Understanding blood pressure

  • Health
  • Sunday, 01 Jun 2003


DOC, what is hypertension? I’m sure every physician is often faced with this question. We also call it high blood pressure.  

Every time I water my plants, which is not very often, I am reminded of a simplistic view of hypertension. In its simplest form, understanding hypertension is a bit like understanding the water that flows through the garden hose. The pressure within the hose will depend on the size of the hose, the material and the amount of water flowing within it.  

The narrower the hose, the higher the pressure. The larger the hose, the lower the pressure. Also, if the hose were elastic and can expand, the more it expands, the lower the pressure. Additionally, as more water surges through the hose, the more the pressure.  

In other words, the larger the amount of water flowing through it, the higher the pressure and of course the pressure would be even higher within a smaller hose.  

In simple terms, understanding blood pressure is akin to understanding the pressure of water flowing through a hose.

In reality, within our body, the fluid is blood and the “hose” is the blood vessel. Now, under certain circumstances, the blood volume may increase and in others, the vessels can become narrower or larger. Hypertension is when this pressure inside our blood vessels is too high and exceeds 140/90mmHg when measured repeatedly. The higher the pressure, the more severe the hypertension.  

Why don’t we take a look at these two things separately. The volume of blood can actually increase and this can happen with increased sodium (salt) uptake. This is the reason why when someone with high blood pressure sees a doctor, he is almost always told to cut down his salt intake.  

Salt actually draws water. Our body has evolved to conserve salt as land creatures because salt was a commodity in short supply in the wild. We still do lose salt with everyday activity, for example through our sweat, in our body fluids and stomach acid. However, the amount is very small and with the prevailing high intake of processed foods, the amount of salt intake is far higher than what is lost.  

We should not exceed 6g of salt per day. Otherwise, in the end, there will be consequent nett salt gain. The blood volume actually increases. Of course, hormonal effects will also come into play to achieve this effect.  

The second point involves our blood vessels. Some of these can change in diameter depending on certain factors. We call it the vessel tone. The vessel tone is actually a way of regulating the diameter or calibre of the blood vessels. When we look at the bigger picture, small changes in the diameter of the vessels can make a lot of difference.  

The total amount of blood vessels that you have in your body when opened out can be quite extensive, stretching several kilometers! They start with larger vessels called arteries, and branch out into smaller vessels called capillaries. These arteries are able to relax and widen. They can also contract and become smaller in size.  

The other type of blood vessels, called veins, are unable to do so. These vessels are controlled via nerves or hormonal mechanisms that in turn control the calibre. Stress, which may be physical or mental, is a good example of a condition that can narrow blood vessels and increase blood pressure.  

Picture this – increased tone decreases the diameter, making it narrower. Of course, with decreased diameter, there is increased resistance. This results in an increase in blood pressure within the blood vessel.  

Moreover, it follows that if you have vessel walls which are stiffer, as happens with age, the blood vessel is less able to expand with each surge in blood flow within it. It is rather like squeezing water in a long balloon. In the soft balloon, squeezing one end will force water to the other end while maintaining the same pressure inside. However if it were rigid, the increased surge of blood does not allow expansion of the vessel and therefore the pressure within will increase.  

Remember in your body, blood flows in a pulsatile way, as many times a minutes as your heart beats. Blood surges forcefully forward with each beat and flows less forcefully between heart beats.  

The pulsatile flow of blood gives rise to the "top" and "bottom" values in a blood pressure recording.

This pulsatile flow gives rise to the “top” and “bottom” values in a blood pressure recording. We call it “systolic” and “diastolic” pressure. It follows then that each time a heart beat causes the heart to contract, forcing blood out, this will be measured as a systolic reading. In between beats, the vessels return to the original diameter and in doing so continues to push the blood forwards although at a slower rate and lower pressure – hence the “diastolic” recording. What determines the diastolic pressure is the vessel tone and size of the smaller vessels and capillaries.  

Some of the issues which have come out is the importance of the systolic or diastolic blood pressure, or perhaps it is the difference between the two, called “pulse pressure” that is important. Over the years, many studies have shown the diastolic blood pressure reading as being important in its association with the development of long term complications.  

However these values are different in the strength of their association with the development of complications. Examples of complications are heart attacks, stroke and kidney failure.  

Some studies suggest that the top blood pressure (systolic) is more reliable in terms of forecasting possible complications. In other words, the higher the systolic blood pressure, the more association there is with the likelihood of developing cardiovascular disease or stroke. Every effort must therefore be made to try and lower this value.  

The risk for developing these events in patients with systolic blood pressures of more than 160 mmHg is 2 ½ times higher than those whose systolic blood pressure was less than 140 mmHg. Those with systolic blood pressures between 140 and 160 mmHg, corresponding to a mild increase in blood pressure, had a two-time increase in mortality or risk of dying. What this means is that blood pressures should be as low as can be comfortably tolerated by an individual, regardless of age.  

This is contrary to the popular belief, at least when I was a younger doctor, that it was appropriate for someone to have a blood pressure of 100 plus his age. In other words, for a 70-year-old man, it was mistakenly believed that he was not at risk walking around with a systolic pressure of 170 mmHg. This is especially true of older patients, who, as their vessels became less elastic, tend to have higher systolic readings.  

Younger hypertensives more often tend to have higher diastolic readings. In a 1996 Health and Nutrition Survey in the US (NHANES III), it was found that 26% of hypertensives were below 65 years and had more diastolic hypertension whilst 74% were over 65 years and had systolic hypertension.  

So what is the ideal blood pressure? The blood pressure should really be reduced to about 140 mmHg and less for systolic and less than 80 mmHg for diastolic pressure. Remember, the top value reflects the elasticity of the blood vessel, and as one ages and as the blood vessel becomes less elastic, there is more tendency for the systolic reading to be higher.  

Those who have an elevated systolic value would be called systolic hypertensives. The lower pressure represents the resistance of the blood flow to the system and this can be high, usually in younger persons. It can of course be high in older patients too. How did this value of 140/80 mmHg come about? 

This is interesting. Blood pressure was actually recorded amongst a population of individuals in Framingham, a town in the US. The number of complications such as stroke and heart attacks were recorded for each corresponding blood pressure reading. It was found that above 140/90 mmHg, there was a marked increase in the number of events such as heart attacks and stroke. It was therefore decided that the best blood pressure would be below this value.  

Remember this was a population study involving westerners. It is not known what a truly “normal” blood pressure of a human being should be. Amongst “primitive” cultures in South America and Malaysia, the systolic blood pressure can be as low as 90 to 100 mmHg systolic. The general rule therefore is that the lower blood pressure is, the better it is, providing it does not cause dizziness or fainting.  

Most physicians try to aim for as low a blood pressure as possible. It is not always possible because some individuals become giddy below a certain level of blood pressure.  

Why don’t we now look at hypertension in the Malaysian context. The problem with hypertension here is that many patients who have hypertension are undiagnosed. In Malaysia, just under 30% of hypertensives are never diagnosed and only present themselves to hospitals when complications arise.  

This was the finding in the latest National Health and Morbidity Survey. Worse still, 4.1% are never on medications. The complications can be life-threatening and include stroke, heart disease, and of course kidney failure and even heart failure.  

In fact uncontrolled hypertension is the single most important predictor of developing heart failure later on in life. It contributes to 27% of all kidney failures requiring dialysis; second only to diabetes as a cause. It follows therefore that the sooner one discovers and treats blood pressure elevations, the better the outlook is.  

It is rather like investing in your future health. Therefore it is important for everyone to have their blood pressure checked at least once as teenager and of course as you reach adulthood, perhaps once every year. It should be done more often if it is abnormal.  

After the age of 40, a yearly check-up may be a good idea. The object of the whole exercise is to actually detect blood pressure increases as early as possible and to find out any treatable causes of high blood pressure. We will discuss the various causes of hypertension later. 

The Malaysian Hypertension Club and website was recently set up by Malaysians for Malaysians to enable the public to be informed about the latest on this condition. It will be launched on August 2, 2003, by the Minister of Health. 

  • This article is a contribution of The Star Health & Ageing Panel, a group of panellists who are not just opinion leaders in their respective fields of medical expertise, but have wide experience in medical health education for the public. 

    This group of specialist doctors and members of the academia are committed to public education, and the weeks ahead will see numerous articles on aspects of healthy ageing and the prevention of disability in men and women. 

    The members of the panel include: Datuk Dr Tan Hui Meng, consultant urologist; Dr Yap Piang Kian, consultant endocrinologist; Dr Azhari Rosman, consultant cardiologist; A/Prof Dr Philip Poi, consultant geriatrician; Dr Hew Fen Lee, consultant endocrinologist; A/Prof Sarinah Low, psychologist; Dr Nor Ashikin Mokhtar, consultant obstetrician and gynaecologist; Dr Guna Sittampalam, consultant obstetrician and gynaecologist; Dr Ting Hoon Chin, consultant dermatologist.  

    The Star Health & Ageing Advisory Panel provides this information for educational purposes only and it should not be construed as personal medical advice. Information published in this article is not meant to replace, supplant or augment a consultation with a health professional regarding the reader’s own medical care.


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