What Causes High Blood Pressure?

Alas, most of the time they have no idea.

Kidney problems can cause abnormal secretion of hormones and enzymes that raise blood pressure, and kidney problems can also interfere with electrolyte balances, altering the vital sodium/potassium/magnesium balances that govern the retention of water in the body. Thyroid problems can cause excess secretion of thyroid hormones, which can raise blood pressure. For these reasons, the first thing doctors typically do on detecting ongoing elevation of blood pressure is to request blood tests for kidney function and thyroid stimulating hormone. Adrenal gland diseases (the adrenal glands sit atop the kidneys) can also directly cause hypertension.

If the blood pressure problems stem from these identifiable problems, the condition is termed "secondary hypertension" (because the problem is secondary to another primary disease process). Secondary hypertension accounts for an estimated 5-10% of cases of high blood pressure.

And what of the other 90-95%, the cases of so-called "primary" or "essential" hypertension?

Nobody knows. There are a lot of associations, suspicions, and conjectures, but the origins of most high blood pressure are obscure, and multiple factors may be interacting to cause the condition.

There are three major interacting systems that directly govern blood pressure:

1) The barorecptor system. Baroreceptors (pressure receptors) aren't 'receptors' in the modern biological sense of the word, which usually means a chemoreceptor. Instead, baroreceptors are complexes of blood vessels and nerves located along major blood vessels. The high-pressure baroreceptors, located along the carotid arteries and the aortic arch, respond to increases in blood pressure, while the low-pressure baroerecptors, located in the right atrium of the heart and along major veins, respond to decreases in blood pressure.

2) The autonomic nervous system. The sympathetic nervous system is responsible for arousal, and prepares the body for exertion, stress, or emergencies. An overstimulated sympathetic nervous system can contract arteries, speed the heart rate, and steer blood away from the stomach and intestines to the muscles. All of these effects increase blood pressure. Signals from the sympathetic nervous system that cause jumps in blood pressure when someone is alarmed or frightened. The other half of the autonomic nervous system, the parasympathetic system, is responsible for lowering blood pressure. Neither system is under direct conscious control, but both of them respond to what is happening in the mind.

3) The renin-angiotension-aldosterone systems. The renin-angiotensin hormones, secreted by the kidneys, cause constriction of arteries. Aldosterone, secreted by the adrenal glands (in response to high levels of angiotensin, among other factors), causes sodium retention, fluid retention, and potassium excretion.

Here are a list of factors that may cause, or contribute to, high blood pressure:

High-pressure Baroreceptor Errors. Sometimes you need your blood pressure to increase. For example, when you stand up, blood pressure needs to rise somewhat or you will faint. If you need to lift a heavy object, sprint for a bus, or engage in hot sex, your blood pressure needs to climb suddenly and sharply. The high-pressure baroreceptors are designed to accomodate these kinds of transient changes, and then go back to monitoring the blood pressure against a more reasonable baseline. But when high blood pressure continues over a period of days, the barorecptors "reset," so that the higher blood pressure becomes the new normal. This, and other kinds of baroreceptor problems, are suspected of being involved in many cases of hypertension.

Sympathetic Nervous System Overactivity. Overstimulation or overactivity of the sympathetic nervous system can raise blood pressure. Ongoing fear, stress, panic, or just plain tension can raise blood pressure directly (as well as speeding heart rate).

Too Much Stress. In addition to the sympathetic nervous system activity engendered by stress, various kinds of hormones (notably cortisol and noradrenaline) generated by stress and sympathetic nervous system activity act to raise blood pressure. Chronic elevation of these chemicals in the bloodstream can cause hypertension.

Oversecretion of angiotensin or other hormones. Attacking angiotensin (by means of drugs such as ACE inhibitors) is a common approach to lowering blood pressure, but the fact that this approach is taken doesn't necessarily confirm that the cause of a given case of hypertension was oversecretion. Since inhibiting Ancgiotensin Converting Enzyme lowers blood pressure, ACE inhibitors are often employed even though the hypertension might be unrelated to oversecretion.

Too Much Sodium. This topic requires a whole post of its own, but as almost everyone knows, too much salt can raise blood pressure in at least some people. (In some people, too little sodium results in overstimulation of the sympathetic nervous system, which raises blood pressure, but the medical establishment has chosen to ignore this inconvenient fact.)

Too Little Potassium. Potassium acts to lower blood pressure, and can be thought of as the counterweight to sodium. Recommended intakes of potassium are very high--three to four grams a day--but very few Americans eat enough fruits and vegetables to get anywhere near this amount. Because potassium is so potent at lowering blood pressure, however, the FDA has made potassium supplements above 99 mg (about 3% of the typical daily requirement) available by prescription only.

Too Little Magnesium. Magnesium also acts to lower blood pressure, and plays a vital role in cardiovascular health. Unlike sodium and potassium, however, where a simple blood test gives a good indication of any deficiencies or excesses, magnesium deficiencies are difficult to identify with serum tests. Most of the body's magnesium is sequestered in cells and bone, and these reserves can be called upon to keep blood levels reasonably stable. By the time blood magnesium drops, the total body deficit is very large. Furthermore, the body can only absorb so much magnesium at a time, so returning the body to magnesium balance can take months.

Sleep disturbances. Sleep apnea (interruption of breathing during sleep) has been shown to be a clear cause of some cases of hypertension. In addition, sleep deprivation unrelated to apnea also appears to raise blood pressure. Even in subjects with normal blood pressure, cutting back on hours of sleep raises blood pressure on the subsequent day, and many scientists now believe that prolonged sleep deficits may result in the development of hypertension.

Obesity. There is a high correlation between overweight and hypertension, but the relationship is far from perfect; there are many overweight people with normal blood pressure, and many trim people with hypertension. Losing weight can often lower blood pressure, sometimes dramatically, but since losing weight usually involves changing diet and exercise, it is unclear whether it is the weight loss per se that causes the reduction on blood pressure. The mechanisms by which obesity causes hypertension are unclear, although Syndrome X and/or liver problems (see below) may be the root causes.

Arteriosclerosis/atherosclerosis. Clogged arteries increase blood pressure by requiring the heart to pump harder to move blood around the body.

High cholesterol. High cholesterol, particularly high LDL cholesterol, is commonly assocaited with hypertension, but there is considerable doubt whether it directly causes hypertension. LDL is a building block involved in clogging arteries, which raises blood pressure, but the mechanisms of laying down arterial plaque now appear to be more complicated than the mere presence of high levels os LDL.

Sedentary lifestyle. Lack of exercise predisposes to the development of hypertension, and hypertension can sometimes be reversed by exercise. Once again, the mechanism is unclear. Exercise may help reset baroreceptors (see above), and can also reverse aspects of Syndrome X (see below).

Alcohol. The link between alcohol and high blood pressure has long been a puzzle to researchers. One or two alcoholic drinks per day is associated with lower blood pressure than found in abstainers, but higher levels of consumption are associated with increased blood pressure. Moreover, the blood-pressure-increasing effects of alcohol are reversible as soon as intake is reversed. Recent research suggests that the association between high alcohol consumption and high blood pressure may not be an effect of the alcohol as such, but rather an effect of developing a fatty liver [1], which is common with high levels of alcohol intake (see below).

Upper cervical spine problems. Recently it was discovered that misalignment of the top cervical vertebra, the "Atlas" vertebra, can cause high blood pressure, presumably by pressing on nerves in the brainstem. Techniques for adjusting this misalignment through procedures developed by NUCCA (National Upper Cervical Chiropractic Association) techniques have been decisively shown to normalize blood pressure in individuals with neck injuries[2].

High insulin levels. Insulin causes high blood pressure in at least three ways. First, it instructs the kidneys to retain sodium and thereby retain body water. Second, it enlarges the smooth muscle cells of the walls of the arteries, stiffening them and also constricting them. Third, it stimulates the release of noradrenaline, which has many of the same effects as hyperactivity of the sympathetic nervous system.

Diabetes. When most people think of diabetes, they think of an insufficiency of insulin. But the most common form of diabetes, Type II diabetes, involves a long stage where there is a superabundance of insulin, co-existing with high blood sugar (a state called insulin resistance). About 70% of diabetics have hypertension. The exact link is not certain, but insulin is quite likely one of the culprits. In fact, diabetes may not be a cause of hypertension, but just another symptom of an underlying problem.

Syndrome X/Metabolic Syndrome. In the late 1980s, Gerald Reaven at Stanford University wrote a groundbreaking paper in which he contended that a whole host of health-damaging characteristics tended to occur in synchrony:

1) High insulin levels
2) Insulin resistance
3) High blood sugar
4) High blood pressure
5) Elevated VLDL cholesterol
6) Low HDL cholesterol

He dubbed this cluster of symptoms"Syndrome X" (though it is now often referred to as "metabolic syndrome"), and argued that insulin resistance was the real root of the problem. Syndrome X is a precursor to Type II diabetes, is associated with obesity and rapid weight gain, especially around the abdomen, and often is associated with fatty liver (see below). In effect, Reaven says that insulin resistance is a major cause of hypertension (as well as many other diseases).

Chronic inflammation. Inflammation has become the latest suspect in a great many diseases, ranging from arteriosclerosis to cancer to many autoimmune disorders. Recently it discovered, for example, that women with psoriasis (and inflammatory skin disorder) were far more likely to go on to develop Type II diabetes and high blood pressure.

Fatty liver. Fat accumulation in the liver causes the liver to become inflammed (and often progresses to hepatitis or even cirrhosis). An inflammed liver produces high levels of a protein called C-reactive protein (CRP). Although the exact role of CRP in the body is obscure, big jumps in CRP are seen during infections. Chronic elevation of CRP is associated with the development of arteriosclerosis; in fact, some medical scientists argue that CRP and another protein, homocysteine, rather than high cholesterol, cause atherosclerotic damage to arteries (the cholesterol is merely used as a building block). In any case, high CRP levels were recently shown to raise blood pressure directly by acting on the artery walls in some fashion [3]. Since fatty liver is associated with Syndrome X, obesity, high alcohol intake, and high cholesterol levels, fatty liver and elevated inflammation proteins may be the link between all of these conditions, and may point to the true root cause of many cases of hypertension.

[1] Stranges, S. et al. Body Fat Distribution, Liver Enzymes, and Risk of Hypertension. Hypertension. 2005;46:1186

[2] Bakris, G, et al. Atlas vertebra realignment and achievement of arterial pressure goal in hypertensive patients: a pilot study. Journal of Human Hypertension (2007), 1–6.

[3] CRP Liver Protein Induces Hypertension. Medical News Today, Feb 22, 2007.