You're at the doctor's office, feeling the blood pressure cuff squeezing your arm—then you wait as the nurse lets the air whoosh slowly out and listens to your pulse through the stethoscope. Next, you find out the verdict: the two numbers that could mean your blood pressure is too high. So what do these two numbers represent, exactly? What do your blood pressure readings mean—and why should you care about them? What happens during a blood pressure check? To measure your blood pressure, your doctor or nurse will put a cuff around your upper arm and inflate it, essentially cutting off blood flow in the brachial artery carrying blood from your shoulder to your elbow. Next, the provider slowly releases this pressure, which lets blood flow back into your artery. He or she listens to this blood flow via a stethoscope, and also watches the pressure reading on a gauge attached to the cuff. This gauge indicates the pressure in the artery in units of millimeters of mercury (mmHG). Your blood pressure reading includes two numbers. The first, or top, number is the systolic pressure, which reflects the force of your blood inside your arteries when your heart muscle contracts. The number is recorded when the first heartbeat is heard through the stethoscope. The second, or bottom number, is your diastolic pressure, which shows what happens in your artery between heartbeats—the degree to which your artery relaxes and opens to allow the heart muscle to refill with blood. The number is recorded the moment the heartbeat becomes too quiet to hear. A healthy blood pressure reading is one where the top number is less than 120 mmHg and the bottom is less than 80 mmHg. This is more typically reported this way: 120/80. What’s considered high blood pressure? High blood pressure, called hypertension, is broken into several diagnostic stages, based on pressure readings: Prehypertension If your top number (systolic pressure) is between 120 and 129 and your lower number (diastolic pressure) is higher than 80, you are considered to have prehypertension. This means your blood pressure is higher than is healthy, but not high enough for a diagnosis of hypertension. You don't usually get prescribed medication for prehypertension—but your doctor will warn that you are at risk of developing high blood pressure. You’ll be advised to make lifestyle changes, such as losing weight, quitting smoking, exercising and eating a healthier diet, which includes cutting down on salt. Stage 1 hypertension If your top number on at least two consecutive readings is 130-139 or your lower number is 80-89, you are considered to have stage 1 hypertension. Your doctor will usually recommend medication at this stage. Stage 2 hypertension If your pressure is 140 or higher for a top number or 90 or higher for a lower number, you are considered to have stage 2 hypertension. This also will be treated with medication. Hypertensive crisis If you have a reading of 180 or higher for a top number and 120 or higher for a lower number, you are considered to be in a hypertensive crisis. Your blood pressure is high enough to cause damage to your organs or other serious problems, so emergency treatment is needed. Call 911 or go to the hospital right away if you get this reading on a home monitor. How can I make sure my blood pressure really is high? Some people have what's called "white coat" syndrome. Their blood pressure goes up due to anxiety about being in the doctor's office. Other people relax more in a medical setting and have pressure readings that are lower than in their daily lives. If a doctor suspects the office readings aren't representative of your true pressure, he or she may suggest measuring your blood pressure with a home monitor or at a pharmacy or other public site to compare with your office readings. You may also be tested with a monitoring device that you wear for 24 hours. This device will automatically take your blood pressure at various times to get a more accurate average reading. Blood pressure varies for most people throughout the day. Pressure in the morning is generally higher, but it can range quite a bit depending on activities, such as exercise or eating; stress or excitement; or whether you've had caffeine or other stimulants. Taking a sampling of pressures during your day can give a better picture than a one-shot reading in the doctor’s office. The American Heart Association recommends people older than 20 with normal blood pressure—less than 120/80—get their blood pressure checked by their doctor at least every two years. If your pressure is higher, your doctor will likely advise you to have it checked more frequently so it can be kept under control.