With each heartbeat, the circulatory system pushes blood through the vessels with measurable force. A sphygmomanometer — the cuff wrapped around your upper arm — captures that force in two readings. As you slowly release the cuff's pressure, the first sound through the stethoscope marks the systolic pressure, when the heart pumps. When the sounds stop, that quieter number is the diastolic pressure, when the heart rests. Measuring different groups of people reveals how much those two readings vary from one person to the next.

A blood pressure reading gives two numbers, and each one matches a different phase of the heart's pump cycle. You wrap a cuff around someone's upper arm, inflate it, and then slowly release the air while listening through a stethoscope. The first sound marks the systolic pressure — the force when the heart pumps. When the sounds stop, that reading is the diastolic pressure, the force when the heart rests between beats. What you are hearing are the brief moments when the brachial artery, squeezed shut by the cuff, opens and closes again with each heartbeat as the pressure drops.

A blood pressure reading captures the heart's pumping rhythm as two separate numbers. The first is systolic pressure — the force measured when the heart pumps. The second is diastolic pressure — the force measured when the heart rests and fills. To take the reading, you wrap a cuff around the upper arm and inflate it until it squeezes the brachial artery shut. As you slowly release the air, listen through a stethoscope: the first sound marks systolic pressure, and when the sounds stop, that reading is diastolic.