In engineering, the word “pressure” is used to describe the amount of force applied per unit area. It is a calculated parameter given by the equation
p = F / A
where
p = pressure
F = force applied normal to the surface
A = area over which the force is applied.
In this case, the word “normal” means perpendicular in a three-dimensional space (note that “perpendicular” is a two-dimensional term).
Pressure is commonly expressed with a variety of different units such as pounds per square inch (psi). The metric unit is the Pascal (Pa), which is equal to one Newton per square meter (N/m2). Because a Newton is small (about 1/4th of a pound) and a square meter is large (1550 in2) pressure is often expressed in kilopascals (kPa) or megapascals (MPa), which are 1000 Pa and 1,000,000 Pa, respectively. Pressure is also expressed in millimeters of mercury (mmHg). This is an old term used to state the pressure required to lift a column of mercury a specified height and the first accurate means to measure pressure. Actual mercury gauges have generally been replaced by modern instruments due to mercury toxicity, but the measure still remains in use. Another common unit of pressure is the atmosphere (atm), the pressure exerted by the air at sea level. Atmospheric pressure is caused by the weight of the tall column of air above the point where the measurement is taken. Atmospheric pressure is reduced at high elevations due to a reduction in the height of the air column above. Pressure may be expressed as absolute pressure, which uses a vacuum as the reference or gauge pressure. Note that gauge pressure uses atmospheric pressure as the reference. Pressure is generally assumed to be gauge pressure unless otherwise specified.
The term “pressure” is used in a variety of different medical applications. Angioplasty balloons used to open arteries are inflated with 8–13 atm (810–1320 kPa) for low-pressure balloons and 15–20 atm for high-pressure balloons. Normal blood pressure is 120/80 mmHg (16/10.7 kPa), indicating the range of pressure in the blood during a normal heart beating. Maximum normal air pressure is about 75 cm H2O (7.4 kPa) when air forced in or out of the lungs. Peak pressure in the knee joint during a squatting movement has been measured at 14 MPa (14,000 kPa) in cadaver studies. Normal eye pressure range is 12–22 mm Hg (1.6 – 2.9 kPa). From these examples, it is clear that pressures in medical application very greatly and there are a variety of different units that are commonly used to express these pressures.
