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In healthy individuals, respiratory and circulatory functions
are linked to tissue metabolic activity by a responsive regulatory
system that translates biochemical and neural signals from the tissues
into adjustments in cardiac output, vascular tone, and minute ventilation. The
purpose of this system is to assure that all the cells in the organism
receive a supply of O2 commensurate with their metabolic
needs without accumulating excessive amounts of CO2. The
system relies both on local circulatory reflexes, which alter the
caliber of the supplying blood vessels in accordance with tissue
metabolic activity, and on central circulatory and respiratory reflexes,
which adjust the pumping function of the heart and the intensity
of the respiratory effort in response to changes in the concentration
of the respiratory gases in the blood.1
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Chemoreceptor reflexes play a singularly important role in the
genesis of the manifestations of respiratory disease (Fig.
102-1). Alterations in the blood’s PO2, PCO2,
and pH are sensed by specialized chemo-sensitive cells located in
the carotid bodies (peripheral chemoreceptors, PO2) and
reticular nuclei of the medulla oblongata (central chemoreceptors,
PCO2 and pH). These cells relay the information to a medullary
neuronal network of premotoneurons, which also receives inputs from
mechanical and chemical sensors distributed throughout the lungs,
airways, and chest wall. Chemical and mechanical inputs are integrated
to define the amount of ventilation needed to sustain adequate gas
exchange and the manner in which the respiratory muscles are applied
to achieve this ventilation most efficiently. The physiologic basis
of respiration is reviewed in more detail in Chapter 503.
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A variety of developmental factors render the infant, and especially
the newborn at increased risk for difficulties with respiration.
These include immaturity of the neural ...