Blood pressure is the force blood exerts on artery walls as the heart pumps. When this pressure is persistently too high — a condition called hypertension — it silently damages blood vessels and greatly increases the risk of heart attacks and strokes, the leading causes of death in the UK.
What is blood pressure?
Blood pressure is measured in millimetres of mercury (mmHg) and is always expressed as two numbers:
- Systolic pressure — the higher number; the pressure when the heart ventricles contract and force blood into the arteries
- Diastolic pressure — the lower number; the pressure when the heart relaxes and refills between beats
A reading is written as systolic/diastolic, for example 120/80 mmHg (read as "120 over 80").
| Category | Systolic (mmHg) | Diastolic (mmHg) |
|---|---|---|
| Optimal | Below 120 | Below 80 |
| Normal | 120–129 | 80–84 |
| High normal | 130–139 | 85–89 |
| Hypertension (stage 1) | 140–159 | 90–99 |
| Hypertension (stage 2) | 160+ | 100+ |
| Low blood pressure | Below 90 | Below 60 |
How is blood pressure measured?
A sphygmomanometer is used to measure blood pressure. A cuff is wrapped around the upper arm and inflated to stop blood flow. As the cuff deflates, a doctor or automated device detects two key sounds (or pressure changes) in the artery:
- The pressure at which blood first pushes through the compressed artery as the heart contracts = systolic pressure
- The pressure at which blood flows continuously because the artery is fully open = diastolic pressure
Modern automatic devices detect these electronically; traditional manual devices use a stethoscope over the brachial artery. Blood pressure varies throughout the day — it rises with exercise, stress, and caffeine, and falls during sleep. Multiple readings over time give a reliable picture.
What are the risk factors for high blood pressure?
Several factors raise blood pressure, increasing the risk of cardiovascular disease:
Modifiable (lifestyle) factors:
- High-salt diet — excess sodium causes the body to retain water, increasing blood volume and pressure
- Low physical activity — exercise strengthens the heart and keeps arteries flexible
- Excess alcohol — raises blood pressure over time
- Smoking — nicotine causes immediate vasoconstriction (narrowing of blood vessels) and long-term arterial damage
- Obesity — more mass requires more blood volume; fatty deposits (atherosclerosis) narrow arteries
- Chronic stress — elevated cortisol and adrenaline sustain higher pressure
Non-modifiable factors:
- Age (arteries stiffen with age)
- Family history / genetics
- Ethnicity (people of African Caribbean heritage have higher average blood pressure)
How does high blood pressure damage the cardiovascular system?
Atherosclerosis is the key disease process linking high blood pressure to cardiovascular disease:
- High-pressure blood damages the inner lining (endothelium) of artery walls.
- Cholesterol accumulates at damaged sites, forming a fatty deposit called a plaque (or atheroma).
- The plaque hardens (calcifies) and narrows the artery lumen, reducing blood flow.
- Blood pressure rises further as the narrowed artery offers more resistance.
- The rough, narrowed artery is more likely to develop a blood clot (thrombosis) that may completely block it.
Consequences of blocked coronary arteries (the arteries supplying the heart muscle):
- Angina — chest pain from insufficient oxygen to heart muscle
- Heart attack (myocardial infarction) — part of the heart muscle dies when blood supply is cut off
Consequences of blocked arteries in the brain:
- Stroke — brain cells die rapidly without oxygen; this may cause paralysis, speech problems, or death
How is cardiovascular disease prevented and treated?
Lifestyle changes: Diet low in saturated fat and salt, regular exercise, stopping smoking, and moderating alcohol all reduce risk significantly.
Medicines:
- Statins lower blood cholesterol, slowing plaque formation
- Antihypertensives (e.g. ACE inhibitors, beta-blockers) lower blood pressure
Surgical interventions:
- Coronary angioplasty — a tiny balloon is inflated inside a blocked artery to widen it; a stent (metal mesh tube) is left to keep it open
- Bypass surgery — a vessel from elsewhere in the body is grafted to route blood around the blockage
Frequently asked questions
Why is blood pressure highest in arteries and lowest in veins?
Arteries receive blood directly from the ventricles during each contraction — they experience the full force of the heartbeat. Capillaries are very narrow and numerous, so collectively they offer great resistance; pressure drops significantly as blood moves through them. By the time blood reaches veins, much of the pressure energy has been used overcoming resistance in the capillaries. Veins return blood at low pressure and rely on skeletal muscle contractions and one-way valves (not pressure) to push blood back to the heart.
Can blood pressure be too low?
Yes — hypotension (low blood pressure, below 90/60 mmHg) can cause dizziness, fainting, and in severe cases shock. It can result from dehydration, blood loss, certain medications (especially antihypertensives taken at too high a dose), prolonged standing (orthostatic hypotension), or heart failure. Unlike hypertension, which is symptomless for years, low blood pressure usually causes obvious symptoms because the brain and organs receive insufficient blood flow.
Why do statins reduce cardiovascular disease risk?
Statins inhibit an enzyme (HMG-CoA reductase) that the liver uses to make cholesterol. Reducing blood cholesterol slows the deposition of LDL-cholesterol (low-density lipoprotein — "bad" cholesterol) in artery walls, reducing plaque formation and the rate of atherosclerosis. Statins also have anti-inflammatory effects that stabilise existing plaques, reducing the risk of rupture and thrombosis. Large-scale clinical trials show statins reduce heart attack and stroke risk by about 25–35% in people with known cardiovascular disease.
Is there a link between salt intake and blood pressure?
Yes — strong evidence from population studies, intervention trials (particularly the DASH trials), and experimental studies shows that dietary sodium raises blood pressure. Sodium causes the kidneys to retain water, increasing blood volume and therefore blood pressure. The effect is stronger in people who are "salt-sensitive" (about half the hypertensive population) and in people over 60. UK guidelines recommend no more than 6 g of salt per day (about one teaspoon); the average UK adult consumes about 8 g. Reducing salt intake is one of the most cost-effective public health interventions for reducing cardiovascular disease.
For Socratic GCSE biology with Professor Darwin — tracing pressure from heartbeat to arterial damage at the molecular and system scale — visit aitutors.me.