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Overview of the Respiratory System

By Ned F. Kuehn, DVM, MS, DACVIM, Michigan Veterinary Specialists

The respiratory system performs several functions. Most importantly, it delivers oxygen to the cardiovascular system for distribution to the body and it removes carbon dioxide. Gas transfer occurs in the alveoli of the lungs, where the air-blood barrier is a thin, permeable membrane. Failure or major dysfunction of gas transfer due to disease processes that compromise this membrane or its air or blood supply have serious effects. In addition to gas exchange, the respiratory system performs numerous other functions, including maintaining acid-base balance, acting as a blood reservoir, filtering and probably destroying emboli, metabolizing some bioactive substances (eg, serotonin, prostaglandins, corticosteroids, and leukotrienes), and activating some substances (eg, angiotensin). The respiratory system also protects its own delicate airways by warming and humidifying inhaled air and by filtering out particulate material. The upper airways also provide for the sense of smell (olfaction) and play a role in temperature regulation in panting animals.

Large, airborne particles are usually deposited on the mucous lining of the nasal passages, larynx, trachea, and bronchi, after which they are carried by the mucociliary “blanket” to the pharynx to be swallowed or expectorated. Small particles may be deposited as deep as the alveoli, where they are phagocytized by macrophages. Defense against invasion by microorganisms and other foreign particles is provided by anatomic structures and by both nonspecific and immunologic mechanisms (both cellular and humoral). These are the factors that determine species and individual susceptibility to disease and that may be manipulated by using various management techniques, vaccines, antimicrobials, and other agents such as interferons and lymphokines. Other mechanical factors include the tortuosity of nasal passages; presence of hairs, cilia, and mucus; the cough reflex; and bronchoconstriction. Cellular defenses include macrophages, which phagocytize invaders and present them (or at least their important antigens) to lymphocytes for stimulation of an immune response, and neutrophils, which often die in their fight against invaders and must be removed along with their potentially damaging enzymes. Secretory defenses include interferon for antiviral defense, complement for lysis of invaders, surfactant lining the alveoli to prevent their collapse and to facilitate macrophage function, fibronectin to modulate bacterial attachment, antibodies, and mucus.

The respiratory system must perform many functions, preferably while expending minimal energy. The required effort is increased by processes that oppose expansion of the lung (eg, fibrosis or hydro-, chylo-, pneumo-, or hemothorax), impede the flow of air (eg, obstructive nasal disease, bronchiolitis, bronchoconstriction, laryngeal paralysis, or pulmonary edema), or thicken the air-blood interface (eg, interstitial pneumonia due to viruses or toxins, pulmonary edema).

The anatomy of the respiratory tract differs markedly among species in the following features: 1) shape of both the upper and lower respiratory tract; 2) extent, shape, and pattern of the turbinate bones; 3) branching patterns of bronchi; 4) anatomy of terminal bronchioles, including collateral ventilation; 5) lobation and lobulation; 6) thickness of pleura; 7) completeness of the mediastinum; 8) relationship of pulmonary arteries to bronchial arteries and bronchioles; 9) presence of vascular shunts; 10) distribution of mast cells; and 11) blood supply to the pleura. Each variation in anatomic structure implies variation in function, which can influence the pathogenesis of respiratory disease in a particular species. The three main groups of species that have similar subgross anatomy of the lung are 1) cattle, sheep, and pigs; 2) dogs, cats, monkeys, rats, rabbits, and guinea pigs; and 3) horses and people.

Marked physiologic variations also exist between different species. For example, cattle are prone to retrograde drainage from the pharynx, are predisposed to pulmonary hypertension and reduced ventilation in a cold environment, have relatively small lungs with low tidal volume and functional residual capacity, and are more sensitive to changes in environmental temperatures than are most other species. These anatomic and physiologic differences largely determine why some pathogens affect only some species (eg, Mannheimia haemolytica affects cattle but not pigs) and why pneumonia is very important in some species (cattle, pigs) but less so in others (dogs, cats).

Hypoxia (lowered oxygenation, often termed anoxia) causes clinical signs of respiratory disease. It can result from the following: 1) reduced oxygen-carrying capacity of the blood (anemic anoxia, as in carbon monoxide or nitrite poisoning, or true anemia due to various causes); 2) reduced blood flow (stagnant anoxia, as in congestive heart failure or shock); 3) insufficient alveolar ventilation, mismatching between ventilation and perfusion, shunt or diffusion impairment (hypoxic anoxia, as in pneumonia, pulmonary edema, chronic congestion, pneumothorax, or paralysis of respiratory muscles); or 4) inability of tissues to use available oxygen (eg, histotoxic anoxia, as in cyanide poisoning).

Compensatory mechanisms for hypoxia include increased depth and rate of breathing, which is mediated by chemoreceptors located in the carotid and aortic bodies; contraction of the spleen, which forces more RBCs into the circulation; and increased cardiac stroke volume and heart rate. If cerebral hypoxia develops, respiratory function may be reduced even further due to depression of neuronal activity. Erythropoiesis is also stimulated with chronic hypoxia, although the degree of polycythemia is species dependent. In addition, myocardial, renal, and hepatic functions may be reduced, as may motility and secretions of the intestine. If compensatory mechanisms are inadequate, a vicious cycle may begin in which all body tissues function less efficiently.