Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: Igor on 07/12/2007 10:59:29
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In a pound-for-pound comparison does the brain use more blood than the average for the whole body,
i.e. do nerves have above average needs for oxygen and glucose ?.
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The brain is supposed to be the major consumer of glucose in the body (partly because it cannot use fats as an energy source, but it is also very energy hungry), but that does not mean it will necessarily have a greater blood supply.
The blood neads to pass through the rest of the body also, for very good reasons. There is absolutely no point in lots of blood going to the brain if it has not passed through the lungs first (to pick up the oxygen that needs to be transported elsewhere), and ofcourse, the blood goes nowhere unless it passes through the heart in order to pump it to wherever it is going, and then there in the required cleansing of the blood by the liver and kidneys. Thus, simply sending lots of blood to the brain, unless it also goes to lots of other places as well, will do no good for the brain at all.
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The brain is supposed to be the major consumer of glucose in the body (partly because it cannot use fats as an energy source, but it is also very energy hungry), but that does not mean it will necessarily have a greater blood supply.
I had heard that the brain has 20% of the blood flow from the heart, but the brain is only 2% of body weight.
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The brain is supposed to be the major consumer of glucose in the body (partly because it cannot use fats as an energy source, but it is also very energy hungry), but that does not mean it will necessarily have a greater blood supply.
I had heard that the brain has 20% of the blood flow from the heart, but the brain is only 2% of body weight.
I had expected the brain size to be a higher percentage of body mass, but checking elsewhere, it seems to also suggest about 2% of the body mass is brain mass (probably very much larger for young children).
But the point I was trying to make is, what do you mean by 20% of the blood flow from the heart?
Firstly, the heart itself mist have 100% of the blood flow, so if one is just looking at amounts of flow, it is not a net sum 100% - you add all the flows to various parts of the body together, and you will get considerably more than 100%, because the blood has to go through various different parts of the body in series.
What is true is that the heart has to use more effort driving blood to the brain than to the feet, simply because human beings stand upright (most of the time), and so the heart has to work against gravity when feeding blood to the head (this situation is considerably worse for a giraffe).
So what exactly does the 20% figure mean?
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Here is a site which uses the 20% figure, twice.
Blood Supply
% brain utilization of total resting oxygen = 20%
% blood flow from heart to brain = 15-20% (Kandel et al., 2000)
http://faculty.washington.edu/chudler/facts.html
The systemic circulation of blood consists of parallel not "series" circuits: e.g. arteries from the aorta branch to head, arms, viscera, legs, (there are no branches in a "series" circuit).
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Here is a site which uses the 20% figure, twice.
Blood Supply
% brain utilization of total resting oxygen = 20%
% blood flow from heart to brain = 15-20% (Kandel et al., 2000)
http://faculty.washington.edu/chudler/facts.html
The systemic circulation of blood consists of parallel not "series" circuits: e.g. arteries from the aorta branch to head, arms, viscera, legs, (there are no branches in a "series" circuit).
OK, I was somewhat simplistic, and we are both right to a point.
The pulmonary circuit is clearly separate from the rest of the system, and so is in effect a parallel system to the rest of the circulation.
Similarly, about 75% of the blood to the liver is venous blood, and so is in series with other organs (particularly the intestines).
http://biology.about.com/library/organs/bldigestliver.htm
# Roughly 75% of the blood entering the liver is venous blood from the portal vein. Importantly, all of the venous blood returning from the small intestine, stomach, pancreas and spleen converges into the portal vein. One consequence of this is that the liver gets "first pickings" of everything absorbed in the small intestine, which, as we will see, is where virtually all nutrients are absorbed.
# The remaining 25% of the blood supply to the liver is arterial blood from the hepatic artery.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fz.about.com%2Fd%2Fbiology%2F1%2F0%2FD%2F3%2Fbloodsys.gif&hash=a32f10c88844a70002b046c8d0ee2fb7)