So there you are, crouched low on the ground, tying your shoe or inspecting some interesting bit of trash or sidewalk art or spring flower...and everything's cool until you stand up too fast. Then comes the head rush. Suddenly you're all dizzy and feeling like you might black out, and just as you think you're gonna fall over, everything's back to normal, and you're left wondering what the heck just happened? 

Well, probably your blood pressure dropped is what happened. Your blood pressure will vary a little throughout the day, like if you're stressed out or relaxing, running or sleeping, but in general it exists within an average range. That said, individual's normal ranges also vary -- for example, athletes typically have lower blood pressure than overweight smokers -- and what's normal for you might be considered low or high for me. We've all heard about how elevated blood pressure, or hypertension, can be dangerous. It puts extra strain on the heart, and can cause arterial walls to leak or rupture -- but low blood pressure, or hypotension, can also cause problems: like dizziness or fainting. Lots of conditions can lead to temporary or chronic low pressure, like pregnancy, dehydration, anemia, or taking various medications. But there are a specific types of hypotension, too, when it's specifically associated with things like eating or nerve damage.

When you stand up too fast and get that wobbly, head-rushy sudden drop in blood pressure, doctors call that orthostatic, or postural hypotension. It happens because even though your blood keeps circulating no mater what position you're in, when you stand, gravity causes some of the blood to collect in your legs. Your body is supposed to combat this by constricting blood vessels and ramping up your heart rate -- effectively increasing the pressure to help keep enough blood flowing through important places like your brain. But sometimes when you jump up too quickly, that sudden change in blood flow and drop in pressure can strike before your circulatory system has a chance to recalibrate, and now you're seeing stars and groping for balance -- which is basically your desperate brain's way of telling you it needs more oxygen. Usually it only takes a few seconds for the adjusted blood pressure to kick in, but if that initial drop is too extreme, you could actually faint. If you're feeling faint all the time, especially when you haven't just stood up quickly ... well, you might want to get that checked out. But lots of people get head rushes now and then, and it isn't a big deal. Still, if you'd rather avoid the dizziness, just remember to take your time standing up the next time you tie your shoes. 

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