What is Blood Pressure?

And What Do the Numbers Mean?

Blood pressure gauge
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Ever noticed how a hose spouting water in your favorite Saturday-morning cartoon always looked like it was a snake vomiting footballs? Despite the fact that the water coming out of the end of the hose was running smoothly, it's still a pretty good representation of how blood flows through our veins: in waves that we call pulses.

The Pressure of the Blood

Blood pressure is the force exerted against blood vessel walls by the blood as it flows through them. Because of the way arteries and veins are used by the circulatory system, arterial walls are much thicker and withstand higher pressures than venous walls do. Arteries have the ability to expand and constrict much more than veins can, which is necessary to adjust blood pressure. Because they exert that control, they have to be sturdy.

When we measure blood pressure, we are measuring the pressure in the arteries. Usually, we measure the pressure in the brachial artery, although it is possible to measure blood pressure in other arteries as well. Blood pressure is manually measured using a stethoscope to listen blood flow turbulence, a cuff to constrict blood vessels enough to stop the flow, and a sphygmomanometer (big, fancy word for a pressure gauge and a squeeze bulb). 

Electronic blood pressure monitors don't need humans (other than the one they're testing) or stethoscopes. There are plenty of blood pressure monitors in homes today. If you have a blood pressure monitor or are considering buying one, you may be wondering what exactly blood pressure is and if you should monitor it.

Why Does It Matter?

Anyone who has left the water on in the garden has seen the hole that rushing water can make under pressure. That erosion can also happen in the body if high blood pressure is not treated. High blood pressure can also lead to strokes and aneurysms. An aneurysm is a weak spot in an artery that swells until it bursts, and hypertension makes that process happen faster.

The Pulse

Blood does not flow smoothly through arteries. Instead, it surges through the arteries each time the heart beats. That surge is known as the pulse and is easily felt through arteries in the wrist and neck. Even though blood is surging through the blood vessels, there is pressure on the vessels at all times. Indeed, the pulse we feel is really the difference between the pressure exerted against the arterials walls during the heart's rest and during the heart's contractions.

Why an Upside Down Fraction?

When blood pressure is measured, we commonly record the pressure as two numbers, one above the other, like a fraction. The difference between a fraction and a blood pressure is that the top number of a blood pressure is always higher than the bottom number (example: 120/80).

  1. The top number is the systolic blood pressure. This is the pressure in the artery during the beating of the heart (systole). This is the pressure that creates the pulse we feel in the wrist or neck.
  2. The bottom number is the diastolic blood pressure. This is the pressure that is always in the artery, even when the heart is resting between beats (diastole).