Smoking-related lung disease can lead to profound changes in the way the brain works, researchers have found.
Brain scans revealed changes
It appears that the brain changes the way it functions in response to a poor oxygen supply.
But it also has the ability to switch back again once oxygen levels rise in the blood.
It has long been known that smoking causes emphysema and chronic obstructive pulmonary disease (COPD), a common disorder in which lung damage over a long period of time impairs the flow of air in and out of the lungs.
This causes breathlessness and reduces the capacity to replace carbon dioxide in the blood with new supplies of oxygen.
Doctors at the Hammersmith Hospitals NHS Trust and Imperial College London have found that this imbalance can lead to biochemical changes in the brain during the later stages of lung disease.
The team used a technique called magnetic resonance spectroscopy (MRS) to examine brain function in eight patients with serious lung disease.
The researchers expected to see a build-up of lactic acid and an acidic environment in the brain, as seen in other organs suffering from oxygen deprivation.
Instead the team found an alkaline pH in the cells, suggesting that the brain had compensated over time by adapting its functions to redress the chemical changes brought about by the lack of oxygen.
Researchers then administered oxygen to the patients and found that within about half an hour the chemical imbalance in the patients' brains returned to normal.
It seems that brain is able to reset its chemical environment to ensure it can continue to function in adverse conditions.
But this change appears to be reversible, and not to lead to permanent brain damage.
Researcher Dr Simon Taylor-Robinson said: "This research is a reminder to us all that smoking not only causes serious lung disease but also low blood oxygen levels which can affect brain function.
"When very severe, this may manifest itself as confusion in some patients.
"Over time the brain in smokers, suffering from the late stages of lung disease, has managed to compensate for the lack of oxygen.
"If you deprive a normal person of oxygen suddenly there would be serious consequences, even death - but the brain appears to be resourceful enough to address this problem in patients with COPD and emphysema, gradually adapting to be able to function in adverse conditions."
Dr John Harvey, of the British Thoracic Society, told BBC News Online the findings were interesting but not surprising.
"This emphasises the dramatic widespread nature of the damage that cigarettes can wreak on the body," he said.
"This is not a natural thing for the brain to do but it does show just how clever it is at adapting to circumstances."
The research is published in the journal Metabolic Brain Disease.