In addition to being a factor that regulates respiration, CO2 is also a local factor which regulates vasoconstriction in various tissues. For example, high levels of CO2 in the brain induce vasodilation to increase the blood flow so that the CO2 can be removed.
A person with anxiety attacks or who knowingly is hyperventilating, is blowing CO2 out of the lungs. O2 is not affected as the blood gas is still 100% saturated.
When CO2 is blown out of the lungs CO2 is removed from the blood, because the diffusion from the blood into the alveoli will increase if the level in the alveoli decreases under increased exhalation. When CO2 levels in the blood decrease, pH increases. The consequence of this may be that CO2 as a stimulus for vasodilation disappears and the blood vessels vasoconstrict. Thus the blood flow to the brain decreases. In addition, with decreasing CO2 in the blood the strongest stimulus to maintain respiration also disappears. Thus, the brain does not detect the need to increase the respiratory rate. This can lead to dizziness and fainting.
Therefore, a person who is involuntarily hyperventilating should be encouraged to breathe into a paper bag, because CO2 is then returning to the lungs instead of being blown out. Thus vasodilation in the brain is sustained and so is the stimulus for respiration. You therefore reduces the risk of fainting.
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