WASHINGTON — Take a slow deep breath, then exhale just as slowly. Can you take fewer than 10 breaths a minute?

Research suggests breathing that slowly for a few minutes a day is enough to help some people nudge down bad blood pressure.

Why would that brief interlude of calm really work? A scientist at the National Institutes of Health thinks how we breathe may hold a key to how the body regulates blood pressure — and that it has less to do with relaxation than with breaking down all that salt most of us eat.

Now Dr. David Anderson is trying to prove it, with the help of a special gadget that trains volunteers with hypertension to slow-breathe.

If he's right, the work could shed new light on the intersection between hypertension, stress and diet.

"If you sit there under-breathing all day and you have a high salt intake, your kidneys may be less effective at getting rid of that salt than if you're out hiking in the woods," said Anderson, who heads research into behavior and hypertension at the NIH's National Institute on Aging.

An estimated 65 million Americans have high blood pressure, putting them at increased risk of heart attacks, strokes, kidney damage, blindness and dementia. Many don't know it. Hypertension is often called the silent killer, because patients may notice no symptoms until it already has done serious damage.

Anyone can get high blood pressure, measured as a level of 140 over 90 or more. But being overweight and inactive, and eating too much salt — Americans eat nearly double the upper limit for good health — all increase the risk. Indeed, losing weight, physical activity and cutting sodium are the most effective lifestyle changes people can make to lower blood pressure. Still, most hypertension patients need medications, too.

While they know risk factors, scientists don't fully understand the root causes of hypertension: What skews the body's usually finely tuned mechanisms for regulating the force of blood pounding against artery walls, until it can't compensate for some extra pounds on a couch potato? Understanding those mechanisms could point to better ways to prevent and treat hypertension.

Enter breathing.

Meditation, yoga and similar relaxation techniques that incorporate slow, deep breathing have long been thought to aid blood pressure, although research to prove an effect has been spotty.

Then in 2002, the Food and Drug Administration cleared the nonprescription sale of a medical device called RESPeRATE, to help lower blood pressure by pacing breathing. The Internet-sold device counts breaths by sensing chest or abdominal movement, and sounds gradually slowing chimes that signal when to inhale and exhale. Users follow the tone until their breathing slows from the usual 16 to 19 breaths a minute to 10 or fewer.