Blood Pressure Determination

Blood Pressure Determination

Blood Pressure Determination(See Merck Manual, 17th Ed, p 1603-1604, 1629-1635.)

The cyclic beating of the heart pumps blood to the lungs via the pulmonary circuit, and then to the body via the systemic circuit. The pressure in the arteries alternates from a maximum when the heart is in full contraction, called systole (Greek for contraction), to a minimum when the heart is relaxed, called diastole. Blood pressure (BP) is determined by measuring these pressures by first constricting blood flow using a sphygmomanometer (an inflatable cuff equipped with a manometer to display the pressure), and then monitoring the flow of blood through the artery with a stethoscope as the cuff pressure is allowed to slowly decline. The pressure at which the first thumping sounds of blood flowing under the cuff marks systole, and the point at which these sounds become muffled is diastole. (The sphygmomanometer is no longer able to close off the artery .)

The average BP in young adults is 120/80 mm Hg, while the upper limit in healthy individuals is 140/90. Individuals with BP above this but below 180/115 are said to have moderate hypertension. BP >180/115 constitutes hypertension. Hypertension, besides forcing your heart to work harder, can impact your circulatory system in much the same way as over-pressurization of a tire: it increases the likelihood of a blowout. Such a rupture is termed a hemorrhage and can cause blindness, kidney failure, or paralysis according to what tissue has its circulation interrupted.

Arteries normally act as blood pressure shock absorbers by stretching during systole. If the arteries toughen or harden, a condition called arteriosclerosis, the loss of elasticity causes the transmission of unmoderated pressure through the arterial system. Hypertension routinely occurs as a function of aging, but diet also plays an important role. Reduction of animal fat is important in ameliorating the sclerotic process, as is regular physical exercise. Reduction of salt intake can also lower BP.

This lab requires QUIET during the activity.

Stethoscope (Wipe earpieces with 70% EtOH to avoid transmission of ear infections.)
Sphygmomanometer (Illustrate and label all bolded functional parts. )


1) Have subject seated comfortably, with non-writing arm bare, unconstricted and supported on a table.

Stethoscope ear pieces point forward

2) Insert ear pieces The ear pieces should point forward to conform with your ear canals.

Practice fine control of air control nut


3) Adjust valve nut for snug closure, but not too tight


4) Fit sphygmomanometer cuff loosely above elbow so the manometer is easily visible. Insert the diaphragm of the stethoscope under the cuff, over the brachial artery (front of arm).


5) Pump up cuff to around 100 mm Hg first and confirm that you can hear heart sounds. Then pump further so that no heart sounds are heard, approximately 150-160 mm Hg for a resting young adult. (Do not go to painfully high pressures of >200 mm Hg unless necessary.)

Practice fine control of air control nut

6) Slightly loosen valve nut so that pressure drops slowly (the manometer’s needle moves lower).

7) Listen carefully for the first heart sound to appear. Note the pressure at which this occurs. This is the systolic pressure. (The needle will begin to pulse just before the sounds can be heard.)

8) Allow the pressure to continue to slowly drop and note the pressure at which the beats become muffled or indistinct, called the diastolic pressure.

9) Allow cuff to deflate completely to reestablish circulation, record the BP just determined, then repeat a second and third time. Average the three systolic and the three diastolic pressures.
Repeat these measurements after physical exertion, administration of various agents (caffeine, sugar, alcohol, etc) to determine their effect on blood pressure.

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