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Examples of the use of derived units in nursing

Let us look at just two examples of how derived units can be used in the context of nursing. Pressure and energy have been selected:


When we study the physiology of respiration during the nursing course, we shall be looking at the way oxygen is absorbed into the blood and carried to the tissues. (The tissues need oxygen for their processes of living.) The uptake of oxygen by the blood will be influenced by many factors, amongst which will be the overall pressure of the air we breath and the proportion of the air that is made up of oxygen (usually about 1/5th). To understand what is going on during normal physiology, and also when high oxygen levels are being administered therapeutically, we shall need to refer to gas pressures. Therefore, the SI unit of pressure - the pascal - will be referred to.

measuring BPHowever, an older way of expressing the pressure in ‘millimetres of mercury’ (abbreviated to mm Hg) is still in use, for example when blood pressure is being measured. It refers to the height of a column of mercury that can be supported by the pressure being measured. Thus, a systolic blood pressure of 120 mm Hg means that a column of mercury 120 mm high could be supported by the blood pressure during systole of the heart - when the ventricles of the heart contract and the pressure is at its highest.

So the SI units of measurement have not yet gained universal acceptance, but are becoming increasingly commonplace.


Have you ever looked at the nutritional information given on food packaging? In amongst the information about the carbohydrates, fats, proteins, colourings and flavourings you will see a figure (or figures) relating to the energy content. Energy content is usually expressed either in kilojoules or in kilocalories. (A calorie is the amount of energy required to heat 1 gram of water through 1 Celcius degree - because this is a small quantity of energy, it more common to use a unit known as the kilocalorie or Calorie - this is equivalent to 1000 calories.) These two units - joules and calories - are directly related: if you need to convert one to the other, you can use the following equation:

nutritional information

the number of calories multiplied by 4.186 = the number of joules

or: the number of kilocalories multiplied by 4.186 = the number of kilojoules

Knowledge about nutritional information is important to nurses, particularly now that people are being encouraged to maintain and enhance their own health through eating a good diet and incorporating exercise into their weekly activities. Nurses are key contributors to this process of health promotion. Good nutrition is also an important for normal growth and development of children, and during recovery from illness or operation.

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