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.
However, blood pressure can become dangerously high, and it can also get too low.
In this article, we will discuss what blood pressure is, how it is measured, and what the measurements mean for our health.
Blood pressure is the force that moves blood through our circulatory system.
It is an important force because oxygen and nutrients would not be pushed around our circulatory system to nourish tissues and organs without blood pressure.
Blood pressure is also vital 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 defenses against tissue damage, the clotting platelets that prevent blood loss following injury.
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.
Our circulation is similar to 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 this law also applies in a garden hose pipe.
Blood flows through our body because of a difference in pressure.
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.
Arteries affect blood pressure in a similar way to the physical properties of a garden hose pipe affecting water pressure. Constricting the pipe increases pressure at the point of constriction.
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.
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 affects blood pressure and flow, and narrowing of the arteries can eventually block the supply altogether, leading to dangerous conditions including stroke and heart attack.
The device used to measure blood pressure is a sphygmomanometer, it consists of a rubber armband – the cuff that is inflated by hand or machine pump.
The reading is expressed in terms of the pressure it takes to move mercury round a tube against gravity. This is the reason for pressure being measured using the unit millimeters of mercury, abbreviated to mm Hg.
A stethoscope identifies the precise point when the pulse sound returns and the pressure of the cuff is slowly released. Using the stethoscope enables the person measuring the blood pressure to listen out for two specific points.
Blood pressure readings consist of two figures - the systolic pressure first and the diastolic pressure second. The reading is given as, for example, 140 over 90 mm Hg.
The systolic pressure is the higher figure caused by the heart's contraction, while the diastolic number is the lower pressure in the arteries, during the brief 'resting' period between heartbeats.
The National Institutes of Health cite normal blood pressure to be below 120 mm Hg systolic and 80 mm Hg diastolic.
However, blood pressure changes naturally, a fact that cardiologists explored while writing about blood-pressure variability in 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)."
The guidelines state that for blood pressures above a figure of 115/75 mm Hg, every rise of 20/10 mm Hg doubles the risk of cardiovascular disease.
Since 2017, the American Heart Association (AHA) has advised that people with high blood pressure should receive treatment at 130/80 mm Hg rather than 140/90 mm Hg.
They also removed the "prehypertension" category between 120-139/80-89 mm Hg. A blood pressure reading of 140/90 mm Hg now qualifies as stage II hypertension and not stage I, as it used to be.
This category now forms two separate ranges:
- elevated blood pressure, from 120-129/less than 80 mm Hg
- stage I hypertension, from 130-139/80-89 mm Hg
In these new guidelines, the AHA also advises that doctors should only prescribe medication in cases of a previous heart attack or stroke, or in the presence of risk factors for these conditions, such as age, a diabetes mellitus diagnosis, or chronic kidney disease.
Treatment at the earlier stages should instead come mainly through lifestyle changes.
The guidelines for doctors list the following measures patients can take to help keep a healthy blood pressure:
- Keep a healthy body weight.
- Eat a diet rich in fruits, vegetables, and low-fat dairy products.
- Cut down on sodium, or salt, in the diet.
- Take regular aerobic exercise, such as brisk walking, for at least 30 minutes a day, most days of the week.
- Moderate alcohol intake. Men should drink fewer than two alcoholic beverages a day for men. Women and men with a lower body weight should consume a maximum of one alcohol drink a day.
Taking these steps can reduce the risk of health problems further down the line.