Obviously, the body needs oxygen, but carbon dioxide (CO2) levels are much more potent. In hypoxia, the body is not getting proper supply. The brain needs lots of O2. But if a pt is hypoxic, meaning they are not getting proper respiration, the CO2 levels will most likely rise. The body reacts a lot more to CO2 than O2. How fast we breath is dictated by our CO2 levels. We have specific receptors sensitive to CO2. If it's too high, we breath faster to blow it off. If it is too low, we breath slower to retain it. Running causes our body to use more O2 but it also produces more CO2 as a by-product, so breathing faster keeps homeostasis. But the body is amazing in how it compensates for different diseases. Some people can live with high CO2 levels, like COPD. CO2 is acidic. Acidosis will cause higher resp b/c we are trying to blow it off. In diabetic ketoacidosis (DKA), the body become so acidic that they have constant tachypnea and long sighs to blow off CO2, and they usually end up with very low CO2 levels. The kidneys try to help by secreting bicarbonate (a strong buffer). But when someone has chronic CO2 retaining, the body is able to compensate and they will have normal blood PH, even with high CO2 levels. If someone becomes hypoxic, less O2 goes to the brain. This causes delerium but high CO2 levels will also cause restlessness. Taking in O2 and blowing off CO2 fixes it.
In everyday life, if you hyperventilate, you will become dizzy b/c Co2 levels will decrease. This is b/c CO2 is also a very potent vasodilator. Baroreceptors interpret CO2 and if you blow off CO2, vasoconstriction ensues. CO2 levels can affect BP related to certain disease processes. If a pt has an increased intracranial pressures ( elevated ICP), hyperventilating them will shortly lower the ICP (b/c of vasodilation).
Usually, doctors don't really care about the O2 level as long as the saturation is fine. They are more concerned about CO2. As said before, the CO2 levels dictate how fast we breath. Any causes of the CO2 being too high or too low can be fatal. However, giving someone lots of O2 can be bad. This is called oxygen intoxication. It can cause seizures, retinal issues, and can develop free radicals. Scuba divers can suffer from oxygen toxicity. But if you give a COPD pt too much oxygen, it can actually stop the pt from breathing b/c it shut down the breathing drive. Any one of these factors can cause someone to become restless, confused, or comatose.