[Geezer (OP)]

Having just been to the doc, I realized I don't have a clue about what my BP measurements actually mean, and for that matter, how the measurement process really works. I understand what the instument does, but how does it measure blood pressure?
 
(Should this be a technology question?)

[RD]

Beware of the white coat effect.

If you are considering buying your very own sphygmomanometer to avoid the above phenomenon, note the upper-arm-cuff type are more accurate than the wrist-cuff type ...

http://www.dableducational.org/pdfs/september05/051-05_Wrist_device_inaccuracy_JBP_Monit_May2005.pdf

[SeanB]

I do not have the white coat effect. My GP thought I had mild hypertension until I was there one day for non medical reasons and asked him to check BP. Was normal, and he agrees that the only time he normally saw me was when I was sick, and had a high pressure from that. Statistically he has a higher risk of heart attack than me,  and needs to look out for himself that he does not have one - good doctors that are good conversationalists as well are hard to find.

[Geezer (OP)]

Yes, yes! Thanks all, but in words of less than two syllables, what the f does it all mean?

[CliffordK]

You will get two blood pressure readings.
Systolic and Diastolic

When you put the cuff on, you don't hear anything until the pressure reaches Diastolic (the lower number), and can hear the pulse up to the Systolic reading (the upper number).  If you pay attention, you can also feel the different phases while the pulse is being taken.

Essentially Systolic is the maximum pressure that your heart puts out. 
Diastolic is essentially the residual pressure in the arterial system between pumps.

You may feel "normal" with high blood pressure, and perhaps light-headed with very low blood pressure.  High pressures are associated with heart attacks and other complications.  Medications can reduce those risk factors.  You should find guidelines online.

Your blood pressure obviously can vary with exercise and activity.  The standard is to take it at "rest".
It can also vary with laying down, sitting, and standing.  If you have too much of a drop from laying to standing, it can also be indicative of light headedness, and perhaps low water volume.

[Mazurka]

What Clifford said - maximum and baseline presure in the arterial side of the circulatory system.

It also naturally varies through the day - so if you have high BP in the docs, but want to resist being put on tablets so test yourself at home to produce a more useful set of data, do it at the same time of the day...

Stress is another cause of high BP

(Isostatic pressure from water can also make a significant difference to BP - although this is because of the physiological effects of immersion.  The key consequence of this is that a casualty should be removed from water horizontally - and it is also why if you get up out of deep bath suddenly that you can feel faint.)   

[Geezer (OP)]

Ah right - thanks everyone!

So, translating to "engineering speak", would the systowotsit number be the static pump head and the diawotsit number be the dynamic pressure in the plumbing?

[CliffordK]

Ah right - thanks everyone!

So, translating to "engineering speak", would the systowotsit number be the static pump head and the diawotsit number be the dynamic pressure in the plumbing?

I thought it was the other way around, but in plumber speak...  yes.
Or...  the systolic - diastolic = the pressure difference.

[Nizzle]

Basically, diastolic BP is the pressure of the blood on your vascular system 'between' heart beats, or the arterial pressure if your heart would not be beating.
Systolic BP is the pressure right after a heart beat radiating from the heart through arteries -> arterioles -> cappillaries in which the BP get's reduced to just above zero when it enters the venous system.
The systolic BP 'moves your blood forward', the diastolic BP is the 'static BP'.

The arterial system is a system of flexible stretchable pipes. Through various causes, your arterial system may become less flexible/stretchable and more rigid/stiff. This causes hypertension.