Normally blood flows through the arteries because of a pressure gradient - the heart pumps blood into the arteries and at the moment of most forceful contraction, the pressure in the arteries is about 120 mm Hg - blood pressure is measured in millimetres of mercury (mm Hg) and 120 mm Hg is the pressure required to push a column of mercury up 120 millimetres in a blood pressure measuring machine.
When the heart relaxes, the pressure in the arteries drops to a low of about 80 mm Hg, before the heart contracts again and forces it back up to 120 and blood pressure is measured as a set of two readings; the systolic (normally around 120 mm Hg) and the diastolic (normally around 80 mm Hg).
Blood pressure can fluctuate - during periods of physical activity the heart pumps harder and faster and the systolic blood pressure can go much higher than normal – to 180 mm Hg for example and can also be higher during times of stress - it also commonly increases with age.
However in some people, blood pressure stays abnormally high, even at rest and in 95% of cases the exact cause is unknown - the small arteries in the body become narrower and the heart has to pump harder to force blood through the narrowed arterial channels, which pushes up the blood pressure.
Because the cause is unclear this is often called primary or 'essential' hypertension. ...but in a relatively small - about 5% of cases – hypertension is a by-product, of another disease process such as kidney or hormonal diseases like Cushing's syndrome or Lupus. Rare hormonal diseases can cause the release of abnormal amounts of the hormones adrenaline and noradrenaline, making the heart pump harder and causing blood vessels to contract.
Some drugs can also cause hypertension, including non-steroidal anti-inflammatory drugs (NSAIDs), oral contraceptives, corticosteroids and some antidepressant, as can illegal stimulants, such as cocaine, amphetamine and crystal meth.