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Unit V Promoting Health in Patients With Oxygenation Disorders
INTRODUCTION All body functions require a constant supply of oxygen to support their many metabolic activities. As a consequence of supplying continual oxygen, the respiratory tract maintains a persistent interface with the external physical environ- ment, resulting in a high degree of direct exposure to micro- organisms. It is for this reason that the respiratory tract is a common site of infection by pathogens. Fortunately, there is a series of complex, comprehensive, and efficient protective and defense mechanisms against harmful pathogens for both the upper and lower respiratory tracts. The Upper Respiratory Tract A wide variety of pathogens can produce infection within the respiratory tract, including bacteria, viruses, and fungi. As environmental air is inhaled, the first line of defense lies within the upper respiratory tract, which consists of the nasal cavities, the pharynx, and the larynx. The nares and nasal cavities are equipped with coarse hairs (vibrissae) and a mucous layer that filter out and trap macroparticulates (large particles). In addition, the nasal cavities are lined with epithelial tissues and blood vessels that filter and warm the inspired air. The surface epithe- lium secretes antimicrobial peptides that exert a bacte- ricidal effect on a variety of pathogens. The endothelial lining of the nasal cavities contains hairlike projections, called cilia, that, through their wavelike motion, transport any particles trapped by the mucous lining through a mech- anism called the mucociliary escalator . This allows the particulate matter to be expelled from the respiratory tract by the protective sneezing and coughing reflexes. After inhaled air moves through the nasal cavities, the anatomy of the upper airway changes direction, causing any remain- ing large particles to come in contact with the back of the throat. The tonsils and adenoids (lymphoid organs) play an integral role in the development of an immune response to pathogens remaining in contact with the mucoid surfaces of the throat through trapping and filtering. Next, the larynx (voice box), which houses the epiglottis, provides mechanical protection of the airways. During breathing, the epiglottis remains open to allow air to pass into the trachea. While food and fluids are swallowed, the epiglottis closes, directing solid material into the esophagus and pre- venting aspiration , or movement of gastric contents into the airways. Additional protective mechanisms of the upper respira- tory tract are provided by the colonization (a collection of a number of bacteria small enough not to cause infection) of resident bacteria and some viruses. Examples of resident bacteria and viruses include but are not limited to forms of Staphylococcus, Streptococcus, spirochetes, mycobacteria, Pseudomonas, Proteus, and Enterococcus . This “normal flora” functions to maintain a healthy respiratory status by com- peting with pathogens for attachment sites in the respiratory mucous lining and producing bactericidal substances that destroy harmful microorganisms.
The Lower Respiratory Tract The trachea marks the beginning of the lower respira- tory tract, along with the bronchi, bronchioles, and alve- oli. A layer of ciliated cells and the mucous-secreting cells within the trachea, bronchi, and bronchioles protect the lower respiratory tract via the mucociliary escalator from smaller-size particles that have avoided the upper airway defenses. Any pathogens that reach this area are trapped in an additional ciliated mucous layer and are driven upward via ciliary motion to the larynx and oropharynx, where they are swallowed and eventually destroyed by digestive enzymes in the stomach. The lower respiratory tract houses no resident flora and is considered a “sterile site.” This is in part due to the efficiency of the upper respiratory tract’s ciliated epithelial lining in eliminating the majority of inhaled pathogens. Should pathogens gain access to the lowest portion of the respiratory tract (the alveoli), alveolar macrophages are the most important means of eliminating microorganisms from this area by phagocytosis. It is of the utmost impor- tance that the alveoli remain free of pathogens because it is here that the vital exchange of gases occurs (Fig. 24.1). It is only when the respiratory epithelium becomes damaged or the sheer numbers of inhaled pathogens exceed the ability
Deoxygenated blood from pulmonary artery
Oxygenated blood to pulmonary vein
Terminal bronchiole
Alveolar- capillary interface
Capillaries
Alveolus
Deoxygenated blood cell
Carbon dioxide
Capillary wall
Oxygen
Alveolus wall
Oxygenated blood cell
FIGURE 24.1 Gas exchange at the alveolar level: gases moving from areas of higher to lower concentration. Oxygen moves into the blood; carbon dioxide moves out of the blood.
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