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.
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.
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.
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. ...