We are often being warned about the perils of high blood pressure. But what really happens when it is too high?

Blood pressure (BP) is the pressure exerted against the vessels which lead away from the heart (the arteries). It is measured on a major artery- usually the radial artery in the arm (see picture). There are two figures on the measurement the units of which are expressed as millimetres of  mercury (mm Hg). The top or first figure is always higher as it reflects the point at which the heart is pumping whilst the bottom/second/lower figure reflects the heart at rest. Current guidance underlines the importance of a blood pressure below 140/90 mmHg. However if you have heart diseae or diabetes it should be 130/80.

BP can be changeable from moment to moment dependent on whether you are awake or asleep, active, resting, happy, depressed angry, relaxed and in a comfortable posture etc.

But what can really happen if the numbers are too high?

• When the body’s arteries tighten to cause a higher BP the heart has to work harder to pump the same amount of blood. This can cause the heart to grow more muscle cells that require more oxygen, this means that if the arteries feeding that muscle become narrowed angina pains can easily occur. If the problem is not treated in time then the enlargement can go on   and the muscle tissue loose its’ elasticity and pump poorly. In turn heart failure is likely to develop where fluid can be pushed into areas such as the ankles, around the liver or even the lungs causing breathing difficulties. 

• The arteries feeding the body’s tissues and organs will harden and narrow (atherosclerosis) in time reducing the blood supply to them (ischaemia). Angina, strokes caused by the bursting of minor vessels feeding the brain, kidney failure and leg pains (claudication) as a few examples can all follow. Diseased areas known as atherosclerotic plaques can also themselves be damaged and debris from them can break-off to occlude a finer artery closer to the muscle tissue again leading to events like heart attack or stroke.

• The formation of clots completely cutting the blood supply causing strokes and heart attacks.

Diastolic versus Systolic hypertension.

Diastolic hypertension occurs where the bottom/second/lowest BP number is too high. A healthy level of diastolic is about 75 mm Hg and an increase above that number is linked with worsening disease of blood vessels.  
Approximately every 10 mm rise in diastolic pressure causes a doubling of incidence of adverse events such as heart attack, stroke, kidney failure and heart failure.
It should be noted that changes in diastolic blood pressure as a concern tends to be relevant to those under 50 years of age as with age BP tends to rise anyway in the absence of other problems (like heart failure) that will bring it down.
For systolic elevations, it takes a 20 mm Hg rise to double the incidence. Thus every 20 mm rise in systolic pressure doubles the incidence of adverse events.