All About Blood Pressure.

 

So, just what exactly happens when the Doctor takes your blood pressure? What is the Doctor doing and what is blood pressure anyway? Read on to find the answers.

 

Blood pressure is, surprise surprise, exactly what it says. It is the pressure the blood is under inside the body. Just like air in a car tyre is pushed in under pressure so the blood inside us is under pressure all the time to keep it moving. If you pump more air into a car or bicycle tyre it gets less and less soft. In the same way if you increase the blood pressure the force on all the arteries that carry the blood around increases.

Taking things to the extreme, if you pump up a tyre too much, it will, like a balloon, simply go `pop`. If the blood pressure rises very high and the artery that the blood is flowing through has a weak spot it too will go `pop`. This is the basis for the main type of stroke which occurs when an artery in the brain goes `pop` and bursts. This is the main reason why it is important to avoid high blood pressure. The higher the pressure gets, the more likely you will have a stroke.

Having said this of course, the presure can sometimes get too low. Just as you can get a flat tyre so you can get low presure in the blood which means the arteries are not filled up properly, blood does not get around the body and you feel faint and unwell.

Don`t let this worry you too much though, the body can cope with a wide range of pressures and both the above events do not happen very often in practice, but things like smoking greatly increase the risk of problems with high blood pressure.

 

 

So, how do we measure the blood pressure? Well, you use an instrument like the one in the picture at the top of this page; its proper name is a `sphygmomanometer`, which really is just a long word meaning `thing for taking blood pressure that goes around the arm`, but Doctors like long words!

 

The tube at the front of the machine is full of mercury. This is a liquid metal and is used because it is very heavy. This liquid is used to balance the pressure in the blood. The Doctor puts the fabric cuff around the arm and blows it up by pumping air into it. (just like a tyre). The air squeezes the arm and to exactly the same amount it also squeezes the mercury. The more air is pumped in, the harder the arm is squeezed and the higher the mercury is pushed up the tube.

 

The height the mercury rises in the tube (millimetres of mercury) is what blood pressure is measured in. The higher the mercury is pushed up, the tighter the cuff is pressing against the arm. It would not have to be mercury, the tube could just as well be full of water but mercury is much heavier than water so it is pushed up less distance. If water was used the Doctor would need to carry around a tube several feet high.

Have you followed this so far? Good. I bet you are wondering how this helps the Doctor to tell the pressure Of the blood. Easy!

Using his stethoscope, the Doctor can hear blood as it flows through the arm. The stethoscope is placed against one of the main arteries in the arm and the Doctor listens until he can hear the blood flowing through it. As the pressure in the cuff rises and the arm is squeezed tighter and tighter, the blood has more and more of a struggle getting down the arm. The pressure eventually becomes so great that the force of the heart is not enough to pump the blood past the squeezed bit and it stops flowing. This can be heard through the stethoscope as the noise of blood stops. (It can also usually be felt by the patient). Whatever pressure the cuff is being squeezed at when the flow stops must be the same pressure the blood is being pumped at by the heart. Bingo! The Doctor writes down the pressure at the point where the blood stops flowing and he has the blood pressure.

But, I hear you all asking, why are there two numbers in a blood pressure reading? (eg 160/90)? The blood is pumped around the body by the heart. Each heartbeat involves the heart pushing out a load of blood then relaxing and filling up with the next load (like a bicycle pump). The higher number is the pressure of blood when the heart is pushing, the lower number is the pressure when it is relaxed and filling up.

 

So, just what is a normal blood pressure? Well, that is not so easy because everyone has a different blood pressure. It varies by age and also its importance depends on whether you have any other illnesses such as diabetes or a heart problem. There is no one single reading that is `normal`. Your Doctor or Nurse will, however, be able to tell you whether your own blood pressure is right for you.



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