Monday, 30 July 2012

Blood pressure continued

Part two: why do we need such high blood pressure?

So we have seen how blood pressure can be used as an indicator of largely chronic effects, and that it is a very useful measurement to keep track of. But what about the more acute side of medicine? 

I was lucky enough to stay in cardiothoracic critical care in my local hospital for a while as part of a work experience scheme with a consultant anaesthetist (a very underrated speciality, I have decided). While I was there, I noticed that they were measuring blood pressure on almost every patient. None of the people on the ward was there for chronic problems, so why were they bothering to measure the blood pressure? 

The answer is that they needed to see whether the heart was working properly, as almost everyone there had just been in or was about to go into surgery for a heart operation. If the heart is pumping blood around the body, it will push against the outside of the arterial walls, which is what we call blood pressure. If the pressure decreases, it is likely that the heart is not pumping the blood so well, and the patient might need to be looked at in more detail.

But that is a very crude way of getting a very vague answer. What I wanted to know was what the blood pressure actually signified about the heart that the doctors were interested in. When I asked the consultant this, he answered me with another question: why do we need such high blood pressure? I'll admit, I didn't understand the question when he first asked me it. 

My first thought was that some of the organs need high blood pressure, such as the kidneys: in the nephron, blood needs to be at high pressure in order to undergo ultrafiltration. But there are very few bits of the body that are like that, so it doesn't make sense to have the whole system running under unnecessary stress. What could be the advantage of having high pressure in the system? Does it have an advantage in itself? No. The advantage of running any fluid system at high pressure is to make the fluid move fast. That's what really matters: how quickly can the blood transport the necessary nutrients to the organs, and take away the waste products.

Again, I thought I'd got it, that we needed to run at such high pressure in order to move the blood around our body fast enough to work properly. Sadly for me, it turns out that the pressure needed to move the blood around our body is much lower than the average blood pressure. All I'd done was work out the question: why do we have such high blood pressure, when the same blood flow could be achieved with much lower pressure?

Looking at it from a purely physical standpoint, it makes no sense that two different pressures in the same system can provide the same blood flow. Where is the pressure being lost? There must be some kind of resistance that we are artificially providing in order to keep flow lower than its maximum. The pressure could be coming from the organs, or the arteries. We already know that only a few organs need the maximum pressure, so it must be coming from the arteries before the blood reaches the tissue it's going to. That can easily be provided by the muscle layer in the vessels, but I still hadn't worked out why it happened, just how it happened.

Why would the body have adapted to provide more resistance in the vascular system, making more work for the heart? Because it's not a fixed system: the muscles in the arteries can relax, reducing the pressure for various tissues, increasing the flow to those organs. This means when any tissue needs to increase the rate it gets its nutrients, the resistance of the vessels supplying that tissue can drop, which makes the flow spike immediately.

So to answer the question, we need high blood pressure so that blood flow to any part of the body can be increased straight away. That doesn't really explain why they measure blood pressure. That'll be in part three.

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.