What Is Blood Pressure?

Whether blood is in the heart or in blood vessels, it has a certain pressure associated with it. In fact, blood moves through circulation from an area of greater blood pressure to an area of lower blood pressure. As  mentioned earlier, when the heart contracts the pressure of the blood in the ventricles increases. This establishes a blood pressure gradient that drives the movement of blood through the blood vessels. This is somewhat like turning on a garden hose. When you turn on a garden hose, the water pressure is greatest close to the faucet (versus toward the open end of the hose). The result is that water moves from the area of greater water pressure  toward the area of lesser water pressure and out the end of the hose.
 

We define pressure as a force exerted upon a surface and can measure it in millimeters of mercury (mmHg). If we apply this definition to our blood, we can say that blood pressure is the force exerted by blood upon the walls of a blood vessel. When blood pressure is measured, two numbers are provided, for instance 120/80 or “120 over 80.” What this means is that the pressure exerted by the blood is 120 mmHg during heart contraction and 80 mmHg when the heart is relaxing between beats. The first number is the systolic or blood pressure when our heart contracts. The second number is the diastolic pressure and it is blood pressure when our heart is relaxing. Blood pressure is typically measured in the large artery of the arm because of its accessibility.