Smoking appears to reduce a key enzyme in the lungs which helps regulate blood pressure, research suggests.
Smoking can damage health
The researchers used sophisticated scans and a chemical tracer to show levels of the enzyme monoamine oxidase were 50% lower in the lungs of smokers.
It is thought that reduced levels of the enzyme could impair lung function, as well as blood pressure control.
The study, by the US government's Brookhaven National Laboratory, appears in the Journal of Nuclear Medicine.
Lead researcher Dr Joanna Fowler said: "The effects of smoking on human health are enormous; yet, little is known about the pharmacologic effects of smoking on the human body apart from the effects of nicotine."
The researchers analysed monoamine oxidase (MAO A) levels in the bodies of nine smokers and nine non-smokers.
The scans revealed that levels of the enzyme were similar in all of the peripheral organs of both groups - except the lungs.
Analysis also revealed that smokers' lungs held onto the tracer chemical much longer than non-smokers, and the delivery of tracer into the arterial blood supply was much lower for smokers, particularly for the first few minutes after being injected.
This finding could imply that smokers and non-smokers respond differently to other substances that enter the body via the bloodstream - including therapeutic drugs, anaesthetics and abused substances.
Dr Fowler said: "The role played by MAO in other conditions associated with smoking may also be significant and deserves further investigation, considering the differences observed in the enzyme level between smokers and non-smokers."
Another piece in jigsaw
Dr John Moore-Gillon, president of the British Lung Foundation, said: "It is a very interesting piece of research, which provides another piece in the jigsaw.
"We already know that smoking is enormously harmful to many different body systems, and this might help to show how it is damaging to blood pressure."
Dr Moore-Gillon said it was already known that nicotine constricted the blood vessels - and thus tended to raise blood pressure.
He said smoking was also known to damage the balance of different enzymes responsible for maintaining the tissues of the lung in a healthy state.
Smoking also damaged the lining of the lungs, causing them to secrete excessive amounts of mucus which could lead to chronic bronchitis.
In addition, damage to the ability of cells to multiply and repair themselves could lead to lung cancer, he said.
Judy O'Sullivan, a cardiac nurse at the British Heart Foundation said: "While this study is small in size, it adds weight to the existing and overwhelming evidence that smokers are risking their health and even their lives by continuing with their deadly habit.
"We cannot emphasise enough the need for smokers to stop smoking to protect themselves from premature death and disability."