BLOOD PRESSURE, the hydrostatic pressure under which the blood exists in the arteries and veins of human beings and animals. This pressure was first measured by the Rev. Dr. Stephen Hales, in 1733. Hales connected the femoral artery of a horse to a glass tube 9 ft. in length. When the blood was permitted to flow into this tube, it rose 8 ft. 3 in., showing that the pressure of the blood due to the pumping of the heart-beat was equal to the weight of a column of blood of this height. When a similar tube was attached to a vein, the blood rose only 1 2 inches. In 1828 was invented the mercury manometer, which consisted of a U tube, with mercury resting at the same level in both arms of the tube. When the pressure of blood from an artery was applied to the top of the mercury column in one arm, it forced the mer cury column in the other arm of the tube to rise. The differ ence in height of the two mercury columns then represented the number of millimetres which the blood pressure had lifted the mercury column. Since then, blood pressure has been given in terms of millimetres of mercury.
The sphygmomanometer was devised by von Basch, in 1887, for measuring the blood pressure in the arteries of man. The mod ern form of this instrument consists of a rubber bag, into which air can be pumped by means of a hand-bulb, with a pressure gauge communicating with the air inside the bag. (The pressure gauge may consist of a spring device, calibrated to correspond with mercury pressure, or of a column of mercury which can be read on a millimetre scale as its level changes.) The bag is wrapped around the subject's arm (usually the left), above the elbow, in a position appropriate to shut off the flow of blood in the brachial artery, when the bag is inflated. Air is then pumped into the bag until the pulse in the radial artery, at the wrist, disappears. The reading of the pressure gauge at this moment gives the systolic blood pressure ; i.e., the pressure necessary to suppress the maximum pressure of blood in the artery.
Three phases of arterial blood pressure are customarily dis tinguished during a single beat of the heart : systolic during the heart's systole, when the arterial blood pressure is at its maxi mum ; diastolic, when the heart is in diastole, with arterial pressure at its minimum ; and the mid-point, or mean between these two extremes, called the "pulse-pressure." If the pulse beat be re corded graphically, as the air pressure compressing the artery is diminished, the pressure at which the maximum pulse wave is obtained marks the diastolic pressure. By the auscultation method of obtaining blood pressure, a stethoscope is placed over the brachial artery just below the cuff, the air pressure in the cuff is raised above that necessary to obliterate the artery and is then allowed to fall slowly. The first sound heard through the stetho scope gives the systolic blood pressure; since this marks the moment when the maximum arterial pressure is just able to break through the closed artery. The pressure at which the sound is last heard is the diastolic pressure.
It is a common belief that systolic blood pressure in adults equals loo plus the individual's age ; but research has not verified this idea. Erlanger reports that the average adult (2o to 25), when wholly free from emotional excitement, has a systolic pressure (brachial artery) of 116 mm., and a diastolic of 65 millimetres. Psychologists have found that normal adults have systolic blood pressure varying between 90 and 15o mm., with little or no uni formity, or correspondence with the physical age of the individual (up to 35 years). Blood pressure changes very greatly under emotional excitement, and the excitement caused by undergoing a physical examination, or having the blood pressure taken for such a purpose as obtaining a life insurance policy, has been found to raise the systolic blood pressure 5o to 6o millimetres.
Blood pressure is of value to physicians in diagnosing various diseases, especially such disorders as arteriosclerosis, or dis tinguishing between hyper-thyroid and hyper-adrenal conditions. Systolic blood pressure has been found useful by psychologists as a means of detecting emotional excitement (see DECEPTION TESTS).
Knowledge of the pressure of blood in the veins is not as fre quently required as is arterial pressure. The simplest method of determining venous pressure in man is that of Gaertner. The subject's arm is slowly raised until the veins on the back of the hand just disappear. The height above the heart at which this happens gives the venous pressure in the right auricle, the entire vein constitutes, in essence, a manometer tube leading from heart to hand, with the height of the hand above the heart thus measur ing the height to which the venous blood-column is supported by the pressure exerted upon it.
BIBLIOGRAPHY.-E. A. Schafer, Text Book of Physiology ; T. C. Bibliography.-E. A. Schafer, Text Book of Physiology ; T. C. Janeway, The Clinical Study of Blood Pressure (19o4) ; W. M. Marston, Sex Characteristics of Systolic Blood Pressure Behavior, Journal of Experimental Psychology, volume 6, No. 6, December, 1923 (see Arteries, Diseases of). (W. M. m.) BLOODROOT (Sanguinaria canadensis), a North American plant of the poppy family, Papaveraceae, called also red puccoon, common in rich woods in eastern North America. It is a low, smooth, bluish-green perennial which blossoms usually before its own leaves or those of its native woodland unfold.
The showy white or pinkish flowers, about 2 in. broad, are usually borne singly on stalks about 8 in. long. The flower stalks and the large, rounded, many-lobed leaves rise from a horizontal rootstock, often an inch thick and several inches long. The rootstock, together with other parts, is surcharged with an acrid, orange-red juice, whence the name. The plant con tains an alkaloid, sanguinarin (see ALKALOIDS), used in medicine. Throughout its range the bloodroot is one of the most attractive early bloomers and is often transplanted for ornament. In Great Britain the name bloodroot is sometimes applied to the tormentil (Potentilla Tormentilla) and the crimson crane's bill (Geranium sanguineum).