Our brain loves to eat. It weighs only about 2% of our body weight, but it requires as much as 20% of our cardiac output. All that blood has to bring oxygen and glucose to the brain.
Another problem is that our brain doesn't know how to store nutrients. It hovers out there somewhere in its fantasies and thoughts, completely unconcerned about provisions. Therefore, the brain is totally dependent on blood flow.
Fortunately, the brain's arteries are so arranged that they reflexively regulate blood flow on autopilot. These vessels literally have muscles that can make the lumen of the artery narrower or wider. Blood flow to the brain is directly related to the diameter of these arteries
If the brain is under pressure from high blood pressure, the arteries inside the head narrow. That way less blood gets to the brain and it won't swell. It's not good for the brain to swell. It's trapped inside the skull. The swelling will cause the brain to bump itself against the skull bones.
If the blood pressure drops, the arteries dilate, and a bunch of sluggish erythrocytes will waddle reluctantly through them, bogging down in puddles of glucose and other nutrients. If the arteries were narrow, this whole procession would get stuck halfway through, and the brain would receive neither glucose nor oxygen. So when the pressure is low, the arteries widen as much as possible and let in all comers, in the hope that someone will bring the brain some goodies.
This regulation is effective in the average blood pressure range of 60 to 150 millimeters of mercury. Remember the story about average blood pressure? You have to subtract the lower pressure from the upper pressure, divide by three, and add the lower pressure.
In this range, the arteries of the brain provide the oxygen and nutrient needs of the brain. But if you go out of the range, the brain is damaged.
At low pressure, the red blood cells no longer even weave, but barely crawl through the arteries. Part of the brain can starve to death in this case.
If the pressure is too high, the liquid part of the blood will flow through the artery wall and soak the brain. It will swell and spread out between the bones of the skull. Also a bad thing.
And that, brothers, we haven't yet been distracted by all sorts of blood clots flying into the brain from the heart, or atherosclerotic plaques in the arterial wall.
In short
If your average blood pressure will be in the range of 60 to 150 millimeters of mercury, the blood supply to the brain will support itself. There's nothing for you to do there. Everything runs on autopilot.
And if your average blood pressure is out of that range, you'll already be rushing around in an ambulance, and nothing will depend on you, either. Convenient, isn't it?
Exceptions to the rules
There are, of course, exceptions. First, this cherished range only averages between 60 and 150 millimeters of the mercury column. Everyone is different, and there may be variations.
Second, if you stubbornly ignore the increase in blood pressure, the arteries of the brain will get used to this ugliness and shift the cherished range somewhere higher.
After that you will not achieve normal blood flow regulation. It will turn out that high blood pressure slowly flattens your brain, and when you try to lower it, you will move out of the cherished range, and your brain will suffer from lack of blood.
So the conclusion is: control your blood pressure from the beginning. If then at your old age you want to lower it to live longer, your blood vessels will not always be ready to meet you. They will simply refuse to regulate blood supply to the brain and leave you to stroke.
Do you think your blood pressure of 190/100 would be in the range where your brain arteries can still handle it?
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