Blood pressure is the force exerted by circulating blood against the walls of blood vessels, with clinical blood-pressure measurements referring principally to pressure within the arteries. It is generated largely by the pumping action of the heart and modified by vascular resistance, arterial elasticity and circulating blood volume. Blood pressure is not constant: it rises and falls during each cardiac cycle and varies with posture, activity, stress, temperature, sleep and numerous other physiological influences. It is normally expressed in millimetres of mercury, abbreviated mmHg, and is recorded as two numbers representing systolic and diastolic pressure.
The systolic pressure is the higher number and represents the peak arterial pressure associated with ventricular contraction and blood ejection. The diastolic pressure is the lower number and represents arterial pressure during the relaxation and filling phase between ventricular contractions. A reading such as 120/80 mmHg therefore means approximately 120 mmHg systolic pressure and 80 mmHg diastolic pressure. The difference between systolic and diastolic pressure is called pulse pressure. Mean arterial pressure is a related physiological concept representing the average effective pressure driving systemic tissue perfusion over the cardiac cycle, although it is not simply the arithmetic average of systolic and diastolic values.
Blood pressure is determined by interactions among cardiac output, vascular resistance and blood volume. Cardiac output depends on heart rate and stroke volume, while systemic vascular resistance is strongly influenced by the diameter of arterioles. Vasoconstriction increases resistance and can raise arterial pressure, whereas vasodilation generally decreases resistance. The kidneys influence longer-term blood-pressure regulation by controlling sodium and water balance and through hormonal systems such as the renin-angiotensin-aldosterone system. The autonomic nervous system provides rapid regulation by modifying heart rate, cardiac contractility and vascular tone.
Blood pressure naturally changes from moment to moment. Physical exercise generally raises systolic pressure because cardiac output increases, while vascular changes in active muscles can reduce resistance in those vascular beds. Emotional stress can temporarily increase blood pressure through sympathetic activation. Blood pressure is usually lower during sleep than during waking activity, although abnormal patterns can occur. Standing from a lying position also requires rapid cardiovascular adjustment to prevent excessive reduction in blood flow to the brain. These normal variations are one reason that a single measurement may not completely characterise an individual's usual blood pressure.
Blood pressure is measured using an inflatable cuff around the upper arm, with automated electronic devices now widely used and manual methods using a cuff and stethoscope also possible. Correct cuff size, appropriate positioning, adequate rest and repeated measurements improve reliability. Home monitoring or ambulatory monitoring over approximately 24 hours can provide additional information when blood pressure varies substantially or when a diagnosis needs confirmation. NHS guidance states that blood pressure is usually considered high when it is 140/90 mmHg or higher when measured by a healthcare professional or 135/85 mmHg or higher when measured at home.
High blood pressure, or hypertension, is persistent elevation of arterial blood pressure above clinically defined thresholds. It often produces no obvious symptoms, which is why it can remain undetected for years. Persistent hypertension increases mechanical stress on arterial walls and can contribute to damage involving the heart, brain, kidneys, eyes and blood vessels. It is associated with increased risks of cardiovascular disease, heart attack, stroke, heart failure and kidney disease. Because symptoms are unreliable, measurement rather than how a person feels is essential for identifying high blood pressure.
Low blood pressure, or hypotension, refers to blood pressure that is unusually low for the individual or situation and may be clinically important when it reduces adequate organ perfusion or causes symptoms. Dizziness, light-headedness, weakness or fainting can occur, particularly when pressure falls rapidly on standing. Causes can include dehydration, blood loss, medications, endocrine disorders, severe infection and disorders affecting cardiac function or autonomic regulation. Low blood pressure is not automatically harmful; some healthy people naturally have lower readings without symptoms. Clinical significance depends on the pressure, symptoms, underlying cause and adequacy of tissue perfusion.
Blood-pressure control involves lifestyle measures and, when indicated, medication. Factors associated with higher blood pressure include increasing age, excess body weight, high salt intake, smoking, excessive alcohol consumption, physical inactivity, family history and certain medical conditions. Lifestyle approaches can include a balanced diet, regular physical activity, maintaining a healthy weight, limiting excess salt and alcohol, and stopping smoking. When these measures are insufficient or cardiovascular risk is substantial, medicines may be prescribed, with different drug classes acting through mechanisms such as reducing vascular resistance, lowering fluid volume or altering hormonal regulation. Blood pressure is therefore both a measurable physiological variable and an important clinical indicator of cardiovascular and overall health.
