The Net Resistance of the Systemic Circulation Is Called the Total Peripheral Resistance
As with any other resistance, total peripheral resistance ( TPR) is defined as a pressure difference (perfusion pressure) divided by a flow. In a calculation of the resistance of the systemic circulation, the perfusion pressure is the pressure in the aorta minus the pressure in the venae cavae.
The flow is the total amount of blood that flows through the systemic circuit, which is equal to the cardiac output, as follows:
For a typical dog at rest, the mean aortic pressure is 98 mm Hg, the mean vena caval pressure is 3 mm Hg, and the cardiac output is 2.5 L∕min. Under these conditions, TPR is 38 mm Hg/L/min, which means that it takes a driving pressure of 38 mm Hg to force 1 L/min of blood through the systemic circuit.
Because the pressure in the venae cavae is usually close to zero, it is sometimes ignored in the calculation of TPR. The resultant simplified equation states that TPR is approximately equal to the aortic pressure divided by the cardiac output. Usually, this equation is rearranged to form the statement that the mean aortic blood pressure (Pa) is approximately equal to the cardiac output (CO) multiplied by TPR:
This equation expresses one of the central concepts in cardiovascular physiology: the mean aortic blood pressure is determined by two, and only two, factors. Thus, if the aortic pressure is increased, it must be because the cardiac output increased, the TPR increased, or both. There are no other possibilities.