|MadSci Network: Medicine|
Thanks for your question about the brain. Personally, it's my favorite organ of the body. It does so many things, and we don't even understand how it all works.
What we DO know about the brain is that it depends on the heart to get energy. You see, the brain runs on the energy from a sugar called glucose. We eat glucose in all kinds of foods, and the glucose gets from our stomachs to our brains through the blood. Without the heart to pump blood up to the brain, there's not enough energy for the brain to keep working very long.
The brain also needs oxygen, and oxygen comes into our bodies through our lungs. The lungs transfer the oxygen to the blood, and the heart pumps the blood to the brain.
When the heart stops pumping, the brain uses up the glucose and oxygen that it has on hand, but once that's all gone it stops working.
You could think of a situation where the heart is a power plant and the brain is your house. If the power goes out, you can use rechargeable flashlight batteries to light the house for awhile, but then they will run down, and then it's "lights out".
The energy reserves in the brain can last for about four minutes once the heart stops. After that, the brain runs down, too.
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