by 
The Curious Bag
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Systemic blood pressure is an important measure of overall cardiovascular health. However, the two numbers given to you by health care professionals may seem like medical jargon without an explanation. The numbers are by no means arbitrary.
Number Meanings
The numbers given for blood pressure are in mmHg, the amount of pressure exerted by a certain amount of liquid mercury. For instance, 120 means the amount of pressure exerted by a 120-millimeter column of mercury.
First Number
The first number given, the larger of the two, refers to systolic blood pressure, according to the textbook Human Anatomy and Physiology. Systolic pressure is the point when the heart pumps blood from the left ventricle into the aorta.
Normal Range
A healthy human systolic blood pressure usually falls between 110 and 140 mmHg. A consistent reading of 140 or more is an indicator of high blood pressure, or hypertension.
Second Number
The second number refers to diastolic pressure, the time during the cardiac cycle when the heart is at rest. Blood continues to flow during this time because the arteries are elastic and still exert pressure on the bloodstream.
Normal Range
A healthy human diastolic pressure is roughly 80 to 90 mmHg. Although not as directly related as systolic pressure, a higher diastolic pressure can eventually contribute to hypertension.
Source:
"Human Anatomy and Physiology, Sixth Edition;" Elaine N. Marieb, R.N., Ph.D.; 2004
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