Updated 08 February 2018

What is blood pressure? A quick explanation

Your blood pressure is the force exerted by your heart, against the resistance created by the arteries, to keep blood flowing through your body.

Your blood pressure is the force exerted by your heart, against the resistance created by the arteries, to keep blood flowing through your body. Your blood pressure is high (hypertension) when the force is excessive.

A sophisticated pump and pipe system
Blood pressure is the pressure exerted by the blood, on the walls of the arteries. Your heart functions as a muscular pump, that contracts rhythmically and squirts blood into your arteries. From there your blood is channeled to your entire body, through a circulatory system of smaller vessels. In this way, oxygen is delivered to all your living tissue.

The resistance offered by these arteries and the smaller arterioles is very significant. Constriction of the muscle in the artery wall causes it to narrow, which increases the resistance and hence the pressure within. This can be compared to taking a garden hose and reducing the opening at the nozzle. Pressure in a hose, can of course also be raised by increasing the amount of water flowing from the tap. Similarly, the amount of circulating blood, and the strength of the heart muscle contractions, can also influence your blood pressure.

If your blood pressure is too high, your heart must work much harder to maintain adequate blood flow to your body.

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 millimeters of mercury. 120/80mm Hg also happens to be the optimal blood pressure.

Systolic pressure is the pressure generated by each heartbeat. This occurs during the contraction of the heart muscle, which is called a systole.
Diastolic blood pressure is the pressure between the heartbeats when the heart is resting.
Systolic pressure is obviously always higher than diastolic blood pressure. 
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 heart rate and pumping action as result, the blood pressure is raised. 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.

(Dr Kathleen Coetzee, MBChB)

Read more:
Hypertension and your weight
Obesity: Simple solutions


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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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