What Is Blood Pressure? Two Numbers From a Pump and a Pipe
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A pump pushing fluid through pipes creates pressure, and the two numbers written down at a health check are that pressure at two different moments: the peak as the heart contracts and the trough as it refills. Everything interesting about them comes from what makes the pipes easier or harder to push against.
Where the two numbers come from
The upper number, systolic pressure, is the peak reached when the left ventricle contracts and ejects blood into the aorta. The lower number, diastolic pressure, is the trough while the heart relaxes and fills, and it does not fall to zero because the large arteries are elastic and were stretched by the previous beat, so they recoil and keep pushing blood onward between contractions, which converts an intermittent pump into a near-continuous flow. Both numbers are conventionally expressed in millimetres of mercury, a unit inherited from the original measuring instruments, which is a reminder that the quantity is literally the height of a mercury column the pressure could support. Mean arterial pressure, the average over the cardiac cycle, is closer to the diastolic value than to the midpoint because the heart spends more of each cycle relaxed than contracted. The pressure depends on how much blood the heart pushes per minute and on how much resistance the small arteries offer, and the body regulates both continuously through nerve signals, hormones and kidney control of fluid volume.
How it is measured and why it goes wrong
The standard cuff method inflates until flow stops and then listens or senses as it deflates, and a long list of avoidable errors distorts readings routinely:
- •A cuff too small for the arm, which reads falsely high, and one too large, which reads falsely low
- •The arm unsupported or below heart level, which raises the reading, since a column of blood has weight
- •Talking during measurement, crossed legs, a full bladder, recent caffeine or smoking, and an unsupported back, all of which raise it measurably
- •Insufficient rest before measuring, when several minutes of quiet sitting is the standard
- •The white coat effect, where pressure is genuinely higher in a clinical setting, and masked hypertension, where it is higher outside one
- •Single readings generally, which is why diagnosis increasingly relies on home monitoring over days or on a twenty-four hour ambulatory monitor that records during normal activity and sleep
What raises it
In the large majority of cases no single cause is identifiable and the condition is called primary hypertension, arising from the interaction of genetics, age, weight, salt intake, alcohol, physical inactivity, sleep and chronic stress. Arteries also stiffen with age, which raises systolic pressure specifically while diastolic may fall, and a widening gap between the two numbers in older people reflects that loss of elasticity rather than a stronger heart. A minority of cases are secondary to an identifiable cause including kidney disease, narrowing of the renal arteries, hormonal disorders, obstructive sleep apnoea and certain medications, and these are worth finding because treating the cause can resolve the problem. Salt sensitivity varies considerably between individuals, which is part of why population advice on salt is less dramatic in effect for any single person than the population statistics suggest. The condition is usually symptomless, which is the central clinical problem, since there is nothing to prompt someone to seek treatment and the damage accumulates silently over years.
Why it matters and where the thresholds came from
Sustained high pressure damages arteries throughout the body, accelerating the process that narrows them and increasing the load on the heart, and the consequences are stroke, heart attack, heart failure, kidney failure, and damage to the retina and to cognition. The relationship between pressure and risk is continuous and has no threshold, meaning risk rises steadily across the range with no natural cut-off, so any diagnostic boundary is a decision about where treatment does more good than harm rather than a discovery about the body. Those boundaries have moved downward over time as trial evidence accumulated, and an American revision in 2017 that lowered the definition reclassified a large fraction of the adult population as hypertensive overnight, which was criticised as medicalising normal variation and defended as reflecting where the evidence pointed, with other countries declining to follow. Treatment combines lifestyle change with several classes of drug that work through different mechanisms, and combining low doses of two drugs is generally more effective and better tolerated than a high dose of one.
The takeaway
The upper number is the peak as the heart contracts and the lower one the trough while it refills, held up by elastic arteries recoiling between beats. Cuff size, arm position, talking and insufficient rest all shift readings enough to matter, which is why home and ambulatory monitoring are displacing single clinic readings. Risk rises continuously with no natural threshold, so diagnostic cut-offs are judgements about when treatment helps.