The left side of the heart takes the oxygenated blood and sends it to the rest of the body. When you take a blood pressure reading, you measure pressures in the systemic circulation . The highest pressures occur during systole as blood is ejected into the aorta and subsequent arteries of the body. Diastole , the heart's resting phase, follows systole. During diastole the ventricles fill with more blood.
Systemic pressures fall until more blood is ejected during systole.
The blood pressure cuff lets you apply external pressure to a circumferential ring around the upper arm. When the pressure is great enough, it forcibly closes the main artery of the upper arm - the brachial artery . The stethoscope allows you to hear when the underlying artery opens and closes as you release the air in the bladder. As you release the air out of the cuff, the external pressure falls. Once the arterial pressure falls below 119mm, however, the artery shuts again.