Why does the doctor take your blood pressure? Because doing so gives him or her valuable information about the state of your cardiovascular system.
If your blood pressure is too low (known as hypotension), you may feel dizzy and even pass out. If it's too high (known as hypertension) over a long period of time it can put undue stress on you heart and blood vessels and ultimately result in a heart attack or a stroke.
Think of a hose with the nozzle shut off but the water turned on. Over time, the elastic hose swells due to the pressure. Expansion gradually damages the "vessel wall"? of the hose; eventually it may even burst. This is analogous to the danger of long-term hypertension, only the "hose"? consists of your heart and the blood vessels in your brain. Control of hypertension is one of the 5 steps to a longer life I've discussed previously.
Hypertension is not a new disease. Our ability to treat and control it is relatively new. As recently as 1945 the President of the United States (Franklin Roosevelt) died from a stroke caused by uncontrolled hypertension. Today, 67 million Americans have hypertension. But 36 million people are not being adequately treated, according to the New York Times. Some of these people do not even know they are hypertensive, while others choose to ignore this dangerous, silent, and even painless (until it's too late) condition. A disturbing third group is receiving treatment inadequate to control the disease.
Systolic pressure (the first number) is the pressure during cardiac contraction. Diastolic pressure (the second) is the pressure measured when your heart is at rest, in between beats (contractions).
When is your blood pressure too high? This is actually not such an easy question. Blood pressure varies depending on your age, physical state (at exercise or rest), and even your position (standing versus lying down). Yet we do know that there is a direct relationship between blood pressure and life expectancy.
The lower your blood pressure (down to about 100/60 -- below which point it would be hard to stand up), the longer your life. If your blood pressure is 200/110 it is clearly too high. If you have blood pressure that high and are experiencing chest pain, a headache, shortness of breath, or blood in the urine, take an ambulance to the ER immediately. You are experiencing a hypertensive crisis! (Not defined by the specific BP but rather by the symptoms of organ damage.)
But what about BP of 140/90, previously regarded as the "cutoff"? for the diagnosis of hypertension? Doctors are beginning to realize that even much of the middle ground, between this number and 120/80 (so-called "normal"? or "ideal"? blood pressure) results in cardiovascular damage over long periods of time.
So what should a person do? Have your BP measured periodically and record the numbers to see trends. Avoid table salt ("sea salt"? is no better - it's the same NaCl) and salty foods (pickles, chips, etc. -- read food labels). Keep your weight within the normal range. Talk to your doctor about treatment when indicated and the effectiveness of treatment when implemented.
Enjoy your life in good health.