Blood Pressure

Image Caption : Healthy and Unhealthy Blood Vessel : Some blood pressure medications work by increasing vessel dilation and blood flow. Blood pressure medications are grouped into several classes. Each class works in a different way to lower blood pressure. One example is the ACE inhibitors (e.g., enalapril, lisinopril, ramipril). This class of medications works by blocking the body's production of a substance that tightens the blood vessels. As a result, the blood vessels become more relaxed, and the pressure inside the vessels decreases. Angiotensin II receptor blockers (e.g., candesartan, losartan, valsartan) is another group that works in a similar way as the ACE inhibitors. But instead of blocking the production of the blood vessel-tightening substance, it stops the substance from working on the blood vessels. This causes the blood vessels to relax and the blood pressure to decrease.

blood pressure

force exerted by the blood against the wall of a vessel or heart chamber; can be described with the more generic term hydrostatic pressure

Measurement of Blood Pressure

Blood pressure is one of the critical parameters measured on virtually every patient in every healthcare setting. The technique used today was developed more than 100 years ago by a pioneering Russian physician, Dr. Nikolai Korotkoff. Turbulent blood flow through the vessels can be heard as a soft ticking while measuring blood pressure; these sounds are known as Korotkoff sounds. The technique of measuring blood pressure requires the use of a sphygmomanometer (a blood pressure cuff attached to a measuring device) and a stethoscope. The technique is as follows:

  • The clinician wraps an inflatable cuff tightly around the patient's arm at about the level of the heart.
  • The clinician squeezes a rubber pump to inject air into the cuff, raising pressure around the artery and temporarily cutting off blood flow into the patient's arm.
  • The clinician places the stethoscope on the patient's antecubital region and, while gradually allowing air within the cuff to escape, listens for the Korotkoff sounds.

Although there are five recognized Korotkoff sounds, only two are normally recorded. Initially, no sounds are heard since there is no blood flow through the vessels, but as air pressure drops, the cuff relaxes, and blood flow returns to the arm. As shown in Figure, the first sound heard through the stethoscope-the first Korotkoff sound-indicates systolic pressure. As more air is released from the cuff, blood is able to flow freely through the brachial artery and all sounds disappear. The point at which the last sound is heard is recorded as the patient's diastolic pressure.

Blood Pressure Measurement

This image shows blood pressure as a function of time.

When pressure in a sphygmomanometer cuff is released, a clinician can hear the Korotkoff sounds. In this graph, a blood pressure tracing is aligned to a measurement of systolic and diastolic pressures.

The majority of hospitals and clinics have automated equipment for measuring blood pressure that work on the same principles. An even more recent innovation is a small instrument that wraps around a patient’s wrist. The patient then holds the wrist over the heart while the device measures blood flow and records pressure.

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Blood Pressure Numbers: What They Mean

Blood pressure numbers include systolic (sis-TOL-ik) and diastolic (di-a-STOL-ik) pressures. Systolic blood pressure is the pressure when the heart beats while pumping blood. Diastolic blood pressure is the pressure when the heart is at rest between beats.You will most often see blood pressure numbers written with the systolic number above or before the diastolic, such as 120/80 mmHg. (The mmHg is millimeters of mercury-the units used to measure blood pressure.)The table below shows normal numbers for adults. It also shows which numbers put you at greater risk for health problems. Blood pressure tends to go up and down, even in people who have normal blood pressure. If your numbers stay above normal most of the time, you're at risk.

Categories for Blood Pressure Levels in Adults

(in mmHg, or millimeters of mercury)


Systolic (top number)

Diastolic (bottom number)

Normal Less than 120 And Less than 80
Prehypertension 120–139 Or 80–89
High blood pressure
Stage 1 140–159 Or 90–99
Stage 2 160 or higher Or 100 or higher

The ranges in the table apply to most adults (aged 18 and older) who don't have short-term serious illnesses.

All levels above 120/80 mmHg raise your risk, and the risk grows as blood pressure levels rise. "Prehypertension" means you're likely to end up with HBP, unless you take steps to prevent it. -National Heart, Lung, and Blood Institute

Blood pressure (BP) is the pressure of circulating blood on the walls of blood vessels. When used without further specification, "blood pressure" usually refers to the arterial pressure in the systemic circulation. Blood pressure is usually expressed in terms of the systolic (maximum during one heart beat) pressure over diastolic (minimum in between two heart beats) pressure and is measured in millimeters of mercury (mmHg), above the surrounding atmospheric pressure (considered to be zero for convenience).

It is one of the vital signs, along with respiratory rate, heart rate, oxygen saturation, and body temperature. Normal resting systolic (diastolic) blood pressure in an adult is approximately 120 mmHg (80 mmHg), abbreviated "120/80 mmHg".

Traditionally, blood pressure is measured non-invasively using a mercury manometer because these gauges are dependent upon only gravity, thus inherently more accurate than alternative types of pressure gauges. In research, the values obtained by an experienced knowledgeable physician using a mercury manometer and stethoscope listening for the Korotkoff sounds are typically within 10 mmHg of the pressures measured via higher sophistication internal measurements of central aortic pressures at heart level. Other methods, which have become more dominant (for cost, time, convenience and concerns about potential mercury toxicity issues) are inherently less accurate.

Blood pressure is determined, moment by moment, by the balance between heart output versus total peripheral resistance and varies depending on situation, emotional state, activity, and relative health/disease states. It is regulated by the brain via both the nervous and endocrine systems.

Blood pressure that is low due to a disease state is called hypotension, and pressure that is consistently high is hypertension. Both have many causes which can range from mild to severe. Both may be of sudden onset or of long duration. Long term hypertension is a risk factor for many diseases, including heart disease, stroke and kidney failure. Long term hypertension is more common than long term hypotension in Western countries. Long term hypertension often goes undetected because of infrequent monitoring and the absence of symptoms.

The material on this site is for informational purposes only and is not intended as medical advice. It should not be used to diagnose or treat any medical condition. Consult a licensed medical professional for the diagnosis and treatment of all medical conditions and before starting a new diet or exercise program. If you have a medical emergency, call 911 immediately.