Blood pressure is the pressure exerted against blood vessel walls (see graphic) 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. 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.
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 - hypertension makes that process happen faster.
The PulseBlood 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)
- The top number is the systolic blood pressure. This is the pressure in the artery during the beating of the heart (systole). This the pressure that creates the pulse we feel in the wrist or neck.
- 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).
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