airways and lungs frequently cause abnormal sounds within the chest.
• Evaluation of mucous membrane color is also usually performed. The color of the gums of the mouth provides valuable information on the blood circulating to the body tissues. If oxygen levels are low in the blood, then the gums appear blue or cyanotic. If the blood is anemic, the gums may appear pale. If carbon monoxide levels are high in the blood, the gums are usually very bright red. These changes allow indirect assessment of the gaseous exchange functions of the lungs.
• Thorough physical examination of all visible components of the airway system, including the nostrils, front of the nose, nasopharynx, and opening of the larynx is indicated. Palpation of the trachea is also useful. Close examination of the rest of the body for signs of trauma, heart disease, cancer, and other diseases is also beneficial.
• Evaluation of a complete blood count (CBC), biochemical profile and urinalysis may reveal changes suggestive of infection, inflammation, electrolyte and acid-base imbalances, and/or other organ involvement.
• Specialized testing for infectious diseases and heartworm disease may also be recommended. The measurement of oxygen and carbon dioxide (also known as blood gas analysis) may be performed in blood samples from both arteries and veins.
• Thoracic radiographs (chest X-rays) are very useful to evaluate the lungs, thoracic trachea, lower airways, heart, diaphragm and pleural cavity. They may detect evidence of pneumonia, edema, free air, foreign bodies, airway deformities or obstruction, tumors with the lungs or chest cavity, fluid within the chest, intestines within the chest, enlargement of the heart, and abnormalities of the esophagus.
• An echocardiogram (ultrasound of the heart) should be considered in those cases where heart disease is suspected. Your veterinarian may refer your dog to an internal medicine specialist, a cardiologist, or a radiologist to have this test performed.
• When fluid is present within the chest, an ultrasound may help identify its source and detect a diaphragmatic hernia.
• Skull and/or nasal X-rays may be very helpful in diagnosing several causes of rhinitis and sinusitis including tumors, foreign bodies, trauma, pneumonia and cancer.
• Computed tomography (CT scan) or magnetic resonance imaging (MRI) are advanced techniques that are very sensitive in diagnosing nasal disorders, sinus and upper airway disorders.
• Rhinoscopy is a procedure that allows direct examination of the nasal passageway through a small rigid or flexible scope. Biopsies and samples for culture may be taken during this procedure. It is always performed with the animal under general anesthesia.
• Rhinotomy is surgical exploration of the nose and is usually done when a definitive diagnosis has not been achieved with other less invasive techniques.
• Bacterial cultures of the airway may be helpful in determining if there is a bacterial infection present and to define appropriate antibiotic therapy.
• Cytology (microscopic analysis of fluid or cells retrieved from the respiratory system) may confirm fungal, cancerous or parasitic diseases . Samples may be retrieved during rhinoscopy, tracheoscopy, bronchoscopy, transtracheal washings, and aspiration of masses.
• Tracheoscopy is evaluation of the trachea using a rigid or flexible scope, and bronchoscopy is examination of the bronchi. Both procedures may be helpful in cases of suspected tracheal or bronchial inflammation, infection, foreign bodies, tumors, etc.
• A transtracheal wash is a procedure that involves the passage of a small catheter into the trachea and lower airway. It can be done without the use of expensive endoscopes, often under local anesthesia and light sedation. It facilitates the retrieval of cells from the trachea and lungs for analysis and culture.
• A lung aspirate is another way to sample the lung tissue or a lung mass. It is done through the body wall with a small needle.
• At times surgical exploration of the chest is also needed to define and/or correct problems that are occurring within the chest.