Blood pressure is the range of pressures in your cardiovascular system exerted by the heart pumping blood through your system. it is normally expressed in units of mmHg (millimeters of mercury). So a value of 120 is enough to support a column of mercury 120 mm high. Blood pressure has two values diastolic and systolic. The systolic (higher value) is the transient pressure in your arteries experienced during a heart beat. The diastolic (lower value) is the recovery or resting pressure experienced in-between heartbeats. Blood pressure is normally expressed as systolic over diastolic, e.g. 120/80.
It is very common to find articles that talk about how high blood pressure is associated with heart disease. It is also common to see articles that say that high blood pressure causes stroke and heart disease. E4E wonders about this and set out to research it.
High Blood Pressure from an Engineering Perspective
Think of the circulatory system and the heart as a pump--perhaps something like a diaphragm or a peristaltic pump. It pushes out the fluid with one stroke, then brings in fluid with the next. Each time the heart pushes out fluid, there is pressure created downstream from the heart. This rapidly dissipates for two main reasons: the arteries themselves are elastic and therefore expand when they receive pressure, the blood flows out of the arteries and into tissue and through the kidneys (and ultimately into the veins for return to the lungs and heart)--it's like having a hose without anything to stop the flow.
From this simplistic perspective then, removing the elasticity of the arteries or restricting the flow from the arteries can cause high blood pressure. Arteriosclerosis, the thickening, hardening, and loss of elasticity of the walls of arteries reduces the elasticity of the arteries, and so is a cause of high diastolic blood pressure. The loss of elasticity can be from either changes in the structure of the artery walls or by deposition of plaques inside the innermost lining of the artery, a condition known as atherosclerosis.
Additionally, renal artery disease, a blockage of the artery that brings blood to the kidneys for filtering, prevents the blood from exiting freely from the arterial system.
After the pulse of pressure from the heart, there is then a brief period for the pressure to dissipate before the next beat. So the systolic pressure indicates your body's ability to handle and dissipate the initial pressure pulse, while the diastolic pressure indicates a more general dissipation of blood into the capillaries.
Is High Blood Pressure Bad?
Medical people have said that high blood pressure (hypertension) increases the risk of kidney failure, stroke, cardiovascular disease, chronic kidney disease, and events such as an abdominal aortic aneurysm (AAA). From what I can determine, there is actually very little evidence that high pressure by itself is the cause of those issues.
The problem is more likely with the factors that cause the high blood pressure; the pressure is just a symptom. There is something called hypertensive heart disease, which is a thickening of the heart muscle caused by high blood pressure. The heart has to work harder to pump blood, so it bulks up. This larger heart needs a larger blood supply, and if it can't get it, problems ensue. From what I interpret in my readings, it is unclear if the enlarged heart is a primary cause, or if obstructed coronary blood vessels are what prevents the heart from getting blood supply. Wikipedia has a good article on Hypertension.
The hypothesis that hypertension causes these issues goes something like this from MedicineNet:
"High blood pressure makes your heart work harder and, over time, can damage blood vessels throughout your body. If the blood vessels in your kidneys are damaged, they may stop removing wastes and extra fluid from your body. The extra fluid in your blood vessels may then raise blood pressure even more. It's a dangerous cycle."
E4E thinks that the arrow of causality may be reversed. The above quote never addresses what caused hypertension in the first place. I believe that blood factors damage arteries (inflammation), which causes hypertension. The damage to the blood vessels also damages the blood vessels in the kidneys. Once the blood vessels are damaged, I suspect that elevation of blood pressure can rupture blood vessels, leading to events like AAA and stroke. So if your blood vessels are already damaged, it might be worth using drugs to lower BP. In my paradigm, though, that does not prevent further damage to the blood vessels, but might prevent a catastrophic event in the short term.
I would guess that the "high blood pressure causes bad things" paradigm is a relic of the history of medicine. They could measure it, so they did. They found a correlation, and then rationalized a causality story.
I wonder if somewhere, there is primary research, perhaps in animals, in which the researchers elevated blood pressure prior to causing damage to the circulatory system, for example, with a small pump just past the exit from the heart to somehow elevate overall pressure. That might give us a sense of whether elevated BP actually causes damage to otherwise healthy blood vessels.
[Edit: For several days after this post, the issue of blood pressure has been rattling around in my brain. There is a circularity that has confounded me somewhat, so here is what I am thinking:
A damaged vascular system causes high blood pressure. Damaged blood vessels are more prone to failure. The damage creates the danger, and also elevated blood pressure. High blood pressure can, in turn, cause failure in damaged blood vessels.
In the absence of damage, high blood pressure is not dangerous; after all, systolic blood pressure can go over 300 in times of intense exercise. Moderately high blood pressure, e.g 140/90, is indicative that there is damage, but it may not have progressed to dangerous levels. It is important to do everything you can to prevent further damage that would drive your BP higher. Clean up your diet, lay off junk food, eat naturally nutritionally dense foods (not barren foods that are enriched), stick with natural fats (rather than highly-processed ones), layoff sugar, exercise
Drugs that lower BP do not repair or reverse the damage. They may help prevent or delay the catastrophic failure, but they do not fix the real problem, which is the damage to your arteries.
When a water main blows and floods city streets, we don't blame the pumps for creating the pressure, we blame the pipes. Blood pressure is analogous.]
Notes and Sources
Blood Pressure During Exercise
Salt and Blood Pressure
The press and medical establishment have, for years, warned us to reduce salt intake to reduce our incidence of high blood pressure. Unless you have some kind of physical problem processing salt, your blood pressure might rise by up to 6 mm Hg after ingesting salt. Notice "might" and "up to" in the last sentence. Salt is not what causes your pressure to go from 120/80 to 160/100.
Recent New England Journal of Medicine paper on reductions of death that could result from a reduction in salt intake.
This paper on the Hypertension Paradox addresses the fact that despite improved means to treat hypertension, it continues to increase in the population. It seems they are considering hypertension as a disease rather than as a symptom. This is the problem with aggressive drug treatment of cholesterol as well.
Gary Taubes wrote an article in Science magazine entitled "The (Political) Science of Salt." (That is a free link. Science charges for the article.) The article is written in the vein of Good Calories, Bad Calories, and Taubes received the 1999 National Association of Science Writers' Science in Journalism Award award for writing it. His conclusions were that the research supports the concept that salt is, at most, the cause of a few point of blood pressure.
Here's what he said in this article in the Daily Bell:
"Daily Bell: Can you explain more about salt in diet and blood pressure, etc?"
"Taubes: For fifty years, researchers have been trying to causally link salt consumption to hypertension and the data has continued to be, at best, ambiguous. It's a nice hypothesis, but it just hasn't panned out in human trials or even, really, in the observational studies. On the other hand, it's been known since the 1870s that carbohydrates cause water retention and the more water you retain, simplistically speaking, the higher your blood pressure will be. It's been known since the 1950s that when people go on carbohydrate-restricted diets their blood pressure drops dramatically because of that water loss, and it's been known since the 1980s that one of the many things insulin does is regulate blood pressure. Moreover, hypertension is associated with obesity and diabetes so, in one sense, whatever causes obesity and diabetes also causes hypertension, and obesity and diabetes, as I explain in GCBC, are almost assuredly caused by the quality and quantity of carbohydrates in our diet."
From everything I can tell, high blood pressure is associated with cardiovascular disease--it is not a cause of the disease. Tom Naughton (creator of Fat Head) expressed a number of alternate hypotheses on the Fat Head blog.
- "Refined carbohydrates produce high blood sugar and high levels of insulin, which in turn are both bad news for your arteries. Refined carbohydrates also cause water retention, which raises your blood pressure. (So if you really want to reduce your blood pressure, give up the sugar and starch.)"
- "Blood pressure tends to go up as we get older. (Mine hasn’t, but bear with me here.) We’re also more likely to suffer heart attacks and strokes as we get older.
- "Stress causes your body to produce more cortisol, which can damage your arteries. Stress also raises your blood pressure.
- "Eating lots of vegetables may be good for your heart. Vegetables are also high in potassium, which lowers blood pressure."
John Briffa - Blood pressure is a symptom. Fixing the symptom may not fix the underlying problem.
John Briffa - is lower blood pressure always better? This report on a study that shows that lowering blood pressure below140/90 did not result in any changes to outcomes. The pressure was changed through chemical/medicinal means.
There has recently been a call to arms to reduce salt in food, under the premise that doing so will save "between 44,000 and 92,000" lives every year. The articles I read didn't go into detail, but the thinking in the past has gone something like this. (WebMD, New York Times, LA Times)
1. Salt causes high blood pressure
2. High blood pressure causes heart attack and stroke
3. Reduction in blood pressure of x points reduces probability of heart attack or stroke by y%
4. Therefore, reduction of salt will reduce deaths by total susceptible population * y%
Of the above points, the basic premises (1 and 2) are not supported by research and science. The extrapolation (3) is based on a correlation in the data with no implicit causality. The conclusion is just speculation that follows from 1, 2, and 3. Repeat after me "Correlation is not Causation."
The Diet Doctor blog
Bad Science leads to Bad Government
The British Medical Journal published a paper recently called Hypertension: empirical evidence and implications in 2014. Their finding is that there is no evidence that reducing blood pressure in people with mild hypertension with antihypertensive therapy will reduce cardiovascular events or mortality.
Some Additional Links:
Chris Kresser post on Blood Pressure and how to lower it naturally
Insulin and Hypertension from an evolutionary perspective
Tierney NYT article
Blood Pressure reduction does not reduce cardiovascular risk in Diabetics
There is a question about the impact of Isolated Diastolic hypertension, i.e. the high number's low and the low number's high. The Wiley article below concludes that there is little evidence of danger from IDH. There is evidence that using aural measurement of BP (with a stethoscope) can result in systematically higher readings of BP. E4E wonders whether, with accurate readings of the diastolic, this might be shown to have a high risk.
Isolated Diastolic Hypertension -
Another Isolated Diastolic Hypertension (Wiley) article - doesn't find much association with adverse issues.