Blood Pressure
One of anatomy's great mysteries was solved when William Harvey explained the mechanics of blood circulation in 1628. Remaining to be investigated was blood pressure, the force of the blood pressing against the inner walls of the blood vessels. Blood pressure is higher during cardiac systole, when the heart's ventricles contract, pumping blood into the arteries. Blood pressure is lower during cardiac diastole, when the ventricles relax.
Harvey had noted that when an artery was severed, blood was ejected in spurts. Stephen Hales, an English clergyman with an avid interest in physiology, became intrigued: how great was the pressure the arterial blood appeared to be under? Hales began exploring the mysteries of blood pressure and its measurement while a student at Cambridge University in the early 1700s and continued around 1712-13 at his parish in Teddington, England. He conducted a series of experiments on domestic animals, beginning with dogs. Using a glass tube connected via a brass pipe to an animal's blood vessel, Hales was able to measure blood pressure by the height to which the blood rose in the tube, most dramatically 9 ft. 6 in. (2.75 m 15.2 cm) in a white mare. Hales reported these first-ever blood pressure measurements in his Haemastaticks of 1733.
Hales continued his blood pressure investigations by studying the rate of blood flow through the capillaries, successfully demonstrating that resistance to flow in those small vessels could vary greatly. This indirectly proved that capillaries could dilate and contract. Hales also showed that blood pressure varied according to the size of the animal, and linked blood pressure measurement with rate of flow and peripheral resistance.
In the 1770s, Italian physiologist Lazzaro Spallanzani studied blood circulation and demonstrated that the arterial pulse is caused by the pressure exerted against the vessels' walls by the blood being pumped through.
Hales's method of measuring blood pressure was improved in 1828 by French physician Jean Leonard Marie Poiseuille (1797-1869). In place of Hales's glass tube, Poiseuille used a mercury manometer, a U-shaped tube containing mercury and calibrated in millimeters. Using his device, Poiseuille showed that blood pressure rises and falls as a person breathes out and in, and that blood pressure is the same in vessels both near the heart and far away. Today blood pressure is still expressed in millimeters (mm) of mercury (HG).
Blood pressure measurement became an important diagnostic tool after 1896, when Scipione Riva-Rocci (1863-1937) invented the sphygmomanometer, the prototype of today's standard blood pressure measuring instrument. The sphygmomanometer uses a flexible band around the arm that is inflated until arterial flow stops. Air is released from the band; when the pulse reappears, the pressure reading is taken (systolic pressure, while the ventricles are contracting). A Russian physician Nikolai Korotkoff added the use of the stethoscope to listen to the sound of both the maximum (systolic) and minimum (diastolic) pressure in the elbow's brachial artery. In a blood pressure measurement, the systolic number is placed over the diastolic number; for example, "120 over 80," written as 120/80 mm Hg.
Blood pressure can now be easily and accurately measured and is an important indicator of cardiac and circulatory health or distress. A blood pressure reading less than 140/90 mm Hg is considered normal, and a blood pressure below 120/80 mm Hg is even better. High blood pressure, also called hypertension, is an important risk factor for heart and kidney disease and stroke. About a quarter of all American adults now suffer from hypertension. Maintaining a healthy weight, being more physically active, choosing foods that are low in salt and sodium, and moderating any use of alcoholic beverages are steps that can help prevent this condition.
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