If you've ever had your blood pressure checked, you know the drill. The nurse puts the big cuff around your upper arm, pumps it up, and the blood pressure reads. However, did you ever wonder why it's your upper arm that gets the big squeeze, rather than your wrist or your leg?

There's a large artery on the inside of your upper arm, so it's easy to check your blood pressure there, but there's another reason why we check blood pressure in the upper arm. Your heart isn't the only thing that creates pressure in your circulatory system. You also have to adjust for gravity! If you've ever felt dizzy or seen "stars" after standing up too quickly, you've already experienced the clash between circulation and gravity!

Gravity pulls your blood downward. Your circulatory system has to work harder to push the blood from your legs back to your upper body, so the blood pressure in your legs and feet is usually higher than the pressure above your waist. To get an accurate reading of your blood pressure, it has to be checked at about the same altitude as your heart. If you're lying down, of course, it doesn't matter, any artery would work, since your whole body is at about the same level. If you're sitting or standing, the big squeeze needs to happen at about the same height as your heart, in other words, your upper arm.

The two numbers in a blood pressure reading are actually two pressures. The first, the systolic number, measures how hard your heart is pumping blood into your arteries. The second number, the diastolic, measures your blood pressure in-between heartbeats.