Simply put, Blood Pressure is the amount of pressure inside an artery. Because this pressure varies as the heart contracts and relaxes, it is recorded as a high and a low value. The High value, systolic, value is the maximum pressure as the ventricles of the heart contract. The Low value, diastolic, is the minimum pressure while the ventricles are relaxing.
Blood pressure is measured in millimeters of Mercury (mmHg) which is a unit of pressure derived from the pressure it takes to move a certain amount of Mercury up a glass tube. Most of the actual Mercury manometers have been phased out and replaced by digital or mechanical measurements of pressure, but the units have remained. Blood pressure is typically measured in a sitting position with the arm at heart level. Measuring the blood pressure in a standing position will result in an artificially high value. Measuring the blood pressure in a supine (laying down) position will result in an artificially low pressure. As it is sometimes not reasonable to move the patient to a sitting position, any measurement from any other position is simply noted and accounted for. In reality, most of the time that you are measuring a blood pressure you are looking for a change from that particular patients normal, so if the patient is always lying down, you will be able to see a upward or downward trend even if the initial measurement isn't exact. If you are simply examining the patient for chronic Hypertension it is best to measure from the sitting position.
You will see a lot of different numbers thrown around in reference to blood pressure, but here are some of the common ones that will hopefully help you understand what the numbers mean.
Normal Average: The normal Average blood pressure is 120/80mmHg. This means that if you took and average of all the people who would be considered to have a normal blood pressure, the average would be 120/80. In other words, this is what people shoot for, but it is okay to be slightly above or below this number.
Normal Range: There are actually two sets of normal ranges, one for systolic and one for diastolic. The traditional Systolic normal range is 110-140mmHG, but in the last couple years they have changed the standard, so that 140mmHg actually considered high now. The Diastolic rage is 60-90, and again, 90 is now considered high. There is also a small upper normal range which is now considered "pre-hypertension"
Pre-Hypertension: A Systolic pressure of 130-140mmHg (the upper range of normal) is now considered Pre-Hypertension. This means that while it is not high it is potentially worrisome, and my be treated by your physician. A Diastolic pressure between 85-90 is also considered Pre-Hypertensive.
Hypertension- (High Blood Pressure) If either your Systolic or Diastolic is above the upper limit of the normal range, you are considered to have Hypertension, and this it usually treated with diet and exercise, medication or some combination of the two.
Hypotension- (Low Blood Pressure) If either your Systolic or Diastolic pressure drops below the normal range, then you are considered to have Hypotension. Often this is not of concern, and many people walk around with low blood pressure and don't have any issues whatsoever. Hypo-tension is primarily a concern when it is a sudden drop or the drop is a significant change from the patients personal normal value. In trauma cases, Hypotension is often an indication of a high volume of lost blood.
Pulse Pressure: Probably the least discussed of the numbers associated with blood pressure, the pulse pressure is a calculation of the difference between the systolic and diastolic pressures. A low pulse pressure indicates that the compression of the heart is having a small impact on the pressure in the artery. I high Pulse pressure indicates that the pressure is dropping drastically between compressions of the heart.