Diastole vs. Systole: A Guide to Blood Pressure


When you visit your doctor, the first thing they often do is check your blood pressure. This is important protocol because your blood pressure is a measure of how hard your heart’s working.

Your heart is a muscle about the size of your fist. It has two sides, an upper and lower chamber, and four valves. The valves open and close to let blood through. According to the National Heart, Lung, and Blood Institute, your heart beats 100,000 times a day. As it beats, blood is forced against your artery walls.

Your systolic blood pressure is the top number on your reading. It measures the force of blood against your artery walls as the ventricles of your heart squeeze and blood is pushed out to your body. Your diastolic blood pressure, the bottom number on your reading, measures the force of blood against your artery walls as your heart relaxes and the ventricles are allowed to fill. Diastole is also the time that your coronary artery is able to supply blood to your heart.

Blood pressure ranges

Your blood pressure may be normal, high, or low. High blood pressure is also referred to as hypertension, and low blood pressure is called hypotension. The different blood pressure ranges are:

  • normal: less than 120 systolic and less than 80 diastolic
  • prehypertension: 120–139 systolic or 80–89 diastolic
  • stage 1 hypertension: 140–159 systolic or 90–99 diastolic
  • stage 2 hypertension: 160 or higher systolic, or 100 or higher diastolic
  • hypertensive crisis: higher than 180 systolic or higher than 110 diastolic
  • hypotension: 90 or less systolic, or 60 or less diastolic

Your doctor may diagnose high blood pressure if either your systolic or diastolic number is high. Some doctors pay closer attention to systolic blood pressure because it’s a risk factor for heart disease in people over age 50. A high systolic blood pressure with a normal diastolic blood pressure may indicate artery stiffness, a heart valve issue, or hyperthyroidism.

Systolic blood pressure may be more difficult to control than diastolic. Treating a high systolic blood pressure can also be problematic, as medications that treat the condition may lower diastolic pressure too much.


High and low blood pressure risk factors

If you’re a man older than 45 or a woman older than 65, you’re at an increased risk of high blood pressure. Your risk is also higher if:

  • you have a close relative with high blood pressure
  • you’re African-American
  • you don’t get much physical activity
  • you’re overweight or obese
  • you drink too much alcohol
  • you’re a smoker
  • you experience chronic stress

Sleep apnea, a condition that causes you to stop breathing one or more times during sleep, is another cause of high blood pressure that’s often overlooked. According to Dr. Virend Somers of the National Sleep Foundation, when your breathing pauses, your oxygen levels fall and your blood vessels constrict. This increases your blood pressure. When sleep apnea is persistent, this response may continue during the day when breathing is normal.

If you’re older than 65, you may be at risk for orthostatic hypotension, a condition in which your blood pressure drops when you move from sitting to standing. Endocrine problems, heart failure, or anemia may also cause the condition.

You may also be at risk for low blood pressure if you become dehydrated or take certain prescription drugs such as:

  • high blood pressure medications
  • diuretics
  • nitrates
  • anxiety or depression medications
  • erectile dysfunction medications


Treating high or low blood pressure

Lifestyle changes are often the first line of defense for prehypertension and stage 1 hypertension. These changes may include:

  • eliminating unhealthy foods from your diet
  • eating more heart-healthy foods such as fatty fish, fruits and vegetables, and whole grains
  • cutting back on sodium in your diet
  • exercising regularly
  • quitting smoking
  • maintaining a healthy weight
  • reducing alcohol consumption
  • managing stress
  • monitoring your blood pressure regularly

If lifestyle changes aren’t enough to bring your blood pressure numbers down, or if you have stage 2 hypertension, your doctor may prescribe one or more medications such as beta-blockers, calcium channel blockers, or alpha-blockers.

To treat low blood pressure, your doctor might recommend stopping or adjusting the dosage of any medication that’s causing this condition. Your doctor may also suggest you drink more water, adjust the amount of sodium in your diet, or prescribe a medication to treat low blood pressure.


High or low blood pressure complications

High blood pressure doesn’t cause symptoms unless you’re in hypertensive crisis. It’s actually known as a “silent killer” because it quietly wreaks havoc on your organs, and you may not realize it until the damage is done. Unmanaged high blood pressure can cause:

  • stroke
  • heart failure
  • heart attack
  • vision problems
  • kidney disease
  • sexual dysfunction

Low blood pressure may or may not cause symptoms. Symptoms that may occur include:

  • dizziness
  • fainting
  • falling
  • loss of balance
  • nausea
  • thirst
  • inability to concentrate
  • blurred vision
  • depression
  • fatigue
  • shallow breathing
  • clammy skin

For many people, high blood pressure is manageable. Your outlook is best if you take lifestyle steps that support overall heart health and follow your doctor’s advice about medications to lower your blood pressure numbers.

Because high blood pressure doesn’t cause symptoms, once you’ve been diagnosed it’s critical to measure your blood pressure regularly, even if you’re taking blood pressure medication.

Whether you’ve got high or low blood pressure, tracking your systolic and diastolic numbers is a great way to gauge how well lifestyle changes or medications are working.


Preventing blood pressure problems

You may head off blood pressure problems before they start, or limit your risk, if you live a healthy lifestyle. If you smoke, try to quit right away. Limit alcoholic drinks to one per day if you’re a woman, and two per day if you’re a man.

If you suspect you have sleep apnea, talk to your doctor about a sleep study. Research shows that using a CPAP machine while sleeping can significantly reduce systolic and diastolic blood pressure, and improve arterial tone in people with sleep apnea.

To help prevent low blood pressure, drink plenty of fluids, preferably water, to prevent dehydration. Stand up slowly from a sitting position to help prevent orthostatic hypotension. Notify your doctor right away if you feel a medication is causing your blood pressure to drop. There may be another option that will have less impact on your blood pressure numbers.

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