The heart contracts regularly to pump blood through the arteries under high (systolic) pressure. When the heart relaxes between beats, the blood continues to flow due to the lower (diastolic) pressure exerted by the elasticity of the artery walls. Hypertension occurs when one, or both, of these pressures exceed a safe level. Blood pressure readings are written as systolic pressure/diastolic pressure (eg. 125/70) and are measured with a sphygmomanometer. The numbers are a measure of pressure in millimetres of mercury.
Blood pressure varies with exercise, anxiety, age, fitness, smoking and drinking habits, weight and medications. In a very elderly person 160/90 may be acceptable, but in a young woman, 110/60 would be more appropriate. A blood pressure of 140-160/90-100 would be watched carefully for a couple of months, then treatment started once a persistently high level confirmed. Levels above 160/90 are usually treated sooner, and over 200/120 immediate treatment is necessary.
When treating a patient, doctors will try to keep blood pressure below 140/85, but even lower figures may be desirable in diabetics and those with a bad family or personal history of heart disease. Life insurance companies generally require the blood pressure to be under 136/86 for the person to be acceptable at normal rates.
The majority of patients have “essential” hypertension for which there is no single identifiable cause. The identifiable causes include smoking, obesity, kidney disease, renal artery stenosis (narrowing of the artery to the kidney), oestrogen-containing medications (e.g. the contraceptive pill), hyperparathyroidism, phaeochromocytoma and a number of other rare diseases. High blood pressure may also be a complication of pregnancy (pre-eclampsia), when it can lead to quite serious consequences.