Imagine you are preparing for a water balloon fight. If you fill the balloons only half full, the pressure of the water won’t exert enough force against the balloons and the balloons won’t burst when thrown at an opponent.

If you fill the balloons too full, they’ll burst in your hand when you try to throw them.

What is needed for a good battle is balloons that are full enough to burst on impact, but not so full that they can’t be thrown.

We don’t measure the pressure of each balloon precisely, we judge the optimal fullness by eye and feel.

Unlike a balloon fight, there are situations where measuring the exact pressure a liquid exerts against a surface is important. And to measure, we need a standard to compare to. What scientists came up with is how much does a one-millimeter column of mercury weigh. That is, how much pressure does the weight of it exert.

Mercury is poisonous, so it’s not commonly used to measure pressure any more, but pressure is still noted in millimeters of mercury. Hence your blood pressure numbers are written in this fashion: 120/80 mmHg. The mmHg consists of two parts. The mm stands for millimeters. And Hg is the scientific designation for mercury. Put together, mmHg means the pressure something exerts compared to millimeters of mercury.

Exerts against what? In the case of blood pressure, exerts against the walls of your arteries.

Think of arteries as being similar in a way to water balloons. The pressure in your circulatory system as your heart beats puts pressure on your artery walls. Healthy arteries can accommodate that pressure, like a balloon can. When the pressure eases between heart beats, the arteries are none the worse for wear.

Unlike water balloons, we don’t want our arteries to be just shy of bursting. We want the pressure of our blood to be in a range that our arteries can easily handle.

When blood pressure is taken, two things are measured: how much pressure is put on the arteries each time the heart beats and how much pressure is there between beats.

The first measurement (called systolic) is important because we don’t want too much pressure on our arteries.

The pressure between beats (called diastolic) is important because for the heart to fill back up with blood for the next pump, there needs to be enough pressure in the circulatory system for blood in the veins to travel back to the heart. Blood in arteries is being pumped on its way; blood in veins is dependent on pressure in the system for it to move.

The two numbers in a blood pressure reading — say, 120/80 mmHg — are the measure of how much pressure there is on the arteries during a heart contraction and how much pressure there is between contractions. If either number gets too far out of whack, danger can ensue. Hence the ritual of measuring your blood pressure each time you visit the doctor.

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