Vol. 3, # 38
October 7, 2006
Q: I suffer from hypertension. What can I do to lower my blood presure naturally?- Layperson
A: Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure refers to systemic arterial blood pressure, i.e., the pressure in the large arteries delivering blood to body parts other than the lungs, such as the brachial artery (in the arm). The pressure of the blood in other vessels is lower than the arterial pressure. Blood pressure values are universally stated in millimetres of mercury (mmHg). The systolic pressure is defined as the peak pressure in the arteries during the cardiac cycle; the diastolic pressure is the lowest pressure (at the resting phase of the cardiac cycle). The mean arterial pressure and pulse pressure are other important quantities.
Typical values for a resting, healthy adult human are approximately 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg), with large individual variations. These measures of blood pressure are not static, but undergo natural variations from one heartbeat to another or throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors, drugs, or disease.
Normal values of blood pressure
How blood pressure is defined
Normal ranges for blood pressure in adult humans are:
In children the observed normal ranges are lower; in the elderly, they are often higher, largely because of reduced flexibility of the arteries. Clinical trials demonstrate that people who maintain blood pressures at the low end of these pressure ranges have much better long term cardiovascular health and are considered optimal. The principal medical debate is the aggressiveness and relative value of methods used to lower pressures into this range for those who don't maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality. The clear trend from double blind clinical trials (for the better strategies and agents) has increasingly been that lower BP is found to result in less disease.
In veterinary medicine, blood pressure values for dogs and cats are:
The mean arterial pressure (MAP) or mean blood pressure in the arteries supplying the body is a result of the heart pumping blood from the veins back into the arteries.
The up and down fluctuation of the arterial blood pressure results from the pulsatile nature of the cardiac output. The pulse pressure is determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries.
The larger arteries, including all large enough to see without magnification, are low resistance (assuming no advanced atherosclerotic changes) conduits with high flow rates that generate only small drops in pressure. For instance, with a subject in the supine position, blood traveling from the heart to the toes typically only experiences a 5-mmHg drop in mean pressure.
Regulation of blood pressure
The endogenous regulation of blood pressure is not completely understood. Currently, three mechanisms of regulating blood pressure have been well-characterized:
These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release. Currently, the RAS system is targeted pharmacologically by ACE inhibitors and angiotensin II receptor antagonists. The aldosterone system is directly targeted by spironolactone, an aldosterone antagonist. The fluid retention may be targeted by diuretics; however, the antihypertensive effect of diuretics is not due to its effect on blood volume. Generally, the baroreceptor reflex is not targeted in hypertension because if blocked, individuals may suffer from orthostatic hypotension and suffer from fainting.
Hemoglobin, the familiar oxygen-carrying molecule in red blood cells, may have been leading a double life. A group at the Duke University Medical Center reported in the 21 March issue of Nature that, alongside the familiar respiratory cycle, hemoglobin carries out a second cycle in which it sops up a form of nitric oxide in the lungs and releases it in blood vessels--a shuttle service that helps stabilize blood pressure. Besides giving hemoglobin a surprising new role, the finding could be a boon for efforts to turn cell-free hemoglobin solutions, which now tend to produce dangerous rises in blood pressure, into workable blood substitutes.
Researchers have found that deleting a component of a protein found in smooth muscle cells that surround the arteries can cause a dangerous and prolonged increase in blood pressure. The researchers believe that defects in this ion channel protein may underlie some forms of hereditary hypertension, thereby making the channel an attractive target for new types of blood pressure medications.
Calcium and potassium ions play important roles in regulating blood pressure. In the smooth muscle cells surrounding arteries, calcium-activated BK ion channels control the flow of potassium out of smooth muscle. When a burst of calcium is released within smooth muscle cells, the BK channels in those cells open, potassium floods out, the smooth muscle relaxes and blood pressure is lowered. But calcium can also induce blood vessel constriction, and cause high blood pressure, so researchers wanted to understand more about how these important channels are regulated.
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"BK channels have been known for quite some time as important in blood pressure regulation," said Howard Hughes Medical Institute investigator Richard W. Aldrich, who is at Stanford University. "However, calcium has a paradoxical effect. It not only acts on these channels to produce arterial dilation, but it can also act on the contractile machinery, which leads to blood vessel constriction. Thus, there's a balance involved in regulating blood pressure, and we are hoping to learn how the BK channel acts to tip that balance toward dilation."
BK channels are found in many types of cells, including neurons, smooth muscle cells and skeletal muscle cells. The diversity of cell types that contain BK channels is matched by the architectural diversity exhibited by the building blocks, or subunits, that compose these channels. For example, earlier studies had shown that the various tissue-specific BK channels had only one basic type of a subunit, but each had a slightly different � subunit. The difference in the � subunit may allow BK channels from different types of tissue to respond differently to calcium and voltage signals, says Aldrich.
In a research article published in the October 19, 2000, Nature , Aldrich, Mark T. Nelson of the University of Vermont, and their colleagues created knockout mice that were missing the �subunit of the BK channel that is contained in smooth muscles cells. By knocking out the gene for the � subunit in mice, they targeted the properties of BK channels in smooth muscle without affecting the channels in skeletal muscle or other tissues. The mice lacking the �subunit displayed high blood pressure and other abnormalities caused by prolonged hypertension.
In looking closer at the smooth muscle cells of the knockout mice, the scientists found that the BK channels in those cells showed reduced sensitivity to calcium and were unable to be opened by a drug that interacts with the � subunit. The studies also revealed that the BK channels of the knockout mice were much less able to open in response to calcium release.
Physiological studies of the �subunit-knockout mice revealed that their arterial blood pressure was chronically elevated and they had enlarged hearts, like humans with chronic hypertension. "The finding of elevated blood pressure was important because it showed us that the pressure regulatory system was as simple as we had predicted it to be," said Aldrich.
Altering the �subunit showed such straightforward physiological effects that the subunit may be a promising target for new anti-hypertension drugs. "These findings suggest that drugs that change �subunit function by altering the channel's calcium sensitivity could allow control of blood pressure up or down with fewer side effects than current treatments," he said.
The �subunit's effects on calcium sensitivity offers a new model for researchers who are investigating the molecular basis of hypertension, said Aldrich. "Since we can alter this subunit to affect blood pressure without affecting other systems, we can use it as a model to study hypertension beginning at the molecular level, through cellular physiology and to the pathology and long-term ramifications of hypertension," he said.
Finally, the effects produced by knocking out the �subunit gene suggest that the gene may be involved in inherited forms of hypertension, he said. "Some forms of inherited hypertension so far have not been pinpointed to specific genes, so these findings suggest that this is a good candidate gene to examine to see if humans with hypertension have mutations in this gene," said Aldrich.
Further studies of the various � subunits will also reveal how they function in different tissues. "The organizing principle with these subunits is that they act to fine-tune the BK channels to function in specific tissues," said Aldrich. "For example, studying BK channels in neurons might reveal how the channels limit the duration of neurotransmitter release in controlling nerve impulses."
Effects of high blood pressure
Blood pressure exceeding normal values is called arterial hypertension. It itself is only rarely an acute problem. But because of its long-term indirect effects (and also as an indicator of other problems) it is a serious worry to physicians diagnosing it.
All level of blood pressure puts mechanical stress on the arterial walls. Higher pressures increase heart workload and progression of unhealthy tissue growth (atheroma) that develops within the walls of arteries. The higher the pressure, the more stress that is present and the more atheroma tend to progress and the heart muscle tends to thicken, enlarge and become weaker over time.
Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure, arterial aneurysms, and is the second leading cause of chronic renal failure after diabetes mellitus.
In the past, most attention was paid to diastolic pressure; but nowadays it is recognised that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are also risk factors. In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, due probably to the increased difference between systolic and diastolic pressures.
Effects of low blood pressure
Blood pressure that is too low is known as hypotension. The similarity in pronunciation with hypertension can cause confusion.
Low blood pressure may be a sign of severe disease and requires urgent medical attention.
When blood pressure and blood flow decrease beyond a certain point, the perfusion of the brain becomes critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness and fainting.
However, people who function well while maintaining low blood pressures have lower rates of cardiovascular disease events than people with normal blood pressures.
Factors influencing blood pressure
The physics of the circulatory system, as of any fluid system, are very complex. That said, there are many physical factors that influence blood pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs, and alcohol etc.
Some physical factors are:
In practice, each individual's autonomic nervous system responds to and regulates all these interacting factors so that, although the above issues are important, the actual blood pressure response of a given individual varies widely because of both split-second and slow-moving responses of the nervous system and end organs. These responses are very effective in changing the variables and resulting blood pressure from moment to moment.
Other causes of low blood pressure
Sometimes the blood pressure drops significantly when a patient stands up from sitting. This is known as orthostatic hypotension; gravity reduces the rate of blood return from the body veins below the heart back to the heart, thus reducing stroke volume and cardiac output.
When people are healthy, they quickly constrict the veins below the heart and increase their heart rate to minimize and compensate for the gravity effect. This is carried out involuntarily by the autonomic nervous system. The system usually requires a few seconds to fully adjust and if the compensations are too slow or inadequate, the individual will suffer reduced blood flow to the brain, dizziness and potential blackout. Increases in G-loading, such as routinely experienced by supersonic jet pilots "pulling Gs", greatly increases this effect. Repositioning the body perpendicular to gravity largely eliminates the problem.
Other causes of low blood pressure include:
Shock is a complex condition which leads to critically decreased blood perfusion. The usual mechanisms are loss of blood volume, pooling of blood within the veins reducing adequate return to the heart and/or low effective heart pumping. Low blood pressure, especially low pulse pressure, is a sign of shock and contributes to/reflects decreased perfusion.
If there is a significant difference in the pressure from one arm to the other, that may indicate a narrowing (e.g., due to aortic coarctation, aortic dissection, thrombosis or embolism) of an artery.
Venous pressure is the blood pressure in a vein or in the atria of the heart. It is muc h less than arterial blood pressure, with common values of 5 mmHg in the right atrium and 8 mmHg in the left atrium. Measurement of pressures in the venous system and the pulmonary vessels plays an important role in intensive care medicine but requires invasive techniques.
How can I prevent hypertension? Besides taking drugs, if that becomes necessary, how can I control it if I have it?
It's not certain that you can prevent it, but it's reasonable to think that the same practices that help control it might also prevent or postpone it. Here's what to do:
�/SPAN> Maintain a healthy weight. Losing even a few pounds if you're overweight can reduce blood pressure. Weight loss (achieved through diet and exercise) can sometimes bring hypertension under control without the need for drugs, or with lower doses of drugs.
If you already have high blood pressure and plan to exercise intensely, you should first discuss your program with your doctor. Neither exercise nor any other life-style modification is a panacea, however, or a substitute for antihypertensive drugs if these are needed.
�/SPAN> Eat a diet rich in fruits, grains, vegetables, and low-fat dairy products. For details the "DASH" diet.
What about vitamin supplements?
Researchers at the National Institutes of Health are currently studying the overall effects of diet on hypertension. It's difficult to isolate one nutrient from another and assess the effects that each may have on hypertension. You can't go wrong by increasing your intake of fruits, grains, and vegetables梩hese may have beneficial effects on blood pressure. Recent studies have shown that as produce intake rises, stroke risk drops.
We recommend vitamin C and vitamin E supplements for their antioxidant potential. It's not clear that they help prevent high blood pressure.
What about calcium, potassium, magnesium supplements?
These three minerals are important in blood pressure regulation. But there's no evidence that high doses of them from supplements will lower blood pressure and help prevent hypertension. Calcium supplements are a good idea for postmenopausal women, but we strongly suggest that you get some calcium and all your potassium and magnesium from foods, which also contain other nutrients you need. Nonfat or low-fat dairy products are the best sources of calcium, though some leafy greens are good, too. You need at least 800 to 1,000 milligrams of calcium daily; women over 50 and men over 65 should get 1,500 milligrams daily. Potassium is plentiful in most foods. Magnesium is plentiful in whole grains, leafy greens, meats, milk, beans, bananas, and nuts.
We've recommended a multivitamin/mineral supplement for many older people. If you're on hypertensive medication, your doctor may recommend potassium supplements.
Will taking fish-oil capsules decrease blood pressure?
High doses of fish oil (which contains omega-3 fatty acids) may lower blood pressure on a short-term basis in some people, but are not recommended for preventing or treating high blood pressure. High doses have potential adverse effects, including an increased risk for one type of stroke. Moderate doses have no effect on blood pressure.
Can coffee cause high blood pressure?
Any caffeine-containing beverage (tea, cola, or coffee) can temporarily raise blood pressure, especially if you are not used to caffeine. But caffeine is not known to cause hypertension.
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