Many things can cause your blood pressure to be too low, ranging from normal pregnancy-induced changes to dangerous underlying conditions, like heart problems or hormone disturbances. In some instances, what causes low blood pressure could be a simple case of dehydration brought on by vomiting, intense exercise, or the overuse of diuretics. In fact, even mild dehydration can trigger symptoms such as dizziness, confusion, or other symptoms of low blood pressure.
Blood pressure is expressed by two measurements, the systolic and diastolic pressures, which are the maximum and minimum pressures, respectively.[1] For most adults, normal blood pressure at rest is within the range of 100–130 millimeters mercury (mmHg) systolic and 60–80 mmHg diastolic.[7][12] For most adults, high blood pressure is present if the resting blood pressure is persistently at or above 130/80 or 140/90 mmHg.[5][7] Different numbers apply to children.[13] Ambulatory blood pressure monitoring over a 24-hour period appears more accurate than office-based blood pressure measurement.[5][10]
Your doctor may diagnose you with high blood pressure when you have consistent systolic readings of 140 mm Hg or higher or diastolic readings of 90 mm Hg or higher. Based on research, your doctor may also consider you to have high blood pressure if you are an adult or child age 13 or older who has consistent systolic readings of 130 to 139 mm Hg or diastolic readings of 80 to 89 mm Hg and you have other cardiovascular risk factors.
The value of routine screening for hypertension in children over the age of 3 years is debated.[90][91] In 2004 the National High Blood Pressure Education Program recommended that children aged 3 years and older have blood pressure measurement at least once at every health care visit[89] and the National Heart, Lung, and Blood Institute and American Academy of Pediatrics made a similar recommendation.[92] However, the American Academy of Family Physicians[93] supports the view of the U.S. Preventive Services Task Force that the available evidence is insufficient to determine the balance of benefits and harms of screening for hypertension in children and adolescents who do not have symptoms.[94]
First-line medications for hypertension include thiazide-diuretics, calcium channel blockers, angiotensin converting enzyme inhibitors (ACE inhibitors), and angiotensin receptor blockers (ARBs).[13] These medications may be used alone or in combination (ACE inhibitors and ARBs are not recommended for use in combination); the latter option may serve to minimize counter-regulatory mechanisms that act to restore blood pressure values to pre-treatment levels.[13][129] Most people require more than one medication to control their hypertension.[111] Medications for blood pressure control should be implemented by a stepped care approach when target levels are not reached.[128]

The kidney can respond to changes in blood pressure by increasing or decreasing the amount of urine that is produced. Urine is primarily water that is removed from the blood. When the kidney makes more urine, the amount (volume) of blood that fills the arteries and veins decreases, and this lowers blood pressure. If the kidneys make less urine, the amount of blood that fills the arteries and veins increases and this increases blood pressure. Compared with the other mechanisms for adjusting blood pressure, changes in the production of urine affect blood pressure slowly over hours and days. (The other mechanisms are effective in seconds.)

Systolic BP lower than 90 mm Hg or diastolic BP lower than 60 mm Hg is considered low blood pressure or hypotension. Unlike hypertension, hypotension is more likely to be due to a primary cause (dehydration, heart disease, drugs, endocrine disease, etc.) and thus therapy is often aimed at treating the primary cause. Borderline BP with no obvious cause can be treated with increased salt and water intake, regular mild exercises, cutting down on alcohol, etc.). Drug therapy can be used if such hypotension is not amenable to these measures.
Orthostatic hypotension is caused by a sudden change in body position. This occurs most often when you shift from lying down to standing. This type of low blood pressure usually lasts only a few seconds or minutes. If this type of low blood pressure occurs after eating, it is called postprandial orthostatic hypotension. This type most often affects older adults, those with high blood pressure, and people with Parkinson disease.
tilt table test – usually recommended if your doctor suspects you might have orthostatic hypotension or NMH (During this diagnostic test, a person lies on a table and then the table is tilted to raise the upper part of their body. This simulates the change in position from sitting or lying down to standing up. People with orthostatic hypotension or NMH may feel dizzy, lightheaded, or even faint when their position changes.)

If your systolic and diastolic blood pressure are in two different categories, doctors consider the number that is in the higher category. For example, if your blood pressure is 135/91, your systolic blood pressure is in the prehypertensive range and your diastolic blood pressure is in the range of Stage 1 hypertension. Your measurement or 135/91 would place you in the category of Stage 1 hypertension.
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Normal blood pressure is less than 120/80. Knowing your blood pressure and remembering to check it regularly can help save your life. Choosing healthier, heart-conscious foods, maintaining a healthy weight, getting up and staying active, and managing your stress are great ways to help keep your blood pressure at the level it should be. Learn more about what you can do to help control your blood pressure.
As people age, they get plaque buildup inside the blood vessels, and the flexible walls of the arteries become stiff. Now, when the heart squeezes and pushes the blood out, the blood vessels can't expand like they used to do and sustain higher pressure. Over time, the heart has to push so hard against the pressure that it starts to fail, Bauman said.
There are even more medication types that can lower blood pressure. Some of these are alpha blockers, vasodilators, and central alpha agonists. Your doctor may prescribe these medications if other medications have been ineffective or if you have another condition along with hypertension. Side effects can include fast pulse, palpitations, dizziness, diarrhea, or headaches. 

Current strategies for controlling cardiovascular disease (CVD) risk factors, such as high blood pressure and high cholesterol, are not widely used as standard practice. CDC developed this guide to provide health professionals with evidence-based strategies for effective and sustainable CVD prevention, including health and economic impact and potential for reducing health disparities.

Medicines are available if these changes do not help control your blood pressure within 3 to 6 months. Diuretics help rid your body of water and sodium. ACE inhibitors block the enzyme that raises your blood pressure. Other types of medicines— beta blockers, calcium channel blockers, and other vasodilators—work in different ways, but their overall effect is to help relax and widen your blood vessels and reduce the pressure inside the vessel. [See also the free government publication “Medicines to Help You: High Blood Pressure” (PDF) from the US Food and Drug Administration.]

^ Ostchega Y, Dillon CF, Hughes JP, Carroll M, Yoon S (July 2007). "Trends in hypertension prevalence, awareness, treatment, and control in older U.S. adults: data from the National Health and Nutrition Examination Survey 1988 to 2004". Journal of the American Geriatrics Society. 55 (7): 1056–65. doi:10.1111/j.1532-5415.2007.01215.x. PMID 17608879.
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Hypertension is high blood pressure, a very common condition in older adults. Blood pressure is the physical force exerted by the blood as it pushes against the walls of the arteries. Blood pressure readings are written in two numbers separated by a line. The top number represents the systolic blood pressure and the bottom number represents the diastolic pressure. The systolic blood pressure is the pressure in the arteries as the heart contracts pushing the blood forward. The diastolic pressure is the pressure in the arteries as the heart relaxes.

A recent study compared diuretics with other types of blood pressure-lowering medications and found the diuretics were just as effective as the newer drugs in preventing heart attack or death due to heart disease. The new guidelines say these inexpensive drugs should be used as first-line treatment for most people who have high blood pressure without other risk factors such as heart failure, history of heart attack, diabetes, or kidney disease.
Health issues may happen; however, when a person's blood pressure suddenly drops and their brain is deprived of an adequate blood supply. The condition may lead to lightheadedness or dizziness. A sudden drop in blood pressure usually happens in a person who has risen from a prone or sitting position to a standing one. When this occurs it is referred to as, 'orthostatic hypotension,' or, 'postural hypotension.' Another type of low blood pressure may happen when a person stands for extended periods of time; it is referred to as, 'neurally mediated hypotension.' 

African-Americans tend to develop hypertension earlier in life and often experience more severe blood pressure elevation. It is not known for sure why high blood pressure is more common in this group, but researchers theorize it may include higher rates of obesity and diabetes among African-Americans and a gene that makes African-Americans more sensitive to salt. In people who have this gene, even just one-half teaspoon of salt could elevate blood pressure as much as 5 mmHg. (5)

Angiotensin receptor blockers prevent the actions of angiotensin II on the arteries. This means the arteries stay more open and blood pressure is lowered. ARBs can take a few weeks to work. Side effects can include dizziness, muscle cramps, insomnia, and elevated potassium levels. As with ACE inhibitors, women who are pregnant, planning to get pregnant, or breastfeeding should not take ARBs.

Blood pressure (BP) recordings consist of two numbers. The top one is the systolic blood pressure and relates to the contraction of the left side of the heart and the peak pressure achieved when it pumps blood round the body. The bottom number is the diastolic recording and is the lowest pressure achieved in the circulation; this relates to the relaxation of the heart. Blood pressure is measured in millimetres of mercury (mmHg), e.g. 120/70 mmHg.
Adrenal insufficiency, for example, due to Addison's disease, can cause low blood pressure. Addison's disease is a disorder in which the adrenal glands (small glands next to the kidneys) are destroyed. The destroyed adrenal glands can no longer produce sufficient adrenal hormones (specifically cortisol) necessary to maintain normal bodily functions. Cortisol has many functions, one of which is to maintain blood pressure and the function of the heart. Addison's disease is characterized by weight loss, muscle weakness, fatigue, low blood pressure, and, sometimes, darkening of the skin.

Vasovagal syncope can be treated with several types of drugs such as beta blockers, for example, propanolol (Inderal, Inderal LA) and selective serotonin reuptake inhibitors such as fluoxetine (Prozac), escitalopram oxalate (Lexapro), paroxetine (Paxil), sertraline (Zoloft), citalopram (Celexa), and fluvoxamine (Luvox). Fludrocortisone (Florinef) (a drug that prevents dehydration by causing the kidney(s) to retain water) also may be used. A pacemaker can also be helpful when a patient fails drug therapy.

Blood pressure guidelines show the lower the blood pressure numbers the better. As long as no symptoms of trouble are present there is no one number that doctors consider being too low. The guidelines call for an individualized, risk-based approach to managing hypertension, as well as a personal consultation with a health care provider. While the new guidelines mean we are more aggressive about blood pressure control, lifestyle changes are always a part of the treatment plan. A treatment plan is agreed to by patient and provider, and includes ongoing communication to see how the patient is feeling and how their medications are working.
Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT). This NHLBI study began in 1994 and lasted eight years. People with high blood pressure enrolled in one part of the study and people with high blood cholesterol enrolled in another part of the study. ALLHAT’s findings have informed how we treat high blood pressure and high blood cholesterol. Visit Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial for more information about all research activities and advances from this study.
Optimal blood pressure is less than 120/80 (systolic/diastolic). In healthy people, low blood pressure without any symptoms is not usually a concern and does not need to be treated. But low blood pressure can be a sign of an underlying problem -- especially in the elderly -- where it may cause inadequate blood flow to the heart, brain, and other vital organs. 

In hypertensive emergency, there is evidence of direct damage to one or more organs.[27][28] The most affected organs include the brain, kidney, heart and lungs, producing symptoms which may include confusion, drowsiness, chest pain and breathlessness.[26] In hypertensive emergency, the blood pressure must be reduced more rapidly to stop ongoing organ damage,[26] however, there is a lack of randomized controlled trial evidence for this approach.[28]
Many expert groups recommend a slightly higher target of 150/90 mmHg for those over somewhere between 60 and 80 years of age.[99][100][101][105] The JNC-8 and American College of Physicians recommend the target of 150/90 mmHg for those over 60 years of age,[13][106] but some experts within these groups disagree with this recommendation.[107] Some expert groups have also recommended slightly lower targets in those with diabetes[99] or chronic kidney disease with protein loss in the urine,[108] but others recommend the same target as for the general population.[13][103] The issue of what is the best target and whether targets should differ for high risk individuals is unresolved,[109] although some experts propose more intensive blood pressure lowering than advocated in some guidelines.[110]