• A number of factors play a role in controlling alveolar ventilation. The central areas of inspiratory and expiratory control lie in the medulla, and primarily respond to increases in hydrogen ion concentration in the cerebrospinal fluid. Although hydrogen ion concentration is the most important stimulus to the chemosensitive centers in the medulla, these ions cross from blood through the blood-brain barrier to cerebrospinal fluid with difficulty. In contrast, CO2 readily crosses the blood-brain barrier to react with water to form carbonic acid in the CSF, which dissociates to provide the required hydrogen ion necessary for stimulation. Peripheral chemoreceptors transmit signals via cranial nerves ten (to the aortic bodies) and nine (to the carotid bodies) to the inspiratory area of the medulla and pons. These signals help control breathing frequency and lung inflation. The peripheral receptors respond to drops in arterial oxygen pressure.
  • Generally speaking, carbon dioxide pressure plays a much greater influence on ventilation than oxygen pressure. To exemplify the role that the PaCO2 plays in controlling alveolar ventilation, a 50% increase in arterial PaCO2 produces a tenfold increase in alveolar ventilation, while a 40mmHg decrease in arterial PaO yields only a 1.5 fold increase in alveolar ventilation. The high blood flow through the richly vascularized peripheral chemoreceptors allows the needs of the receptors to be almost entirely met by dissolved oxygen, and excess oxygen bound as oxyhemoglobin is not used by the receptors. As a result, the PaO2 determines the level of peripheral receptor stimulation rather than the arterial oxygen saturation (SaO2).