Hi There Hazel,
First we need to define what those numbers :)
Ok first number (Always first number) is the systolic pressure. The second number is diastolic pressure. These are measured in mmHg (millimeters mercury)
Ok so what does this actually mean right?
Some physiology follows:
As blood is ejected from the heart (left ventricle to be specific), the heart has to overcome a valve found on the aorta
(largest blood vessel in the human body, about the size of your thumb), as well as the force exerted by the blood vessel walls which is indirectly determined by the caliber of the lumen (inside) of the aorta and its compliance (elasticity, ability to expand). This is basically called the systolic pressure and it is modified by many factors including heart rate
, constriction of the aorta (size of the tube). So you can think of it this way. Imagine you are trying to squeeze a balloon full of water and that this water must go through a straw you put in it. As you can imagine the larger the caliber of this straw the easier it is for water to pass. Anyway, when the physican/nurse puts the stethescope (thing they wear around their neck), what they are doing is hearing the sound of the aortic valve
This is the pressure that is formed when the heart is under relaxation and dilation of the ventricles.
So just to say it one last way.
The pressure in your blood vessels is at its highest point with each beat of your heart; systolic portion of the heart beat. When your heart is maximally contracted, it has just sent a substantial volume of blood through your blood vessels, namely aorta which is what we read when measuring blood pressure. So the pressure on those blood vessel walls in that moment is going to be as high as it can be. This pressure is called your systolic blood pressure.
The pressure in your blood vessels is at its lowest point whenever your heart is fully relaxed (diastolic phase). Your heart has to fully relax after each full contraction in order to fill up with more blood. This low point of pressure is called your diastolic blood pressure.
According to JNC-7 (Joint National Committee):
Optimal Blood Pressure: <115/80
Normal Blood Pressure: <120/80
Stage 1 Hypertension: 140-159/90-99
Stage 2 Hypertension: >160/100
I really hope this helps you understand your body a little bit more.