The concept of resistance is derived from the Poiseuille equation. This equation can be rearranged to a form in which flow is equal to driving pressure times all of the other factors, **pi/8 x r ^{4} / l x eta**. If these factors represent conductance, then resistance is equal to one over these factors, since resistance is the reciprocal of conductance.

Rearranging the equation, flow becomes equal to the change in pressure (delta P) divided by resistance, or, resistance is equal to P_{1}-P_{2} (delta P) divided by flow **(Table 1.01)**. Thus, we have an expression of resistance.

This equation is similar to Ohms' law for electrical circuits, where resistance is equal to voltage divided by current. The preferred method of calculating resistance is to divide perfusion pressure by flow per second rather than flow per min. As we will see this produces more convenient values close to 1.0.

For example, if the cardiac output is 6000 ml/min or 100 ml/sec, and the mean aortic pressure minus the mean vena caval pressure is 100 mmHg, then the resistance is equal to 100 mmHg divided by 100 ml/sec, or 1.0. The unit used is the **peripheral resistance unit, or PRU**. In this example therefore, the total peripheral resistance (TPR) is equal to 1.0 PRU.

In other words, the resistance of the systemic circulation, from the aorta throughout the body and back to the vena cavae, including all of the body except the pulmonary circulation, is equal to 1.0 PRU. Clearly, resistance cannot be determined from changes in blood pressure alone.