What is blood pressure? How is it measured?

Normal blood pressure is vital to life: without the pressure that forces our blood to flow around the circulatory system, no oxygen or nutrients would be delivered through our arteries to the tissues and organs.

Nor would white blood cells, an essential part of our immune system, get distributed around our bodies without blood pressure. But blood pressure can be unhealthily high, and it can also be too low.


What is blood pressure?

The direct answer to the question, "What is blood pressure?" is a short one:

Blood pressure is the force that moves blood through our circulatory system.

It is a life force because, without blood pressure, the following two basic provisions would not be pushed around our circulatory system to feed tissues and organs:1

  • ·         Oxygen
  • ·         Nutrients.

Blood pressure is vital also because it delivers white blood cells and antibodies for immunity, and hormones such as insulin.

Just as important as providing oxygen and nutrients, the fresh blood that gets delivered is able to pick up the toxic waste products of metabolism, including the carbon dioxide we exhale with every breath, and the toxins we clear through our liver and kidneys.

Blood itself carries a number of other properties, including its temperature. It also carries one of our defences against tissue damage, the clotting platelets that stem blood loss. See What is blood? for more information.

But what exactly is it that causes blood to exert a pressure in our arteries? Part of the answer is simple - the heart creates blood pressure by forcing out blood when it contracts with every heartbeat. Blood pressure, however, cannot be created solely by the pumping heart.


Biology and physics

Our circulation is a highly sophisticated form of plumbing - blood has 'flow' and arteries are 'pipes'. A basic law of physics gives rise to our blood flow, and it applies all the same in a garden hose pipe.

Blood flows through our body because of a difference in pressure - it has 'potential'.2 This is in the same sense of the word as used for another type of flow, that of electricity. 'Electrical potential', or voltage, forces an electrical current through a wire.

Our blood pressure is highest at the start of its journey from our heart - when it enters the aorta - and it is lowest at the end of its journey along progressively smaller branches of arteries. That pressure difference is what causes blood to flow around our bodies.


Just as the physical properties of a garden hose pipe affect the water pressure - constrict the pipe and the pressure is increased at that point - so too do arteries affect blood pressure.

Without the elastic nature of the artery walls, for example, the pressure of the blood would fall away more quickly as it is pumped from the heart.

So while the heart creates the maximum pressure, the properties of the arteries are just as important to maintaining it and allowing blood to flow throughout the body.

The condition of the arteries has effects on blood pressure and flow, and narrowing of the arteries can eventually block the supply altogether (a heart attack is the heart itself being cut off; a stroke is caused when arteries to the brain are blocked or bleed).


Measuring blood pressure

The way that blood pressure is measured today can be traced back to 18553 - the devices show how much pressure is needed before the pulse in our arm stops briefly: the arterial blood pressure is represented at that point.

The devices in use now still have a mouthful of a name - sphygmomamometer (from the Greek for pulse, sphygmo) - but you would recognize the typical rubber armband, the cuff that is inflated by hand or machine pump.

Once the cuff is inflated enough to stop the pulse, a reading is taken, either electronically or on an analogue dial.

The reading is expressed in terms of the pressure it takes to move mercury round a tube against gravity in traditional scientific manometers - hence the unit, millimetres of mercury, that is abbreviated to mmHg.

Systolic and diastolic readings

A stethoscope - another of those longstanding devices used by doctors - identifies the precise point when the pulse sound returns as the pressure of the cuff is slowly released. Using the stethoscope enables the person measuring the blood pressure to listen out for the two points at which to look at the gauge for the BP readings.


What is normal blood pressure?

This is another question that doctors try to give a simple answer to, although blood pressure normality is complex.


Cardiologists do not really talk about a normal range - instead they work on the basis of when blood pressures should be considered too high, figures that continue to be refined by the medical research. The National Institutes of Health cites normal blood pressure to be below a certain level:

''No greater than 120 mmHg systolic and 80 mmHg diastolic.


But blood pressure changes naturally, a fact that is best described in a quote from cardiologists writing about blood-pressure variability in a paper published by Nature in March 2013:

"Blood pressure is characterized by marked short-term fluctuations occurring within a 24-hour period (beat-to-beat, minute-to-minute, hour-to-hour, and day-to-night changes) and also by long-term fluctuations occurring over more prolonged periods of time (days, weeks, months, seasons, and even years)."

Maintaining a healthy blood pressure level

The guidelines for doctors list the following measures patients can take to help keep a healthy blood pressure (from a table of evidence for the measures):8

  1. Keep a healthy body weight
  2. Eat a diet rich in fruits, vegetables, and low-fat dairy products
  3. Cut down on sodium intake (salt in the diet)
  4. Take regular aerobic exercise (eg, brisk walking) at least 30 minutes a day, most days of the week
  5. Moderate your alcohol drinking - keep under a maximum of 2 drinks a day for men and 1 drink a day for women/men of lighter weight (one drink having a half ounce of alcohol). [source]

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