Pressure and Flow

The physical concepts of circulatory pressure and flow are derived from Ohm’s law, although Ohm wasn’t actually concerned with physiology or the circulation. Georg Simon Ohm was a German physicist and mathematician working on electrical circuits and currents in the early 1800s. The law named after him states “V = IR”. In that equation, “I” is the current of electrons moving through the conductor in units of amperes (or “amps”), “V” is the voltage measured across the conductor in units of volts, and “R” is the resistance of the conductor in units of ohms.

Similarly, in terms used for circulatory performance, blood flow (represented as a dotted Q) is analogous to current, blood pressure (BP) is analogous to voltage, and systemic vascular resistance (SVR) is the term applied for the resistance to circulatory flow.

Thus, in terms appropriate for the circulation, Ohm’s law looks like BP = Q x SVR.

The circulation of blood through the cardiovascular system delivers oxygen and nutrients to maintain cellular viability throughout the body. It is very important to understand that it is circulatory flow that delivers the cellular fuel. That flow, in turn, is produced by cardiac function continuously pumping oxygenated blood through the circulation. We commonly monitor blood pressure and heart rate to assess the circulation’s activity. However, neither of these parameters represent the actual fuel delivery (in the form of oxygen and nutrients) necessary for cell viability and organ function.

In fact, maintaining blood pressure itself is not actually the primary goal in circulatory shock resuscitation. Blood pressure is often managed in clinical practice with the administration of vasoconstrictive medications, also known as vasopressors. However, such treatment does not really improve the delivery of oxygen to cells and tissues. Oxygen is primarily transported by being bound to hemoglobin molecules in red blood cells with a small amount of oxygen being dissolved in the plasma. Rather, it is blood flow that determines how much oxygen the tissues receive.

The paradigm shift for many clinicians is to understand the difference in importance between blood pressure and blood flow.