The respiratory system consists of the large and small airways and the lungs. When a dog breathes air in through its nose or mouth, the air travels down the trachea, which divides into the tubes known as the right and left bronchi, then into the smaller airways called bronchioles in the lungs. The bronchioles end in the small sacs called alveoli, where the barrier between the air and the blood is a thin membrane.
The most important function of the respiratory system is to deliver oxygen into the blood, which distributes it throughout the body, and to remove carbon dioxide from the blood. The exchange of oxygen and carbon dioxide occurs in the alveoli. When this exchange fails or becomes inefficient because of disease, the animal can become seriously ill. The respiratory system protects its own delicate airways by warming and humidifying inhaled air and by filtering out particles. Large airborne particles usually land on the mucous lining of the nasal passages, larynx, trachea, and bronchi, after which they are carried to the throat to be either swallowed or coughed up. Small particles and microorganisms are destroyed by the body’s immune system. The upper airways also provide for the sense of smell and play a role in temperature regulation in animals such as dogs that use panting as a way to help keep cool. The respiratory system also protects the body in other ways.
Although the basic functions are the same, the anatomy of the respiratory tract varies among species. For example, the respiratory systems of dogs and cats are somewhat similar to each other, but differ from the respiratory systems of horses and humans. These differences explain in part why some diseases affect only certain species of animals.
When the level of oxygen in the blood is too low (called hypoxia or anoxia), the animal will show signs of respiratory distress. Low oxygen levels can be caused by reduced oxygen-carrying capacity of the blood cells, reduced blood flow through the lungs and body, insufficient movement of gases in and out of the lungs, or inability of tissues to use available oxygen (a condition caused by some poisons). The animal’s body attempts to compensate for low oxygen in the blood by increasing the depth and rate of breathing, increasing contraction of the spleen (to force more red blood cells into circulation), and increasing blood flow and heart rate. If the brain suffers from lack of oxygen, respiratory function may be reduced even further due to depression of nervous system activity. In addition, heart, kidney, and liver functions may be reduced, as may the normal movement and secretions of the intestine. If the body is not able to compensate for the reduced oxygen level, a “vicious cycle” may begin in which all body tissues function less efficiently.
Terms Used to Describe the Respiratory System
Respiratory diseases are common in dogs. Although signs such as coughing and labored breathing are most commonly caused by problems of the respiratory tract, they may also occur because of disorders of other organ systems, such as congestive heart failure.
Both very young and older animals are at increased risk of developing respiratory disease compared to healthy adult animals. At birth, the respiratory and immune systems are not fully developed; this makes it easier for disease organisms to enter and spread within the lungs. In aged animals, a decrease in the animal’s ability to filter out particles and fight off infection may render the lungs more vulnerable to airborne disease organisms and toxic particles.
Lung and airway disorders are often caused by direct infection with viruses, bacteria, fungi, or parasites, as well as by immune-mediated reactions or inhalation of irritants or toxic substances. Trauma (such as being hit by a car) may lead to the collapse of a lung or airway.
A variety of bacteria normally live in the canine nasal passages, throat, trachea, and sometimes lungs, without causing signs of illness. Infections by these usually harmless bacteria may occur when the respiratory defense mechanisms are weakened by another infection (such as distemper or parainfluenza virus), irritant (such as smoke or noxious gases), or disease (such as congestive heart failure or lung tumors). Disease organisms may continue to live in the respiratory tract of recovering animals. When stressed, these animals may relapse; they can also act as a source of infection for other animals. Poor management practices, such as overcrowding, are often associated with poor sanitation and environmental conditions, which can lead to more frequent and more severe infections. Conditions that favor the spread of infections often occur in kennels, pet shops, boarding facilities, and humane shelters.
Abnormalities that are present at birth, such as narrowed nostrils, elongation of the soft palate, cleft palate, and narrowing of the trachea, can cause respiratory dysfunction. Tumors, damage to the airways, and collapse of the trachea can result in difficult breathing and other signs of respiratory disease.
Your pet’s history and the veterinarian’s physical examination will help to determine the possible cause and site of respiratory disease. Chest and neck x-rays may be helpful when obstructive upper airway disease or an airway obstruction is suspected. Chest x‑rays are typically done for dogs with lower respiratory signs such as cough, rapid shallow breathing, or labored breathing. Blood gas analysis or pulse oximetry measure the amount of oxygen in the blood and may help assess the need for oxygen therapy in an animal with severe labored breathing.
When obstructive upper airway disease is suspected, your veterinarian may use various scopes to view the nose, throat, and airways. When lung disease is suspected, the veterinarian will want to examine the contents of the lung and its airways. This can be done by bathing the trachea or air sacs with a sterile fluid and examining the contents of the retrieved fluid; procedures such as transtracheal wash and bronchoalveolar lavage work in this way. When bacterial pneumonia is suspected, bacterial culture of transtracheal wash or bronchoalveolar lavage fluid can reveal which type(s) of bacteria are present. Microscopic analysis of fluid from the lungs can also aid in the diagnosis of fungal, parasitic, or allergic lung diseases.
Another way the veterinarian can collect material is to use a fine needle inserted through the wall of the chest to withdraw a small amount of tissue; this procedure is called fine needle aspiration. Fine needle aspirates of lung often are useful in the diagnosis of fungal pneumonia. In dogs with fluid buildup in the chest cavity, a sample of the accumulated fluid is often removed using a needle (thoracocentesis) to allow microscopic evaluation of the fluid. Ultrasonography through the chest wall can be used to diagnose diseases that occur around the lungs (pleural diseases) and lung disease that occurs close to the chest walls.
A sudden onset of a runny nose, sneezing, or both may suggest the presence of viral or bacterial infection (both very rare in adult dogs) or foreign object in the nose. Persistent cases may require additional examination using x-rays, computed tomography (CT), examination with an endoscope, or a sample of nasal tissue. Microscopic evaluation of nasal tissue may help diagnose fungal infections; however, most bacterial nasal infections of dogs occur because of other nasal diseases. Blood tests for fungal respiratory infections are sometimes used in addition to other tests and examinations.
Sudden dietary changes, weaning, cold, drafts, dampness, dust, poor ventilation, and the mixing of different age groups all play a role in respiratory disease in groups of animals. Stress and mixing of animals from several sources should be avoided or minimized if possible. Immunization can help control some types of respiratory infection; however, it is not a substitute for proper environmental conditions and animal care.
Respiratory disorders often involve the production of excess secretions in the respiratory system (for example, in the nose and lungs) that the affected animal may not be able to remove without assistance. One goal of veterinary treatment is to reduce the volume and thickness of the secretions and to make their removal easier. This can be accomplished by controlling infection, thinning the secretions, and when possible, improving drainage and mechanically removing the material.
Animals with respiratory disorders should normally receive plenty of water unless otherwise directed by the veterinarian. Adding humidity to the air may make removal of airway secretions easier. Cough medicines (expectorants) are sometimes used to thin secretions and make them easier for the animal to cough up; however, they are rarely helpful. If airway obstruction is severe, large amounts of secretions may need to be gently suctioned away by the veterinarian.
When an animal has coughing that is not helping to remove mucus (a nonproductive cough), medicines are sometimes used to suppress the coughing. Increased airway resistance caused by contraction of the bronchial airway muscles is sometimes treated with bronchodilators, which expand the airways and may be prescribed for animals with asthma-like conditions and chronic respiratory disease. Antihistamines can be used to alleviate constriction triggered by allergies. Constriction of the bronchial tubes in the lungs can also be reduced significantly by removing irritating factors, using mild sedatives, or reducing periods of excitement. If a bacterial infection is present, antibiotics are often given. Diuretics (medicines that help the body get rid of excess fluid) are sometimes used when an animal has fluid buildup in the lungs.
When a respiratory illness results in a lack of oxygen in the blood, this can usually be corrected by the administration of oxygen. However, this must be done carefully, because too much oxygen can lead to other problems.
Also see professional content regarding respiratory diseases of dogs.