The Humoral Regulation of Breathing: A concept based on the physicochemical composition of mixed venous and arterial blood is presented

Respiratory compensation and blood pH regulation during variable intensity exercise in trained versus untrained subjects

To determine whether endurance-trained cyclists (T; n = 10) have a superior blood-respiratory buffering for metabolic acidosis relative to untrained subjects (UT; n = 10) during variable intensity exercise (VAR). On three occasions, T and UT pedaled for 24 min alternating high- and low-intensities as percentage of their second ventilatory threshold (VT2): VAR(LOW) 87.5-37.5% VT2, VAR(MODERATE) 125-25% VT2, and VAR(HIGH) 162.5-12.5% VT2 to complete the same amount of work. Before and just after each VAR trial, maximal cycling power (P(MAX)) was assessed. For each trial, the respiratory compensation for exercise acidosis (ventilatory equivalent for CO2) and the final blood pH, lactate and bicarbonate concentrations were similar for T and UT subjects. However, after VAR(HIGH), UT reduced P(MAX) (-14 +/- 1%; P < 0.05) while T did not. Our data suggest that endurance training confers adaptations to withstand the low pH provoked by VAR without losing cycling power, although this response is not due to differences in blood-respiratory buffering.

Evolutionary trends in airway CO2/H+ chemoreception

In many species of air-breathing vertebrates CO2-sensitive airway receptors play an important role in ventilatory control. In ectotherms, olfactory receptors often inhibit breathing and prolong breath holding when environmental CO2 levels are high. CO2/H+ sensitive pulmonary receptors (intra pulmonary chemoreceptors (IPC) and pulmonary stretch receptors (PSR)) regulate breathing patterns in all vertebrates in a manner that reduces dead space ventilation and enhances the efficiency of CO2 excretion under conditions of environmental hypercarbia, and/or reduces CO2 loss from hyperventilation. The greater CO2 sensitivity of IPC may allow them to also serve as a venous CO2 receptor (at least transiently when levels of metabolically produced CO2 begin to rise), prevent alkalosis during hyperpnea/polypnea, and may have contributed to the evolution of the extremely thin air/blood barrier and increased diffusion capacity associated with the rigid avian lung. The presence of all three receptor groups with different degrees of CO2 sensitivity in most reptiles, however, gives rise to what appear to be anomalous responses to environmental CO2.

Contribution of acid-base changes to control of breathing during exercise

The mechanisms mediating the exercise hyperpnea remain controversial; there is no unequivocal evidence that any of numerous proposed mechanisms mediates the hyperpnea. However, a great deal has been learned including the potential role of changes in PCO2, [H+], strong ion differences (SID), weak acids, or any other acid-base component. The contribution of acid-base changes to the hyperpnea during exercise is likely through known or postulated chemoreceptors. Two of these, pulmonary and intracranial chemoreceptors, do not appear critical for the ventilatory adjustments to meet the metabolic demands of exercise. A third, the carotid chemoreceptors, appear to fine-tune alveolar ventilation during exercise to minimize disruptions in arterial blood gases. The role of the fourth chemoreceptors, those within skeletal muscles, is least clear. However, there is evidence that they do contribute to the hyperpnea, and it is quite clear that a muscle chemoreflex contributes to the exercise muscle pressor reflex; thus the contribution of these chemoreceptors to the exercise hyperpnea requires additional study.

Aortico-Pulmonary Glomus Tissue Distribution and Blood Supply in the Adult Cat

A microscopic, three-dimensional study of the aortico-pulmonary glomus tissue in the adult cat indicates a random, continuous aggregation of glomus tissue extending from the left coronary artery to the ligamentum arteriosum. It was not possible to demonstrate unequivocally specific glomera. Serial section and the injection of the pulmonary artery with India ink failed to show a vessel of supply to glomeric tissue.