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The respiratory system is composed of many different cell and tissue types, including airways, lung parenchyma, respiratory muscles, nerves, and blood vessels. These structures must operate in an integrated way to serve the main functions of the system, that is, to deliver oxygen to tissues and remove carbon dioxide. Herein we will examine the component parts of the respiratory system, as well as explore techniques for assessing the function of these components in health and disease.

Structures/Physiology: Respiratory Control Centers

The coordination of breathing, which occurs 24 hours per day, requires a careful integration of automatic respiratory centers with multiple inputs that allow for changes in breathing patterns necessary to accommodate speech, swallowing, and other activities. The automatic control centers are located primarily in the medulla of the brainstem, although other brainstem centers in the pons contribute to finer tuning of inspiration and expiration times. These automatic control centers allow for very fine control over ventilation, maintaining Pco2 and pH in a very narrow range. There are a variety of mechanoreceptors in the upper airway that can be stimulated by airflow or swallowing, as well as receptors in the lung parenchyma that can be stimulated by rapid stretch or deflation. These receptors can modulate an increase or decrease in ventilation.

Testing: P0.1, VCo2 Response, Polysomnography

Assessing the function of the respiratory control centers is not straightforward. The measurement of airway opening pressure very early (100 ms) in inspiration (P0.1 or P100) has been used as an assessment of respiratory drive; depressed drive caused, for example, by narcotics, lowers this pressure. Polysomnography measures a wide variety of physiologic variables during sleep, including nasal and oral airflow, chest and abdominal wall motion, oxygen saturation (SpO2), end-tidal (exhaled) pCO2, and electrocardiogram. Sleep state is monitored by electroencephalogram (EEG), electrooculogram, and electromyogram. Applications of polysomnography will be discussed in more full detail elsewhere (Chakravorty), but abnormalities of respiratory drive (especially central apneas and periodic breathing) can be detected with this testing.

Structures/ Physiology Chest Wall, Respiratory Muscles

The respiratory muscles, notably the diaphragm and intercostal muscles, are the “pump” of this system and are responsible for generation of negative intrathoracic pressure during inspiration. Respiratory muscle weakness can manifest in primary neuromuscular diseases (ie, muscular dystrophy, spinal muscular atrophy, myasthenia gravis) or secondarily due to malnutrition or hyperinflation (which alters the length–tension ratio of the diaphragm).

Most chronic respiratory failure in children results from pump, not lung, failure. The pump is responsible for maintenance of ventilation by their force-generating capacity to overcome the elastic and resistive loads of the lungs and chest wall. Respiratory “success” depends not on pump function alone, but on a balance between pump function and the magnitude of the loads upon which the pump is acting. ...

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