Blood pressure

Lore & Background

Blood pressure is usually expressed as systolic pressure over diastolic pressure, measured in mmHg. The difference between these is pulse pressure, and the average over a cardiac cycle is mean arterial pressure. Blood pressure is influenced by cardiac output, systemic vascular resistance, blood volume, and arterial stiffness, and varies with emotional state, activity, and health. Short-term regulation involves baroreceptors acting via the brain on the nervous and endocrine systems.

Reader's Guide

Blood pressure is a critical clinical measure, with hypertension (consistently high pressure) being a major risk factor for stroke, heart disease, and kidney failure. Hypotension (low pressure) is usually only concerning if symptoms are present. The risk of cardiovascular disease increases progressively above a systolic pressure of 90 mmHg. Ambulatory blood pressure monitoring over 24 hours is advocated by some authorities, such as NICE in the UK, as the preferred diagnostic method for hypertension, as it better predicts cardiovascular events than single office measurements. Blood pressure normally follows a circadian rhythm, and loss of the nighttime dip is associated with greater future risk. Age-related changes include a rise in systolic pressure from early adulthood to at least age 70, with diastolic pressure falling earlier, leading to isolated systolic hypertension in many older adults. Aortic pressure is considered a more accurate predictor of cardiovascular events than peripheral brachial pressure.

Did You Know?

• Blood pressure is most commonly measured in the brachial artery.
• The average blood pressure in the fetal aorta increases from about 30 mmHg at 20 weeks to about 45 mmHg at 40 weeks of gestation.
• If the heart is stopped, blood pressure does not fall to zero; the remaining pressure is termed mean systemic pressure, typically about 7 mmHg.
• Differences greater than 10 mmHg between left-arm and right-arm blood pressure may indicate conditions such as peripheral arterial disease or aortic dissection.

Measurement, Expression, and Contextual Sensitivity

Blood pressure is fundamentally the force that circulating blood exerts against vessel walls, driven primarily by the heart's pumping action. When clinicians use the term without further specification, they are referring to pressure within the brachial artery—the site where measurement is most routine. Readings are conventionally reported as a pair: systolic pressure (the peak during a single heartbeat) over diastolic pressure (the trough between beats), both expressed in millimetres of mercury above ambient atmospheric pressure, or alternatively in kilopascals. The gap between those two figures is called pulse pressure, and the average across the full cardiac cycle is termed mean arterial pressure. Because the number captured depends heavily on context, guidelines distinguish between office (clinic) readings, self-measured home values, and ambulatory readings collected by an automated device over a full 24-hour window. This contextual sensitivity is why a single snapshot in a waiting room may tell a very different story from a day-long recording.

Physiological Regulation and Short-to-Long-Term Variability

In the short term, baroreceptors serve as the body's primary pressure sensors, relaying signals through the brain to modulate both the nervous and endocrine systems. Beyond this rapid feedback loop, blood pressure is shaped by cardiac output, systemic vascular resistance, total blood volume, and the compliance of arterial walls. It is far from a fixed number: it fluctuates from minute to minute, follows a circadian pattern with peaks in early morning and evening and a nadir during sleep, and shifts in response to temperature, noise, emotional stress, food and liquid intake, dietary choices, physical activity, postural changes such as standing, medications, and underlying disease. Loss of the normal nocturnal dip is linked to elevated future cardiovascular risk, and evidence suggests night-time readings predict cardiovascular events more strongly than daytime ones. Longer-term variability spanning months or years also forecasts adverse outcomes. These dynamics have led bodies like the UK's NICE to recommend ambulatory monitoring as the preferred diagnostic tool for hypertension.

Developmental and Age-Related Trajectory

Blood pressure is not static across a lifetime. In the womb, it is the fetal heart—not the mother's—that generates the pressure driving blood through the fetal circulation, reaching roughly 30 mmHg in the fetal aorta at 20 weeks and climbing to about 45 mmHg by 40 weeks. Full-term infants typically show systolic values between 65 and 95 mmHg and diastolic values between 30 and 60 mmHg. In childhood, normal ranges sit below adult levels and are calibrated to the child's height, with country-specific reference tables in use. In most societies, adult systolic pressure trends upward from early adulthood through at least age 70, while diastolic pressure rises initially but begins to decline around age 55. Pulse pressure widens noticeably after 40, largely because arteries stiffen with age. When systolic pressure exceeds the normal range while diastolic remains within it, the condition is labelled isolated systolic hypertension. Notably, this age-related climb is not universal: some isolated, unacculturated communities do not exhibit it, suggesting environmental and lifestyle factors play a role.

Clinical Significance, Disease Risk, and Diagnostic Nuances

Blood pressure sits alongside respiratory rate, heart rate, oxygen saturation, and body temperature as one of the vital signs healthcare professionals rely on to gauge a patient's status. Pressures that are consistently too high—hypertension—carry a well-established risk of stroke, heart disease, and kidney failure, and cardiovascular risk climbs progressively above a systolic reading of 90 mmHg. Observational data show that individuals who keep their arterial pressures at the lower end of the range enjoy markedly better long-term cardiovascular health. Yet a medical debate persists over the optimal drug target, especially in older adults. A consistent inter-arm difference exceeding 10 mmHg can signal peripheral arterial disease, obstructive arterial disease, or aortic dissection and warrants further investigation.