Blood pressure

Part one: problems with extreme blood pressures

As a culture, one of the facets of medicine we're obsessed with is blood pressure. This makes sense: hypertension in particular is a big problem in the obese western world, and hypotension can be equally as serious. It's also very easy to measure, using the fantastically named sphygmomanometer, or blood pressure cuff, of which every GP has at least one.

Hypertension (high blood pressure)

Blood pressure is defined as how hard the blood is pushing against the arteries as it is moved around the body. Arteries have adapted to compensate for being pushed against by developing a layer of elastic tissue, so they can stretch with different pressures. Unfortunately, these elastic layers are not adapted to deal with very high blood pressure, when they are being stretched constantly. Like an elastic band, if you stretch an artery too much and too often, it stops becoming elastic. In the case of arteries, they harden (ateriosclerosis, not to be confused with but similar to atherosclerosis, which is also to do with cholesterol and a build up of fatty substances). Because the arteries harden, the pressure of the blood continues to be high until it reaches the arterioles, which are not developed for that pressure at all, so they burst.
This is a problem particularly in organs like the brain, where a burst blood vessel increases the pressure of the whole cranium hugely, and can leave permanent damage.
Hypertension also leads to heart failure, because the coronary arteries harden, so when more blood to the heart is needed (e.g. running for the bus), they cannot expand to allow that amount of flow, so the heart doesn't get enough glucose or oxygen, so parts of it die; which is a heart attack. Obviously that's a problem. The other way high blood pressure leads to a heart attack is that the left ventricle has to work harder to pump the blood around the body at a higher pressure, so the muscle thickens (hypertrophy). Because the muscle is thicker, it requires more oxygen and gluscose, so the same thing happens as when above.

The reason this is important in the western world is because we are increasingly ticking more and more risk factor boxes: the population is aging, becoming fatter, drinking and smoking. It can also be unsymptomatic for a long time before a huge event happens, so it is important to measure even apparently healthy people.

Hypotension (low blood pressure)

In some people, who exercise a lot, low pressure is a sign of good health and fitness. It can also be caused temporarily by alcohol, other drugs, or a postural change, among others. Mostly, it is harmless.

However, in acute and extreme circumstances, where hypotension is caused by blood loss or heart failure, it becomes very harmful. The brain (as well as other organs) doesn't get enough blood, which contains the glucose and oxygen it needs to survive, so it begins to die. This is a state called shock, which can be life-threatening. The loss of volume of blood is not always as obvious as a bleed; dehydration by diarrhoea or insufficient fluid intake can also cause hypotension, but this is likely to be less serious.