If you ask the internet it will tell you that the causes of high blood pressure are not known. And will then go on to list a load of things that give you a higher risk of high blood pressure. You know, things like smoking, drinking, being overweight, being underfit, not eating enough fruit and veg. Also, if you are older, of particular ethnicities, or if your family (bloody family!) have a history of high blood pressure.
None of which answers the question ‘why do people get high blood pressure?’ which is unhelpful.
What it comes down to, apparently, is that in a lot of cases, the cause of high blood pressure is often not known.
But if you have a thorough and efficient doctor, one who you may have scared into action by mistakenly showing him your todger, you will go through a series of tests to rule out some of the known causes of hypertension. In my case, these tests included an ECG, blood tests, an ultrasound scan of my heart, kidneys and adrenal glands, and the joyful experience of collecting in a large plastic container the accumulated urine from a 24 hour period. At first I thought this last one was a joke, a punishment for exposing myself to the doctor, but it was probably to check for a thing called a phaeochromocytoma.
So, what conditions were ruled out by these tests?
Amongst other things (and I’ve shortened this list considerably, down to the ones I was actually aware of - if you want more then go Googling)…
Adrenal gland disease
Other hormonal problems
I don’t think they tested for excessive liquorice consumption, but that’s on the list as well.
So having done these tests, the end result was that I was almost certainly in the 90-95% of the hypertension cases who just have primary hypertension. One of the reasons why the cause of primary hypertension is so difficult to identify is probably that there are a whole load of things that contribute to it, so you can’t just point a finger at one thing and shout, ‘J’accuse!’ even if that were the sort of thing you were prone to doing.
(and before I go on, I should warn you that these are the most useless non-explanatory theories in the world and sourced from Wikipedia)
1. An overactive system of Renin and Angiotensin. Remember these? The chickens explained all about them last time. Renin released from the kidneys in response to decreased blood flow through the kidneys leads to higher levels of angiotensin 2 in the circulation. Angiotensin 2 does a load of things to bring the blood pressure back up again. So if you’ve got too much of this going on, your blood pressure will go higher. No explanation for why this becomes overactive. But…well, I’ll come back to this.
2. An overactive sympathetic nervous system. Now, I never really talked about this earlier, but I am sure you all know about the nervous system. Your nervous system is like the wiring that controls your body. Part of it is directly under your control. so you can pick up pencils, walk around, engage in conversations with idiots. The other part is not under your control and is doing more important things like controlling your heart rate and rhythm, making sure you go on breathing, sending blood to the bits of your body which need them most. This part is your autonomic nervous system and is split into two parts which balance each other out. These are called the sympathetic and parasympathetic nervous systems. We were always told at university to think of it this way – the sympathetic nervous system is ‘sympathetic to survival.’ Which has helped me remember it but is in almost every other way a stupid thing to say. What does it mean? Well, if you are about to be chopped to pieces by a crowd of angry idiots whose pencils you have just stolen, your sympathetic nervous system gets your body ready to do either ‘fight or flight.’ Physiologists like these little pithy expressions to help you remember stuff. For years and years I thought that ‘fight or flight’ was the choice between sympathetic or parasympathetic but I am a pencil-losing idiot – ‘fight or flight’ are essentially the same option. The sympathetic and parasympathetic systems are constantly active, and they are constantly balanced against each other to give the best results. Sometimes one system is more active than the other, sometimes things inhibit the activity of one more than the other. If you want to illustrate the difference between sympathetic or parasympathetic I feel sure that the chickens will do the job perfectly…
All of these things happen, and some of these have effects that will lead to higher blood pressure. So, if your sympathetic nervous system is in overdrive, it may lead to high blood pressure. Which is where we started.
1. Wikipedia says there are three things. And then it goes off the boil with its list. I think number 3 is supposed to be genetic and hereditary factors which lead to high blood pressure.
Why does being fat give you high blood pressure, then?
Apparently, for various reasons, if you are overweight you are likely to have increased sympathetic nervous system activity. I have studied this intensively (by staring hard at a diagram for 10 seconds) and I think I am just going to have to take somebody’s word for it.
No, come on, you can do it. Let me try again…
Leptin is a hormone released from fat cells to help control how hungry we feel. In theory, if we have enough fat already, the fat cells release leptin into the circulation and it is detected in the hypothalamus (which is part of the brain) which then tells us that we have had enough cake.
For some reason, leptin also acts in the hypothalamus to increase sympathetic nervous system activity. Why would it do this? Well, in a Noddy science sort of way, I suppose this could be happening...
There is a possibility that obesity may be related to resistance in the body to leptin. So the hypothalamus does not respond to leptin in the way that it should and the body never really feels like it has had enough food. So problems with leptin may make you fat, and if the body is producing leptin it could lead to high blood pressure by over-stimulation of the sympathetic nervous system.
Now, this has made me slightly confused, which won’t help you. It sounds like I’m having it both ways – if the hypothalamus is resistant to leptin then you don’t know that you have had enough food, but if your hypothalamus is resistant to leptin surely it won’t stimulate the sympathetic nervous system? And so that wouldn’t lead to hypertension??
But then, swooping out of the sky like a deus ex machina, or like a crazy chicken with a mad idea, comes the notion of SELECTIVE leptin resistance.
Okay. I’m going to have to leave leptin there.
There’s another possible way in which fat may lead to high blood pressure. And I have to say I’m excluding the obvious things – like if you’re fat you have a whole load of fat sitting on top of your organs and around your heart and pressing your blood vessels and all that sort of stuff. And if the fat is squishing everything down, then your heart has to work harder to pump blood through blood vessels which are narrower than they should be. I’ve ignored that so far because if you type, ‘why does obesity lead to hypertension?’ into Google, it never comes up with that sort of thing as an obvious answer.
Okay, so according to a scientific paper that is 10 years old but still being quoted, so it may be correct, fat cells produce angiotensinogen. So if you have more fat cells, you have more angiotensinogen and when renin is produced you will end up with more angiotensin 2. And all of the things that leads to.
So, if you are fat you are more likely to have high blood pressure.
So, all round better to be less fat.
And in the next lesson I will fail to answer this question…
‘How do drugs lower blood pressure?’
(once again, if anybody who knows more about physiology would like to correct any mistakes or misapprehensions, please feel free - I'd rather not spread too much misinformation, but sometimes my brain can't entirely wrap itself around concepts too well)