Continuous respiratory CO2 gas exchange is simulated theoretically by means of a suitably constructed dilution equation of the form (See PDF) where C is instantaneous concentration, Q quantity flow, and V volume flow. When such an equation is constrained in its numerical solutions to yield inflow-outflow equality for CO2 in the steady state, solutions for CO2 as a time series are found to differ according to whether the site of entry of CO2 is entirely by venous blood or is also by inspired CO2. In the case of entry via the venous blood, increasing CO2 is eliminated while keeping mean CO2 constant with increasing fluctuations about this mean. In the case of entry via the lung, increasing CO2 is eliminated by raising the mean and at the same time oscillations diminish. Submitted on August 13, 1959
When rats are infused intravenously continuously with blood enriched in an extracorporeal system with 100% CO2, they are found to excrete the infused CO2 quantitatively. The excretion is accomplished by increasing ventilation in proportion to the infusion rate. Neither mean arterial pH nor Pco2 shows a statistically significant change. This constancy of composition is observed at CO2 infusion rates ranging from zero to an amount equal to six times the resting metabolic CO2 production. Similarity of this hyperpnea to the chief characteristics of the hyperpnea of exercise is notable. Increased oxygen uptake during this procedure is about 3.5 ml/l. of ventilation, a reasonable value for the oxygen cost of ventilation. Since, during this process the animal is quiescent and shows no other metabolic changes it is suggested that the effective signal for the regulation of arterial Pco2 is CO2 itself. The ability of the experimental animal to respond successfully to high rates of loading seems to be limited by failure to make appropriate circulatory adjustments. Removing CO2 by equilibrating blood with 100% o2 in the extracorporeal circuit decreases ventilation, but readily leads to an uncontrolled alkalosis. Submitted on April 13, 1960
The validity of current theories concerning the function of the area postrema ultimately rests on the nature of its microcirculation. Therefore a detailed study of the vasculature of the postremal region was undertaken in the rat.The findings indicate a singular arteriolar supply from branches of the posterior inferior cerebellar arteries. These vessels are caudal, intracranial branches of the vertebral arteries. Networks of enlarged capillaries are prominent features of sections taken through the area postrema. These enlarged capillary channels are re-entrant. At the borders of the structure they are joined by short interconnecting vessels to capillaries of smaller caliber typical of the medullary tissue. The bed is considered to be a portal system because two distinctive, serially connected capillary beds are interposed between artery and vein. No arteriovenous shunts or thoroughfare channels are observed in the rat, contrary to reports of this type of vessel in other species.This microcirculatory pattern would seem to be adequate to current theories of postremal function, which attribute neurosecretory or chemosensory functions to this region.
An estimate of the amount of information transmitted by way of the arterial blood stream in animals is made. Many assumptions are necessary to pose the problem in analyzable form. Taking carbon dioxide as a representative substance, a distribution of maximum entropy is developed. Three points emerge: (1) that homeostatic stability can be related to chemoreceptor sensitivity if both are given statistical interpretations consistent with concepts of information transmission, (2) that the heart acts as a filter which has considerable smoothing effect which depends in a specific fashion upon the cardiac residual volumes, and (3) a numerical estimate of channel capacity is made. The estimate is undoubtedly high. Assuming values typical of man, the calculated channel capacity for CO(2) is 3.5 to 4.7 bits per second. Since some sixty substances of communication importance occupy the blood stream simultaneously, the blood stream has a total capacity near 250 bits per second if every chemical modality has the same properties as CO(2).
Evidence is presented for the localization of central chemoreceptors. Respiratory minute volume was measured in rats after surgical procedures that removed known or presumed chemoreceptor areas in various combinations. In each case the control ventilation was measured during inhalation of 100% O2, and the test response was the steady state ventilation on breathing O2 with either 5% or 10% CO2 added. To test completeness of carotid chemoreceptor denervation, 6–8% O2 in N2 was employed. Our data show that ventilatory response to inhaled CO2 is abolished if three lesions are produced simultaneously. The lesions must destroy the carotid chemoreceptor, a region surrounding and including the area postrema in the medulla, and diencephalic tissue rostral to a line passing through the posterior commissure dorsally and just behind the optic chiasm ventrally. If any of the three remains intact the ventilatory response is indistinguishable from that of the anesthetized, unoperated control animal. Minute volume is not abnormally low in the CO2-insensitive animal, and superficially the respiratory pattern is of normal rate and depth. A speculative hypothesis regarding the mechanism of function of the non-neural tissue of the area postrema is proposed.
The organization of the central nervous system mechanisms involved in respiration in the albino rat as shown by electrical stimulation and surgical transection is similar to that in other species previously studied. Areas of the medulla producing inspiratory or expiratory movements on stimulation are distinct and separated. The inspiratory center occupies about 1.1 cu mm on each side and overlies the rostral one-third of the inferior olive. The expiratory center is somewhat more diffuse and lies dorsal and caudal to the inspiratory center. Serial transections produce the usual phenomena which lead to the identification of the pneumotaxic center and apneustic center. In the medullary rat ataxic breathing shows a dissociation of the inspiratory-expiratory sequence in breathing.
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