Total peripheral resistance (TPR) is the amount of resistance to blood flow present in the vascular system of the body. It can be thought of as the amount of force working against the heart as it ejects blood into the vascular system. Although total peripheral resistance plays an integral role in determining blood pressure, it is an exclusively defined measure of the cardiovascular system and should not be confused with the pressure against arterial walls, which is a measure of blood pressure.
The vascular system, which is responsible for the flow of blood both to and from the heart, can be divided into two components: systemic and pulmonary. The pulmonary system delivers blood to and from the lungs, where it becomes oxygenated, and the systemic vasculature is responsible for the transporting this blood to the cells of the body through the arteries and returning the blood to the heart after perfusion. TPR affects the flow of this system and can, in turn, greatly affect the perfusion to organs.
Total peripheral resistance is calculated by using a specific equation. This equation is TPR = change in pressure/cardiac output. Change in pressure is the difference in mean arterial pressure and venous pressure. Mean arterial pressure is equal to diastolic blood pressure plus one-third of the difference between the systolic and diastolic pressures. Venous blood pressure can be measured using an invasive instrumental technique that physically measures the pressure inside a vein. Cardiac output is the amount of blood pumped through the heart in a one-minute increment.
There are a number of factors that can significantly change the components of the TPR equation, thus changing total peripheral resistance. These factors include vascular vessel diameter and blood property dynamics. The diameter of a blood vessel is inversely proportional to blood pressure, so a smaller vessel would increase resistance, hence increasing TPR. Contrarily, a larger blood vessel equates to a less concentrated volume of blood particles pushing against vessel walls, which translates into lower pressure.
The fluid dynamics of the blood also can heavily contribute to an increase or decrease in TPR. The mechanism behind this is a change in clotting factors and blood components that might change the viscosity of blood. As would be predicted, a more viscous fluid causes more resistance to flow. A less viscous fluid would move more easily throughout the pipeline of the body, causing less resistance. Analogous to this would be the force needed to move water versus molasses.