Pressure and Its Measurement

In physiologic terms, we are exposed to 3 main sources of pressure: 1) the weight of the atmosphere; 2) hydrostatic forces exerted by the weight of body fluids; and 3) mechanical pressure generated by the heart or other muscles that contract around those fluids. Because cardiopulmonary physiology deals so much with pressure measurements, let's start by defining what pressure really is. Simply put, pressure is force divided by area. It's also important to understand what pressure is not. For example, pressure is not energy. Only when pressure is coupled to a volume change (ie, movement or pressure-volume work) is it a component of energy. This is more than just a semantic point. Although we're fond of saying that fluids move from high to low pressure, that isn't always true. The reason why highlights a fundamental difference between pressure and energy. Pressure is surprisingly difficult to measure. Often, when we think we are measuring pressure, we are actually measuring stretch or movement.