It occurs more often in older people who are taking a lot of medication. However, it can cause symptoms in younger people. There may be underlying medical conditions such as joint hypermobility syndrome, diabetes, parkinson’s disease, addison’s disease or autonomic failure. Dehydration, hunger, low body weight and deconditioning (being out of shape/unfit) can reduce blood pressure. 

A sudden fall in blood pressure can be dangerous. A change of just 20 mm Hg — a drop from 110 systolic to 90 mm Hg systolic, for example — can cause dizziness and fainting when the brain fails to receive an adequate supply of blood. And big plunges, such as those caused by uncontrolled bleeding, severe infections or allergic reactions, can be life-threatening.
One especially important cause of low blood pressure is orthostatic hypotension, which is sometimes referred to as postural hypotension. This happens when blood pressure drops rapidly during changes in body position—usually when changing from sitting to standing—inducing classic signs that the blood pressure is too low, like dizziness, blurry vision, and fainting.

High blood pressure is a major risk factor for cardiovascular disease. Without treatment, you can have a transient ischemic attack (TIA) or stroke, heart attack, enlarged heart, heart failure, peripheral vascular disease (such as poor circulation and pain in your legs), aneurysms, kidney disease, and broken blood vessels in your eyes. Treatment includes making changes recommended by your healthcare provider.

^ Qaseem, A; Wilt, TJ; Rich, R; Humphrey, LL; Frost, J; Forciea, MA; Clinical Guidelines Committee of the American College of Physicians and the Commission on Health of the Public and Science of the American Academy of Family, Physicians. (21 March 2017). "Pharmacologic Treatment of Hypertension in Adults Aged 60 Years or Older to Higher Versus Lower Blood Pressure Targets: A Clinical Practice Guideline From the American College of Physicians and the American Academy of Family Physicians". Annals of Internal Medicine. 166 (6): 430–437. doi:10.7326/m16-1785. PMID 28135725.
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Septicemia is a severe infection in which bacteria (or other infectious organisms such as fungi) enter the blood. The infection typically originates in the lungs (as pneumonia), bladder, or in the abdomen due to diverticulitis or gallstones. The bacteria then enter the blood where they release toxins and cause life-threatening and profound low blood pressure (septic shock), often with damage to several organs.
Blood tests may be done to assess risk factors for heart disease and stroke as well as looking for complications of hypertension. These include complete blood count (CBC), electrolytes, BUN (blood urea nitrogen), and creatinine and GFR (glomerular filtration rate) to measure kidney function. A fasting lipid profile will measure cholesterol and triglyceride levels in the blood. If appropriate, blood tests may be considered to look for an underlying cause of high blood pressure (secondary hypertension)including abnormal thyroid or adrenal gland function.

An exception to this is those with very high blood pressure readings especially when there is poor organ function.[79] Initial assessment of the hypertensive people should include a complete history and physical examination. With the availability of 24-hour ambulatory blood pressure monitors and home blood pressure machines, the importance of not wrongly diagnosing those who have white coat hypertension has led to a change in protocols. In the United Kingdom, current best practice is to follow up a single raised clinic reading with ambulatory measurement, or less ideally with home blood pressure monitoring over the course of 7 days.[79] The United States Preventive Services Task Force also recommends getting measurements outside of the healthcare environment.[80] Pseudohypertension in the elderly or noncompressibility artery syndrome may also require consideration. This condition is believed to be due to calcification of the arteries resulting in abnormally high blood pressure readings with a blood pressure cuff while intra arterial measurements of blood pressure are normal.[81] Orthostatic hypertension is when blood pressure increases upon standing.[82]

Blood pressure is the force of your blood pushing against the walls of your arteries. Each time your heart beats, it pumps blood into the arteries. Your blood pressure is highest when your heart beats, pumping the blood. This is called systolic pressure. When your heart is at rest, between beats, your blood pressure falls. This is called diastolic pressure.
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.
^ Jump up to: a b c d e f Chobanian AV, Bakris GL, Black HR, Cushman WC, Green LA, Izzo Jr. JL, Jones DW, Materson BJ, Oparil S, Wright Jr. JT, Roccella EJ, et al. (December 2003). "Seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure". Hypertension. Joint National Committee On Prevention. 42 (6): 1206–52. doi:10.1161/01.HYP.0000107251.49515.c2. PMID 14656957. Archived from the original on 20 May 2012. Retrieved 1 January 2012.
Tilt-table tests are used to evaluate patients suspected of having postural hypotension or syncope due to abnormal function of the autonomic nerves. During a tilt-table test, the patient lies on an examination table with an intravenous infusion administered while the heart rate and blood pressure are monitored. The table then is tilted upright for 15 minutes to 45 minutes. Heart rate and blood pressure are monitored every few minutes. The purpose of the test is to try to reproduce postural hypotension. Sometimes a doctor may administer epinephrine (Adrenalin, Isuprel) intravenously to induce postural hypotension.
"The implications and potential benefits of such healthier lifestyles could be great, particularly since about 22% of the adult population falls into the prehypertensive category," says Chobanian, who is also dean of the Boston University School of Medicine. "In fact, the blood pressure effects of weight reduction or adoption of the DASH eating plan can be comparable to those achieved with any single blood pressure medication."