There are two points, one very important, not yet noted:
(1) Not so important is the fact (I think) that there is less CO2 than O2 dissolved in the blood so a dissolved concentration change of only 0.1% is a big relative change and easier to sense. I think also that a 0.1% increase in the CO2 also make the blood more acidic than even a 1% change in the O2 concentration can. Again easier to sense as many complex proteins fold differently with pH changes.
(2) The really important reason is that the brain has different regions doing different tasks. Thus, when part of the brain is working on a math problem its needs to have increased O2 (and glucose etc.) supply coming to it than the part that recognizes faces does, etc. That means it needs more blood flow to it. As the heart has only one output of pressure vs. time curve that means somehow that active part of the brain needs to expand the diameter of the local blood vessels. Evolution has an optimum system for doing this. In general it is hard to improve on "mother nature's designs."
As I worked a lot with JHU doctors, when two US navy surgeons came to APL, they were sent to talk to me. They wanted to know if APL could help develop an artificial blood. (APL has basically lived for ~70 years by solving many problems for the US Navy.) Fortunately, I was too busy already to help as my ideas about it were all exactly wrong. I thought it would be nice if the artificial blood could carry more O2, and had less viscosity. This is exactly what a company, now nearly bankrupted, thought too when they developed their artificial blood with these characteristics. (In the accident trauma trials with large blood loss, it was worse than simple volume replacement with saline solution!) The lower viscosity means it went thru the capillaries too fast to transfer the O2 to the cells. In the larger vessels, most of the molecules did not even contact the walls. The higher O2 per molecule was also wrong, but I forget why.
Again: It usually dam near impossible to improve on what has evolved. There has been, however, an improvement in stored natural blood. - The part of the hemoglobin molecule that makes for the various bloods of different types can be stripped off so only one modified natural blood serves for all.
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In answer to thread's question: Prolonged hyperventulaltion can so lower the concentration of CO2 that the capillaries in the brain become abnormally small. I think you can lose consciousness as the blood flow in the entire brain is too reduced. Also note that that AFAIK, only the brain's smallest diameter blood vessels change diameter with the CO2 concentration. I.e. they are unique capillaries.
