The mammalian respiratory system begins at the nares; proceeds to the nasal cavity, pharynx, larynx, and trachea; branches into the bronchi and bronchioles; and ends at the alveoli, where gas exchange occurs. The respiratory system can be divided into upper and lower airways.
The upper airway includes the nose, sinuses, and pharynx. The nose is the primary sense organ for smell (olfaction). The nasal turbinates initially humidify and warm inhaled air and filter particulate matter.
The lower airway includes 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 consist of membranes that are only one cell layer thick. Via these membranes, oxygen moves into the capillaries, and carbon dioxide moves into the alveoli from the blood in the capillaries. Failure or major dysfunction of gas transfer as a result of disease leads to respiratory distress or failure.
Additional functions of the respiratory system include the following:
maintaining acid-base balance in the body
acting as a blood reservoir
filtering and probably destroying emboli
metabolizing some bioactive substances (eg, serotonin, prostaglandins, corticosteroids, and leukotrienes)
activating some substances (eg, angiotensin)
In the respiratory process, 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. Smaller particles that are not filtered on inhalation can get deposited in the alveoli, where they are phagocytized by macrophages.
Defense against invasion by microorganisms and other foreign particles is provided by the mucociliary "blanket" and by cellular and humoral immunity. These factors determine species and individual susceptibility to disease and can be manipulated through various management techniques, vaccines, antimicrobials, and other agents, such as interferons and lymphokines.
Mechanical defenses against invasion by microorganisms and other foreign particles include the tortuosity of nasal passages; the 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 the following:
interferon for antiviral defense
complement for lysis of invaders
surfactant lining of the alveoli for prevention of alveoli collapse and to facilitate macrophage function
fibronectin for modulating bacterial attachment
antibodies
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 fenestration
relationship of pulmonary arteries to bronchial arteries and bronchioles
presence of vascular shunts
mast cell distribution
pleural blood supply
Each variation in anatomical structure implies variation in function, which can influence the pathogenesis of respiratory disease in a particular species.
The three main groups of species with similar subgross anatomy of the lung are the following:
ruminants (cattle, sheep) and pigs
dogs, cats, monkeys, rats, rabbits, and guinea pigs
horses and humans
Marked physiological variations also exist between different species. For example, cattle are prone to retrograde drainage from the pharynx, are susceptible to pulmonary hypertension and decreased 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. Such anatomical and physiological 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 a lack of sufficient oxygen in the body (PaO2 ≤ 60 mm Hg) to maintain normal metabolic functions. A patient with hypoxia shows clinical signs of respiratory distress. Hypoxia can result from one or more of the following:
decreased oxygen-carrying capacity of the blood (anemic hypoxia; caused by a decreased number of RBCs)
hypoperfusion (hypoperfusion hypoxia; caused by decreased cardiac output)
hypoxic hypoxia (caused by anatomical shunt, physiological 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)
The body has four major centers of ventilatory control:
respiratory control centers in the brain: pons (fine motor control of breath), medulla (timing of breaths and rhythm of breathing), and cortex (control of respiration)
central chemoreceptors in the medulla
peripheral chemoreceptors in carotid and aortic bodies
pulmonary mechanoreceptors/sensory nerves
If cerebral hypoxia develops, respiratory function can decrease even further as a result of neuronal activity depression. Erythropoiesis is also stimulated with chronic hypoxia; however, the degree of polycythemia is species dependent. In addition, multiorgan dysfunction can result.
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