Hyperventilation increases CO2 loss which decreases arterial PCO2 and increases pH (respiratory alkalosis). Since cerebral blood flow is largely regulated by plasma pCO2, cerebral vascular resistance increases, decreasing blood flow. This relative ischemia decreases oxygen delivery to the brain, the resulting symptoms being light-headedness and syncope.
Because hemoglobin can only carry so much oxygen when fully saturated (Figure 1.09), and because the arterial hemoglobin is normally 96-98% saturated, raising alveolar pO2 through hyperventilation fails to further elevate arterial oxygen content. Thus, hyperventilation does just the opposite of that which is stated.
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