Defense mechanisms of the respiratory system
While animals are grazing or playing outside, there is generally little particulate matter in the air. However, when animals are housed in confined areas or are being transported, exposure to particulate matter can increase dramatically.
Such particulate matter can contain many harmful materials such as bacteria, viruses, and allergens. The respiratory system contains defense mechanisms to minimize the harmful effects from these materials.Nonspecific defense mechanisms
Inhaled air can contain aerosols, airborne particles small enough to float in the air, and unwanted gases. When airborne particles come in contact with the wall of the conducting airway or a respiratory unit, they undergo a process of deposition, which prevents them from becoming airborne again. This can happen in one of four ways:
• Sedimentation, which is settlement by gravity, tends to occur in larger airways.
• Inertial impaction occurs when an airstream changes direction, especially in the nose but also in other large airways. This happens in places where there are bends in the large airways and airborne particles fail to make the turns.
• Interception applies mainly to irregular particles such as asbestos or fibrous dust, which, because of their shape, avoid sedimentation and inertial impaction. However, they are intercepted by collision with walls of bronchioles, especially at bifurcations or if the fibers are curved.
• Diffusion is when very small aerosol particles reach peripheral airways and alveoli and contact the epithelial surface and then move inward in the mucous membrane.
Most particles larger than 20 microns in diameter are filtered within the nose during breathing at rest. However, conditions that increase respiration rate or favor mouth breathing, (e.g., high ventilation rates and obstructive disease of the nasal airways) cause large particles to bypass this filter.
Slow deep breathing favors the movement of particles deep into the lung; rapid shallow breathing favors deposition in the larger airways.Particles trapped in the mucous lining of the respiratory tract are transported back toward the pharynx by the mucociliary escalator. The movement of the cilia pushes the mucus back toward the mouth, thus carrying unwanted particles out of the respiratory system. Allowing a horse to lower its head facilitates such movement and decreases the incidence of pneumonia by preventing the accumulation of bacteria in the lungs.
Specific defense mechanisms
The alveolar lining contains many macrophages. Alveolar macrophages are the cells within the lungs, such as phagocytose microbes and particulate matter, for example, inhaled dust. Alveolar macrophages begin as monocytes produced in the bone marrow, and then they differentiate while in transit through the bloodstream where they leave the capillaries and enter the lungs.
Alveolar macrophages can function in two ways. They can either be fixed, in which case they are trapped within the connective tissue of the alveolar walls, or they can be free, in which they are mobile and can scavenge for particles that are trapped within the surfactant layer. These cells can leave the lungs by ascending in the layer of mucus on the mucociliary escalator
Box 14.5 Chronic obstructive pulmonary disease (COPD)
COPD is the fourth leading cause of death in humans and also occurs in animals. It is a lung disease that resembles human asthma. The condition is also known as heaves, recurrent airway obstruction, broken wind, emphysema, chronic bronchitis, or small airway disease.
Horses are more at risk for COPD if they spend their winters stabled and fed on hay. Small dust and fungal spores, particularly Micropolyspora faeni and Aspergillus fumigatus, enter the airways when the horse eats, causing irritation to the lungs and inflammation and narrowing of the airways.
Treatment first involves lifestyle changes in order to decrease the animal's exposure to allergens.
If this proves ineffective, the animals may need to be given corticosteroids (prednisone, dexamethasone, triamcinolone) to suppress inflammation. Longterm use of corticosteroids can cause problems, particularly heart problems, immune suppression, and decreased ability to heal. The animals can also be given bronchodilators (clenbuterol, pirbuterol, albuterol) to help decrease smooth muscle contraction and reduce mucus production. Although oral administration is the easiest, the most effective treatment is to administer these drugs directly into the airways. This results in minimal systemic side effects, but it requires a special mask.to the larynx, or they can pass through the alveolar wall and pass into alveolar lymphatics.
Alveolar macrophages may phagocytize erythrocytes that have left the pulmonary capillaries as a result of pulmonary congestion. Such macrophages stain positively for iron in the altered pigment hemosiderin. These "heart failure" cells may be coughed up by the animal in its sputum (Box 14.5).
If the lung is injured by infectious agents, it can produce cytokines and chemokines, which come from monocytes, injured epithelial or endothelial cells, or other cells involved in the inflammatory response. These compounds are involved in the inflammatory response.