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CHAPTER 2

What is the Average Blood Pressure in Various Blood Vessels?

For historical reasons, the blood pressure itself is measured using the rather strange units of millimetres of mercury (mmHg – Hg is the shorthand chemical symbol for mercury). That is, how many millimetres of mercury can be pushed up a thin tube by the pressure in the blood vessel. The reason for using mercury is that, historically, it was by far the densest liquid known and, unlike most metals, it is liquid at room temperature.

Mercury is also more than 13 times as dense as water, which means that the column of mercury only needs be one-thirteenth the height of the actual blood pressure you are measuring. If you measure blood pressure using a water sphygmomanometer (the medical name for the blood pressure measuring instrument), it would need to be about 3 metres tall, which is about the height that blood would spurt up if you accidently made a hole in the side of the heart during open heart surgery (do not try this at home). A 3m-long sphygmomanometer would have been a bit inconvenient to carry about in a doctor’s bag, and nowadays measuring blood pressure is almost always done electronically with a hand-held machine. How prosaic it has all become. No skill, no blatant guesswork.

The other point to bear in mind is that the blood pressure in the arteries is going up and down all the time. As the heart squeezes (known as systole), the pressure peaks. When the heart relaxes (diastole), the pressure drops. And that’s why your blood pressure is normally given in two numbers: systolic (the highest pressure reached) and diastolic (the lowest pressure it falls to, before going up again as the heart contracts).

Normal blood pressure, measured in the arm, is around 120mmHg over 70mmHg and will be recorded in your notes as 120/70. Which of these two figures is more important? Books have been written on the matter but they are not, to tell the truth, very interesting. In general, the systolic pressure is normally considered the most important measurement. I await the inevitable howls of protest on this matter from the ‘pointy enders’.

The blood pressure in the coronary arteries is about the same as in the arm, perhaps a little higher. But unlike everywhere else in the body, the blood only flows in the coronary arteries when the heart relaxes (diastole), because when the heart is contracting (systole) the coronary arteries are, in some cases, squeezed shut by the muscle contracting around them.

In veins, however, the blood pressure is very much lower, and does not go up and down with the beating of the heart. It measures around 3–8mmHg, depending on the vein. The blood pressure in pulmonary blood vessels (blood vessels in the lung) is a little higher than in the veins, around 20mmHg over 8mmHg (20/8), i.e. 20mmHg in the pulmonary arteries and 8mmHg in the veins.

The issue of blood pressure in various blood vessels around the body becomes relevant to the entire discussion because (apart from very rare situations), atherosclerosis never develops in the veins and only very, very rarely in pulmonary blood vessels. This is even though these blood vessels are exposed to precisely the same concentration of cholesterol as the arteries. Yes, ponder that fact for a few moments.