The heart and circulation. Blood pressure.




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2. Blood pressure

Picture 2.1. Measuring blood pressure.

The heart is a muscular pump which generates the blood pressure needed to keep the blood flowing. This can be measured using a piece of equipment called a sphygmomanometer (see photo 2.1). There are also modern digital blood pressure meters which take the readings automatically.

Measuring blood pressure

Doctors always give blood pressure as two readings. A healthy person's normal pressure might be "120 over 80 mm of mercury (mmHg)".

The first number is the higher pressure caused as the left ventricle contracts to push blood out of the heart. Doctors call this the systolic pressure. The lower value is the pressure in the arteries during the time that the heart is filling with blood. This is the diastolic pressure. (see animation 2.2).

[The standard unit of pressure is the pascal (Pa) and blood pressure can be measured in kilopascals (kPa). A pressure reading of 120 over 80 mmHg is equivalent to 16 over 10 kPa.]

Picture 2.2 is an animation showing how blood pressure is measured. Click here to see the animation.

Picture 2.3 is an animation showing the effect of beta blockers. Click here to see the animation.

High blood pressure (hypertension)

During activity the blood pressure rises to supply blood faster to the hard-working muscles. This is caused by the body producing adrenaline, the so-called “fear, flight or fright” hormone. When relaxed, the pressure drops as the body's demands are normally much lower.

A person has high blood pressure, called hypertension, if their resting blood pressure is higher than about 140/90mmHg. In the United Kingdom over 10million people have hypertension.

Hypertension is a problem because it puts great strain on the heart and can also cause tiny blood vessels to break. If this happens in the brain it leads to a stroke. A blood clot forms and deprives a part of the brain of blood. This damages nerve cells and can lead to paralysis or even death.

People with hypertension should alter their lifestyle to take more exercise and eat a balanced diet with a high fruit and vegetable intake, low levels of saturated fat and salt and limited amounts of alcohol (see section 6).

Medicines called beta-blockers can be given to slow the heart rate and reduce the force of contractions. This helps to reduce the blood pressure. Medicines known as ACE-inhibitors can also help by opening up the arteries and veins to allow the blood to flow more easily.

Low blood pressure (hypotension)

Low blood pressure can be a sign of heart failure and may be due to a dramatic loss of blood due to severe bleeding after something like a road accident. If it is not treated quickly, oxygen is not delivered to the brain and this is fatal.

At the scene of a car accident, fluids may be put in to the blood stream, using an intravenous drip, to help maintain the blood pressure. Adrenaline can be given to stimulate the heart and during a heart attack a defibrillator gives an electric shock to "jump start" the heart to get it beating properly again.