The respiratory system begins at the nose and ends at the distal alveoli. It is comprised of the upper and lower airways. The upper airway includes the nose, sinuses, and pharynx. The nose provides olfaction and temperature regulation in hyperthermic patients. The nasal turbinates initially humidify and warm air, and filter particulate matter. The lower airways include the trachea, bronchi, bronchioles, and alveoli. The primary function of the respiratory system is to deliver oxygen to the lungs to be exchanged with carbon dioxide.
Gas exchange occurs in the alveoli, which are comprised of one-cell-layer-thick membranes in which oxygen moves into the capillary and where carbon dioxide moves into the alveoli from the blood in the capillary. Failure or major dysfunction of gas transfer due to disease leads to respiratory distress or failure. Additional functions of the respiratory system include 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).
Large, inhaled airborne particles enter the nose and are deposited along the mucous lining of the nasal passages. Cilia move these particles along the mucosal barrier to the pharynx to be swallowed or expectorated. Small particles may not be filtered on inhalation and may be deposited in the alveoli, where they are phagocytized by macrophages. Defense against invasion by microorganisms and other foreign particles is provided by this mucociliary "blanket" and by cellular and humoral immunity. These factors determine species and individual susceptibility to disease and may be manipulated through various management techniques, vaccines, antimicrobials, and other agents, such as interferons and lymphokines. Mechanical factors include the tortuosity of nasal passages; presence of hairs, cilia, and mucus; the cough reflex; and bronchoconstriction. Cellular defenses include neutrophils and macrophages. The latter phagocytize invaders and present them (or at least their important antigens) to lymphocytes for stimulation of an immune response. 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 anatomy of the respiratory tract differs markedly among species in the following features:
shape of the upper and lower airways
extent, shape, and pattern of turbinates
bronchiole pattern
anatomy of terminal bronchioles
lobation of the lungs
pleural thickness
mediastinal completeness
relationship of pulmonary arteries to bronchial arteries and bronchioles
presence of vascular shunts
mast cell distribution
pleural blood supply
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:
ruminants (cattle, sheep) and pigs
dogs, cats, monkeys, rats, rabbits, and guinea pigs
horses and humans
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 is defined as insufficient oxygen to maintain normal metabolic functions; arterial oxygen is 60 mm Hg or less. An animal with hypoxia will show signs of respiratory distress. It can result from the following:
reduced oxygen-carrying capacity of the blood (anemic hypoxia, caused by a decreased number of red blood cells)
hypoperfusion (hypoperfusion hypoxia caused by decreased cardiac output)
hypoxic hypoxia (anatomic shunt, physiologic shunt, decreased inhaled oxygen, ventilation/perfusion mismatch, diffusion impairment, or hypoventilation)
inability of tissues to use available oxygen (eg, histotoxic hypoxia, as in cyanide poisoning)
There for four major centers of ventilatory control:
respiratory control center
central chemoreceptors
peripheral chemoreceptors
pulmonary mechanoreceptors/sensory nerves
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, multiorgan dysfunction may result.
