A. Physiologic Factors Concerned in the Regulation of Respiration
Physiologists now agree that there is a medullary respiratory center which has intrinsic rhythmicity. Nevertheless this center can be influenced profoundly by many chemical and nervous factors. One of the most important of these is carbon dioxide. Under ordinary conditions the medullary center is exquisitely sensitive to changes in carbon dioxide pressure. When the respiratory center is depressed (by deep anesthesia, large doses of morphine or barbiturates, trauma, cerebral edema, increased intra-cranial pressure, severe anoxia or by high concentrations of carbon dioxide itself) it is no longer responsive to carbon dioxide though it may still permit reflex activity and continuation of respiration.
Anoxemia may also stimulate respiration; this occurs through reflexes originating in chemoreceptors of the carotid and aortic bodies. It appears certain that these chemoreceptors are functioning in the normal full-term newborn though they may not be functioning or functioning properly in prematures. When these chemoreceptors are in operation, anoxia will stimulate respiration and oxygen therapy will abolish such hyperpnea. When the chemoreceptors are not in action, one would expect no reflex effects from either oxygen or anoxia; oxygen therapy, however, might relieve cerebral ischemia and permit respiration to improve.
B. Physiologic Methods for Evaluating Respiratory and Pulmonary Function
The function of the lungs is primarily to oxygenate the venous blood and to remove excess carbon dioxide from it. To accomplish this, there must be normal respiratory volumes, normal lung volumes and aerating surface, even distribution of the inspired gas to the alveoli, unimpaired diffusion across the alveolar capillary membrane, and uniform distribution of pulmonary capillary blood flow to the functioning alveoli.
