Modern understanding of the cardiovascular system began with the work of physician William Harvey (1578–1657), who described the circulation of blood in his book "De motu cordis". The English clergyman Stephen Hales made the first published measurement of blood pressure in 1733.[152][153] However, hypertension as a clinical entity came into its own with the invention of the cuff-based sphygmomanometer by Scipione Riva-Rocci in 1896.[154] This allowed easy measurement of systolic pressure in the clinic. In 1905, Nikolai Korotkoff improved the technique by describing the Korotkoff sounds that are heard when the artery is ausculated with a stethoscope while the sphygmomanometer cuff is deflated.[153] This permitted systolic and diastolic pressure to be measured.
Blood pressure is measure in millimeters of mercury (mmHg) and recorded as two numbers, systolic and diastolic. Systolic is the top number (also the higher one), which measure the pressure in the arteries when the heart beats, so when its muscles contract. Diastolic the bottom number (also the lower one), which measures the pressure in the arteries between heart beats, so when the muscles relax and refill with blood.