[MUSIC] Okay, so now, we're going to, we're going to talk about brain lesions. But in order to, to think about how brain lesions happen we have to understand what keeps the brain working. And what keeps the brain working is oxygen, oxygen and then some more oxygen. Even though the brain accounts for about 2% of our body mass, it takes about 25% of our oxygen. It requires about 25% of our oxygen. And when I say it requires, I mean it really does require. If the brain is without oxygen, within minutes neurons start to die. When neurons die, they don't come back. It's not like skin, or hair, or the gastrointestinal tract, where the ner, where the cells can, new cells can be born. And you can heal over a wound in your skin. You cannot remake those neurons. And so, once they die because they haven't had enough oxygen. We're, we're outta luck. And in contrast the heart. Which is, also requires some oxygen can work for on the order of say 30 minutes or so with with low or, or very, or no oxygen. The brain, the neurons are going to start to die within under five minutes. Okay, so one way to understand our requirement for oxygen is to think what happens when we go to high altitudes. When we go to high altitudes the air is the same, but it's under much less pressure. We're much farther away from the earth and so there's less pressure. The air molecules are all much more dispersed. And so, as we take in a breath. For instance, if we were at the top of Mount Everest. We would get in a third as many oxygen molecules at the top of Mount Everest than if we were standing by the ocean. And the consequence of that is that were not it, the brain's not going to work so well. And this is the, this is what's responsible for acute mountain sickness. Is, it's a dramatic example of our requirement, the requirement of the brain to use oxygen. All right. So where do we get our oxygen? We get out oxygen from oxygenated blood. And in everywhere except for the cranium, cerebral blood, I'm sorry. Everywhere except for the cranium blood flow depends on just two variables, the pressure in the arteries and the pressure in the veins. And so, the profusion pressure of say, the liver is a difference between these two values. Or the profusion pressure of the stom, the stomach lining is the difference between these. And the profusion factor, in fact, of the spinal cord is the difference between these two. But in the cranium, we have a different, we have one more factor. And that's called intracranial pressure. We're within this solid, unforgiving skull. And it turns out that that is that pressure is above venous pressure. So this is where intracranial pressure lies. It's about 15 millimeters of mercury. Whereas venous pressure is somewhere between five and ten. Five and ten. So as a consequence the pressure that, that the pressure differential that drives blood flow into the brain. Is the difference between arterial pressure and intercranial pressure. It is not the difference between arterial pressure and venous pressure. So we have less of, less of a, of a pressure differential to work with and we're more vulnerable to either drops in arterial pressure. If we get too, if our blood pressure goes down too much. Let's say this drops to here, well now we only have this much to drive perfusion of the brain. And that's not enough. The other thing that can happen is that intracranial pressure goes up. Because there's a tumor. Because there's a bleed. Because there's swelling for some reason. And again in that case, we don't have enough perfusion pressure to enable us in, to enable in neurons to keep on working. And when this, this differential between arterial pressure minus intracranial pressure is called the cerebral profusion pressure. We can go back over to the tablet, where I've written this out. So cerebral perfusion pressure equals the difference between arterial pressure and intracranial pressure. The problem is going to come if arterial pressure goes down or intracranial pressure goes up. Because if this, if CPP is not kept within an an operating range of say 60 millimeters or so, 60 to 70 millimeters. Then the brain is going to quit. And the, what happens when the brain quits? You faint. And the word for faint in the medical terminology is syncope. So, syncope. If there's not enough oxygen going to brain, a person is going to lose consciousness. They're going to not be there. No communication. And they're going to fall down. They will not be able to continue to oppose gravity and have a posture. So fainting means losing consciousness and falling down. That's called syncope. And [COUGH] and, and so the, the danger of, of for instance getting dehydrated and not having enough arterial pressure. Having your arterial pressure go down is that you might faint. Okay, so in the next segment what we're going to do is we're going to look at the blood supply to the brain. [MUSIC]
