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Pathophysiologic manifestations | |
Atelectasis | |
Bronchiectasis | |
Cyanosis | |
Hypoxemia | |
Disorders | |
Adult respiratory distress syndrome | |
Asbestosis | |
Asthma | |
Chronic bronchitis | |
Chronic obstructive pulmonary disease | |
Cor pulmonale | |
Emphysema | |
Idiopathic respiratory distress syndrome of the newborn | |
Pneumothorax | |
Pulmonary edema | |
Pulmonary hypertension | |
Respiratory failure | |
Sudden infant death syndrome |
The respiratory system's major function is gas exchange, in which air enters the body on inhalation (inspiration), travels throughout the respiratory passages, exchanging oxygen for carbon dioxide at the tissue level, and carbon dioxide is expelled on exhalation (expiration).
The upper airway ― composed of the nose, mouth, pharynx, and larynx ― allows airflow into the lungs. This area is responsible for warming, humidifying, and filtering the air, thereby protecting the lower airway from foreign matter.
The lower airway consists of the trachea, mainstem bronchi, secondary bronchi, bronchioles, and terminal bronchioles. These structures are anatomic dead spaces and function only as passageways for moving air into and out of the lung. Distal to each terminal bronchiole is the acinus, which consists of respiratory bronchioles, alveolar ducts, and alveolar sacs. The bronchioles and ducts function as conduits, and the alveoli are the chief units of gas exchange. These final subdivisions of the bronchial tree make up the lobules ― the functional units of the lungs. (SeeStructure of the lobule.)
In addition to warming, humidifying, and filtering inspired air, the lower airway protects the lungs with several defense mechanisms. Clearance mechanismsinclude the cough reflex and mucociliary system. The mucociliary system produces mucus, trapping foreign particles. Foreign matter is then swept to the upper airway for expectoration by specialized fingerlike projections called cilia. A breakdown in the epithelium of the lungs or the mucociliary system can cause the defense mechanisms to malfunction, and pollutants and irritants then enter and inflame the lungs. The lower airway also providesimmunologic protectionand initiatespulmonary injury responses.
The external component of respiration (ventilation or breathing) delivers inspired air to the lower respiratory tract and alveoli. Contraction and relaxation of the respiratory muscles moves air into and out of the lungs.
Normal expiration is passive; the inspiratory muscles cease to contract, and the elastic recoil of the lungs and the chest wall causes them to contract again. These actions raise the pressure within the lungs to above atmospheric pressure, moving air from the lungs to the atmosphere.
An adult lung contains an estimated 300 million alveoli; each alveolus is supplied by many capillaries. To reach the capillary lumen, oxygen must cross the alveolar capillary membrane.
The pulmonary alveoli promote gas exchange by diffusion ― the passage of gas molecules through respiratory membranes. In diffusion, oxygen passes to the blood, and carbon dioxide, a byproduct of cellular metabolism, passes out of the blood and is channeled away.
Circulating blood delivers oxygen to the cells of the body for metabolism and tr