### Resistance (continued..)

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.

Clinically resistance is determined in the same way with an Ohm's law type of relationship, but the units used are different. In this convention normal systemic vascular resistance is about 1130, while normal pulmonary vascular resistance is 67.