Water rushing through a hose exerts pressure on the hose, and in the same way, blood surging through your blood vessels exerts pressure on the vessels.
It’s important for a doctor to know your blood pressure, because this tells him the strength of your heart, the condition of your arteries, and other important things about your health.
When a doctor takes your blood pressure, he attaches a rubber cuff to your arm, just above the elbow. This cuff is like an empty balloon, and can be filled with air. The doctor pumps up the cuff until it’s so tight that it stops the flow of blood through the artery in your arm. Then he lets the air out of the cuff, and the first rush of blood through the artery is “read” by a tube containing mercury. This mercury tube shows the doctor your blood pressure “reading.”
This first, or maximum, reading tells the doctor your blood pressure when your heart is pumping. The second, or minimum, reading is then taken to show your blood pressure when your heart is in its resting stage. So your blood pressure reading always consists of two numbers, called the ‘maximum/minimum blood pressure.”
The average blood pressure reading of a healthy adult is 120/80. But blood pressure readings can vary in the same person at different times.