WHAT IS BLOOD PRESSURE?
Blood pressure refers to the force (pressure) with which the blood presses
against the walls of the blood vessel. All blood vessels--large or small, artery or vein--
have blood pressure. However, the term blood pressure normally refers to the blood
pressure of a major artery. Unlike the other vital signs discussed previously, it takes
two numbers--the systolic pressure and the diastolic pressure--to describe this vital
sign. Blood pressure is normally measured in millimeters of mercury (mm Hg).
"Millimeters of mercury" is a standard unit for measuring pressure. It refers to how high
a force (pressure) would cause a column of mercury (chemical symbol Hg) to rise in a
tube. Figure 5-1 gives the general idea. The greater the pressure, the more mercury is
forced up the tube.
Figure 5-1. "Millimeters of mercury" as a measure of pressure.
WHAT ARE SYSTOLIC AND DIASTOLIC PRESSURES?
Basically, the systolic pressure is the greatest pressure that the blood exerts
against the walls of the blood vessel while the diastolic is the lowest pressure that the
blood exerts against the walls of the vessel. A person's blood pressure depends upon
the force of the heart's pumping action, the degree to which the blood vessel will stretch,
and the amount of blood in the blood vessel.
a. Systolic. The arteries are under the greatest pressure when the heart pumps
blood into them. The extra blood that is forced into the arteries make them stretch.
(1) In a normal adult male, the systolic pressure should be between 100 and
140 mm Hg, inclusive. (Inclusive simply means to include the ends of the range.
Systolic pressures of 100 and of 140 are within normal range.)