Pumonary Acid Excretion
The main physiologic stimuli to respiration are an elevation in the PCO2 and a reduction in the PO2 (hypoxemia). The CO2 stimulus to ventilation occurs in the chemosensitive areas in the respiratory center in the brain stem, which responds to CO2 induced changes in the cerebral interstitial pH. This effect is important in removing the 15 mol of CO2 produced daily from metabolism of fats and carbohydrates, via alveolar ventilation. In acid base disorders, the initial rise or fall in alveolar ventilation is mediated primarily by the peripheral chemoreceptors in the carotid or aortic bodies, which immediately sense the change in pH. Changes in PCO2 are sensed via central chemo-receptors as CSF ph is altered. In general, PCO2 is regulated by alveolar ventilation. Hyperventilation (increase in alveolar ventilation) enhances CO2 excretion and lowers the PCO2 while hypoventilation (reduction in ventilation) reduces CO2 excretion and raises the PCO2.
The effects of acid base balance on alveolar ventilation and vice versa will be discussed in more detail when specific acid base disorders are described.
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