A Statin Nation. Dr Malcolm Kendrick
how did different part of the body obtain nourishment? If the liver did not make blood from food, where did blood originate? Why was blood blue in veins, but red in arteries? It took two decades for Harvey’s colleagues to acknowledge his achievements.1
It’s a great pleasure of mine, almost a secret vice, to read about the history of influential medical ideas. The passage of time has the great benefit of allowing you to see exactly how and why ideas of the greatest stupidity were so widely believed, and then defended with vigour and venom by the great and the good. At which point, you can draw parallels with the thinking and actions of today. Mentioning no names – yet.
Anyway, it turns out that Galen was wrong and Harvey was right, surprise, surprise. Blood does circulate around the body, travelling from the heart, in arteries, down through arterioles and capillaries, and then back again in venules and veins. How simple everything seems when you know the answer.
One point I need to add here is that the heart also pumps blood through the lungs, where it picks up oxygen and gets rid of carbon dioxide. This means that, in the lungs, blood vessels containing high levels of carbon dioxide are called arteries. On the other hand, blood vessels full of oxygen are called veins. And that’s the exact opposite in the rest of the body, just to add to the general confusion.
The arteries and veins in the lungs (pulmonary blood vessels) also have the same basic structure as the blood vessels elsewhere in the body. However, both the arteries and veins here have thin walls, as the blood pressure in the lungs is relatively low.
Notes
1. Newsletter of the American Institute of Stress, vol. 12, Dec 2008
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.
Atherosclerosis has been described in many ways but, at its simplest, is discrete or patchy thickenings within arterial walls, usually called atherosclerotic plaques. At its most complex, you can read papers discussing seven different types of atherosclerotic plaque, with several subsections in-between. (I remember reading ‘A definition of advanced types of atherosclerotic lesions and a histological classification of atherosclerosis’ – a report from the committee on vascular lesions of the Council on Arteriosclerosis, the American Heart Association – and ending up none the wiser.)
However you choose to define them, plaques start as small areas, usually described as ‘fatty streaks’. Over decades, these streaks grow into bigger and more complex lesions, a lesion being an abnormal/unhealthy thing in the body. At a certain point, they become so big and ugly that they’re termed atherosclerotic plaques. I am not sure when a fatty streak becomes a plaque; it’s a bit like asking when a boat becomes a ship. Nobody knows.
Plaques, in turn, come in many different versions. Some end up hard and calcified (full of calcium). Others have a gooey, fatty centre, known as a lipid core. If the gooey core is covered by a thin, fibrous cap it will usually be called a vulnerable plaque because, if the cap ruptures, the lipid core will be exposed to the bloodstream and triggers an instant, very large blood clot within the artery. More on this later.
DIAGRAM 3
In general, it is thought that calcified plaques represent the end stage of atherosclerosis plaque development and, slightly counter-intuitively, such plaques are probably stronger, better organised and less likely to rupture than earlier stage plaques. It is the transitional phase, the vulnerable plaque with a lipid core, that is the most dangerous and likely to rupture, leading to disaster.
Over the last few years, a test for measuring calcium in the coronary arteries – the coronary artery calcification (CAC) score – has become popular. The higher score, the more calcium in the artery, thus the more atherosclerotic ‘disease’ you have and the greater your risk of a heart attack. This assumes that if you have a lot of calcified plaques you will have a lot of other vulnerable, unseen plaques as well.
Unfortunately, you cannot do a great about the calcified plaques. Once