X-Message-Number: 16420
From: "Jan Coetzee" <>
Subject: Our brains store oxygen 
Date: Mon, 4 Jun 2001 10:17:28 -0400

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Our brains store oxygen 


St. Louis - Scientists have discovered that, unlike many other animals, humans 
have a reserve of oxygen in the brain. This buffer allows the brain to adapt to 
arduous situations without demanding a sharp increase in blood flow. 

"Our finding challenges the previously accepted idea that blood flow increases 
occur during tasks such as reading to raise oxygen levels in the brain," says 
study leader Mark A Mintun. "That idea has been long assumed in brain imaging 
studies that attempt to understand how the human brain functions." 

Mintun is a professor of radiology and professor of psychiatry at Washington 
University School of Medicine in St Louis. His group's findings appear in the 
June 5 issue of Proceedings of the National Academy of Sciences and appeared on 
the journal's website on May 29. 

Imaging has become a critical tool for exploring the brain at work. By measuring
changes in blood flow during different tasks, researchers can see which areas 
of the brain spring into action when, for example, individuals read or memorise 
words. Because blood supplies cells with oxygen, they assumed that blood flow 
increases when a particular area of the brain needs more oxygen. The new 
evidence suggests otherwise. 

"I think we're still very safe interpreting increased blood flow as a change in 
brain activity," says Mintun. "But why flow increases now is unclear. 
Understanding that will probably change our view of the human brain and alter 
the way we design studies." 

An extensive network of small blood vessels called capillaries feeds the brain. 
Because every cell is critical to the organ's function, oxygen must diffuse from
the capillaries to every nook. Current models suggest that, even if the brain 
needs only a small amount of extra oxygen, it takes a large increase in blood 
flow to deliver enough to every cell. 

Using positron emission tomography (PET), Mintun and colleagues examined blood 
flow to the brains of nine healthy volunteers. The subjects were asked to focus 
on a white cross on a black background and press a button whenever the cross 
became dim. They performed this task in a normal atmosphere and under oxygen 
levels resembling those on top of Pike's Peak in Colorado (roughly 4 600 

Current theory suggests that blood flow should increase dramatically if someone 
tries to perform this task when oxygen levels are very low. And Mintun's team 
expected that to be the case. Instead, cerebral blood flow failed to relate to 
the amount of oxygen entering the body. 

"The brain appears to have some sort of built-in insurance policy," explains 
Mintun. "Even when partially deprived of oxygen, it can still take care of 

Armed with this new information, the team retested current mathematical models 
of cerebral blood flow. The models assume that, because the brain requires so 
much oxygen, oxygen from blood diffuses into brain tissue, never to be 
reabsorbed into blood. But Mintun and colleagues left that assumption out of 
their equation. 

"We allowed the oxygen model to develop on its own without assuming that oxygen 
flows in only one direction," says Mintun. "That proved to be the critical 
factor. It turns out that a fair amount of oxygen does go back and forth, 
creating a dynamic buffer. So when the brain needs more oxygen, it simply taps 
into this reserve." 

The researchers also found that the human brain has far more capillaries than it
needs. The extra capillaries might serve as a storehouse for delivering surplus
oxygen, they suggest. 

Other scientists have observed that animals such as rats and even primates are 
more sensitive to low oxygen levels than humans. Mintun now is determining 
whether animals' brains have a smaller capillary content than the human brain 
compared to their use of oxygen. Such a finding might make animal models less 
useful for exploring the relationship between blood flow and cognition. 

The team also is looking for alternative explanations for the increased cerebral
blood flow seen during many mental tasks. "We have to take a step back and 
admit that the blood-flow response we see so crisply when areas of the brain 
become active may serve some other purpose," he says. "Also, other mechanisms 
that drive blood flow in the brain may be waiting to be discovered." 


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