Updated 07 July 2014

How does blood pressure work?

Without it you’d die, but too much of it can kill you. So how does it work, anyway?


Without it you’d die, but too much of it can kill you. So how does it work, anyway?

The terminology
If your blood pressure is recorded as 120/80, the number on top is the systolic pressure, and the bottom number the diastolic. It is measured in millimetres of mercury. 120/80mmHg also happens to be the optimal blood pressure. 

Systolic pressure is the pressure generated by each heartbeat. This occurs during the contraction of the heartmuscle, which is called a systole. Diastolic BP is the pressure between the heartbeats when the heart is resting. Systolic pressure is obviously always higher than diastolic BP. Pulse pressure is the difference between the two readings.

If any of these are significantly elevated, it increases the risk for heart disease, stroke or kidney damage.

Essential to life
Blood pressure is essential to life. In fact when a person dies of so-called “shock”, it usually implies events that cause a fatal drop in blood pressure. This leads to inadequate perfusion of vital organs like the brain and kidneys. Starved from their life-giving source of oxygen, these organs cannot function anymore.

Blood pressure varies during the day
Considerable variation occurs in all people, depending on the demands of the body. When doing exercise, the muscles require more oxygen, and with the increase in heartrate and pumping action as result, raise the blood pressure. Anxiety, when being startled or feeling threatened, also raises the blood pressure through the effects of the “fight or flight” response. Experiencing pain can also raise pressure dramatically.  Alcohol, nicotine and caffeine intake can cause transient elevations of blood pressure.

Some people have high blood pressure only in a clinical setting, so-called white coat hypertension, with normal readings otherwise.

Blood pressure rises when you are active and falls when you are inactive.  During restful sleep, the inactivity reduces the demand for oxygen and therefore blood pressure is usually lowest at night and highest during arousal in the morning.

When is raised blood pressure bad for you?
Blood pressure fluctuations, experienced by all people are not problematic, as long as blood pressure returns to a normal baseline rapidly. It is the sustained increase in blood pressure that causes havoc, especially when accompanied by risk factors and co-existing disease.

Hypertension has no symptoms
However, in people with undiagnosed or untreated hypertension, even the daily fluctuations may pose a health risk. Hypertension often has no symptoms and may go undetected for years. All adults, even if feeling “healthy”, should be screened for high bloodpressure at a regular basis.

So how does blood pressure work?
It’s created by your heart pushing the blood through your veins and it creates pressure against the walls of your blood vessels. The play between the pressure generated by your heart and the resistance of the blood vessels determines your blood pressure level.

Healthcare professionals measure your blood pressure in millimeters of mercury or mm Hg, a unit of measurement that refers to how high the pressure inside your arteries can raise a column of mercury inside a blood pressure measuring machine called a sphygmomanometer.

The rhythmic throbbing of blood in your arteries is in response to the contractions of the heart.

Obviously blood pressure levels differ according to the size of the blood vessel and its proximity to the heart. The pressure is the most powerful in the aorta, leading away from the heart, but the heart is so powerful that you can sometimes see a pulse throbbing in someone’s temple or neck.

This pressure is needed to force blood into capillaries throughout the body. But the heart pumps blood into the great arteries of the body much more quickly than the tiny arterioles and capillaries can absorb it. This lag in flow results in pressure within the arteries.

To get the best reading of blood pressure, the pressure in the brachial artery of the forearm is used as a standard.

Two measurements are taken to work out your blood pressure. The first is taken when your heart pumps and is known as systolic blood pressure. The second is measured between beats, when the heart is resting, and is called diastolic blood pressure.

Your blood pressure will need to be measured several times to work out an average, as it’ll vary according to your activity level. Other factors that can affect your blood pressure include your intake or fluids and alcohol, and your exposure to tobacco smoke.

Read more:
Malignant Hypertension
Types of Hypertension


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Hypertension expert

Dr Jacomien de Villiers qualified as a specialist physician at the University of Pretoria in 1995. She worked at various clinics at the Department of Internal Medicine, Steve Biko Hospital, these include General Internal Medicine, Hypertension, Diabetes and Cardiology. She has run a private practice since 2001, as well as a consultant post at the Endocrine Clinic of Steve Biko Hospital.

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