What Is Normal Blood Pressure?

Blood pressure is primarily produced when the heart muscle contracts - it is the pressure of blood upon the walls of the blood vessels. There are two recordings, a high one (systolic) and a lower one (diastolic). The systolic pressure is measured when the heart contracts, while the diastolic one is gauged just before the heart contracts.

The heart is the muscle that pumps blood throughout the body during every second of our lives. Low-oxygen blood is pumped towards the lungs, where it becomes oxygen-rich again. Oxygen-rich blood is pumped by the heart around to body to supply tissue, muscle, organs and cells. This pumping generates blood pressure.

According to Medilexicon's medical dictionaryBlood Pressure is:

"The pressure or tension of the blood within the systemic arteries, maintained by the contraction of the left ventricle, the resistance of the arterioles and capillaries, the elasticity of the arterial walls, as well as the viscosity and volume of the blood; expressed as relative to the ambient atmospheric pressure."

Hypertension is when our blood pressure is too high, while hypotension is the opposite; it is when our blood pressure is too low.

Taking a blood pressure measurement

Most of us have had our blood pressure taken at some time in our lives. The procedure is straightforward and provides the doctor or other health care professional with vital data regarding the condition of the patient's blood vessels and heart.

As mentioned above, two blood pressure readings are measured:

  • The Systolic Pressure - this is the maximum pressure in an artery. It occurs when the heart contracts; when it is beating, and blood is being pumped through.
  • The Diastolic Pressure - this is the minimum pressure in an artery. It occurs just before the heart contracts; in between heartbeats. It occurs when the heart is resting.

If either the systolic or diastolic pressure is too high, the patient has hypertension (high blood pressure). So, both readings are important.

The patient needs to be relaxed and seated or lying down comfortably when their blood pressure is taken. Their arm must be well supported.

What happens when somebody's blood pressure is taken?

Digital Blood Pressure Monitor
Digital Blood Pressure Monitor - A 122/65 mmHg blood pressure reading, using an electronic sphygmomanometer

A cuff is wrapped around the upper arm and is inflated. Velcro keeps it in place. With an electronic sphygmomanometer, the patient just relaxes and waits and the device does everything.

Clinical Mercury Manometer
A clinical mercury Manometer

  • A Clinical mercury Manometer has a cuff, a tube that leads to a rubber bulb, and another tube which leads to a reservoir of mercury. The doctor wraps the cuff around the patient's upper arm.
  • The cuff is inflated by compressing the rubber bulb repeatedly.
  • The doctor places a stethoscope on the patient's arm and listens to his/her pulse.
  • When the doctor first hears the pulse, the systolic pressure is measured.
  • Gradually the pressure on the cuff is released. As this occurs the sound of the pulse becomes fainter and fainter.
  • As soon as the doctor cannot hear the pulse any more, the diastolic pressure is taken.
  • Blood pressure is measured in mmHg (millimeters of mercury).

Electronic devices are becoming more commonplace, and mercury ones less.

Ambulatory blood pressure monitoring

Also known as ABPMambulatory blood pressure measures the patient's blood pressure at regular intervals throughout a 24-hour period. It is useful, especially with patients who suffer from white coat hypertension - raised blood pressure caused by anxiety and stress when entering a clinical setting. Put simply: some patients see doctors and nurses wearing white uniforms, they become anxious, resulting in higher blood pressure.

The patient's blood pressure is measured as they go about their daily business. They wear a portable device which records BP (blood pressure) information on a chip. The data helps the doctor determine the patient's blood pressure in a normal environment. Doctors may opt for ABPM if the patient's BP readings vary a lot, their blood pressure does not respond to medications, when the doctor suspects current treatment may be causing hypotension (low blood pressure), and for patients with suspected white coat hypertension.

What is normal blood pressure?

Normal blood pressure is generally below 120/80 (one-twenty over eighty). 120 represents the systolic measurement and 80 represents the diastolic measurement.

Desirable blood pressure is:

  • Systolic - from 90 to 119
  • Diastolic - from 60 to 79

High or elevated blood pressure (hypertension)

Prehypertension - this is when the reading is between 120/80 and 139/89.

Hypertension - this is when the reading is at least 140/90.

Health authorities in the UK and USA say that approximately one third of all people withhypertension do not know they have it - meaning, they probably have no symptoms. Experts say we should check our blood pressure now and again because of this, this is especially the case for people who smoke, drink alcohol regularly, are overweight, and are reaching old age.

If symptoms are felt, they may include:

  • Blood in urine
  • Confusion
  • Dizziness
  • Fatigue
  • Headaches, some quite severe
  • Irregular heartbeat
  • Pains in the chest
  • Problems with breathing
  • Vision problems

Click here to read about Hypertension in more detail.

Low blood pressure (hypotension)

Anybody whose reading is 90/60 (ninety over sixty) mmHg or below has hypotension. For some seemingly healthy patients, in fact, a bit of hypotension is thought to protect them from subsequent hypertension. However, hypotension may mean there is an underlying problem.

If the hypotension is not severe, there may be no symptoms.

If symptoms are felt, they may include:

  • Depression
  • Dizziness
  • Eyesight problems, such as blurred vision
  • Light headedness; fainting is possible
  • Nausea
  • Pale, cold and clammy skin
  • Palpitations
  • Shallow panting
  • Thirst
  • Tiredness

Click here to read about Hypotension in more detail.

How do our arteries control blood pressure?

Our arteries control blood pressure by balancing the process that constricts the artery wall against the process that relaxes it. In some people there is more constriction than relaxation, as may be the case in atherosclerosis - thus impeding the free flow of blood.

Eicosanoids are molecules that play a key role in constricting the muscle walls. They are fat-like compounds which are made with the help of enzymes that break down polyunsaturated fats.

Researchers used to think that the eicosanoids are made from cells taken from blood; this is not the case. They are made by muscle cells in the arteries.

Experimenting with rats, researchers from Southampton University, England, deactivated two enzymes that help make the polyunsaturated fats. The result was much less arterial constriction, leading to freer blood flow, and consequently a much lower risk of hypertension.

They also discovered some "epigenetic switches" behind this process. This could explain why the arteries in some people show early signs of causing hypertension.

The researchers said that understanding exactly how arteries control blood pressure, and knowing how to modify this, could lead to better ways of treating cardiovascular diseases.

Written by Christian Nordqvist

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