As the affected individual experiences difficulty filling the lungs with air,(s)he may hyperventilate, because CO2 diffuses about 20 time faster than O2, blood CO2 (PaCO2) will decrease as the CO2 is rapidly expired out of the lungs. If the asthma attack is severe,with significant airways narrowing, CO2 retention can occur, raising the PaCO2 and lowering the blood pH. Blood oxygen (PaO2) will often decrease, because hyperventilation does not allow enough time for oxygen to diffuse from the lung alveoli into the blood. With lowering of PaCO2, the blood pH will rise, because CO2 is a potential source of acid.