Final answer:
The systolic blood pressure measures the pressure during the heart's ventricular contraction, while diastolic pressure measures the pressure during the ventricles' relaxation phase. A sphygmomanometer is used to measure both pressures, with the appearance and disappearance of certain sounds indicating the systolic and diastolic pressures.
Step-by-step explanation:
Difference Between Systolic and Diastolic Blood Pressure
The difference between systolic and diastolic blood pressure is essentially a matter of cardiac cycle phases. Systolic pressure is the higher value usually around 120 mm Hg, reflecting the arterial pressure during the ejection of blood from the heart's ventricular contraction, or systole. In contrast, diastolic pressure is the lower value, typically about 80 mm Hg, representing the arterial pressure when the heart's ventricles are at rest, in between beats, or diastole.
Measuring Blood Pressure
Blood pressure is measured using a sphygmomanometer. This device includes an inflatable cuff to restrict blood flow, a manometer to measure the pressure, and a stethoscope (for manual devices) to hear the first sounds of blood flow, which indicates the systolic pressure. The cessation of sounds indicates the diastolic pressure. When the pressure of the cuff matches the arterial systolic pressure, blood flow resumes, and the first audible sounds via a stethoscope correspond to the systolic pressure. As the cuff pressure continues to decrease and sound is no longer heard, this signifies the diastolic pressure.
The Underlying Principle
The principle behind the measurement involves restricting and then gradually releasing the pressure on the arm using the cuff, while listening to the sounds of the resuming blood flow. This method is based on the changes in arterial pressure as the heart goes through the phases of its beat cycle.