# Total peripheral resistance

Total peripheral resistance (TPR) is the sum of the resistance of all peripheral vasculature in the systemic circulation. Vasculature throughout the entire body can be thought of as two separate circuits - one is the systemic circulation, while the other is the pulmonary circulation.

TPR should not be confused with pulmonary vascular resistance (PVR), which is the resistance in the pulmonary circulation.

## Measurement

TPR is represented mathematically by the formula R = ΔP/Q.

• R is TPR.
• ΔP is the change in pressure across the systemic circulation from its beginning to its end.
• Q is the flow through the vasculature (equal to cardiac output)

In other words:

The numerator can be seen as representing the change in pressure that occurred immediately after having left the heart (i.e., mean arterial pressure) to when it finally returned to the heart via the IVC to the right atrium (i.e., mean venous pressure).

Mean Arterial Pressure (MAP) is calculated from the values of systolic and diastolic pressure, but it is not an average of the two. Diastole lasts almost twice as long as systole does therefore MAP is closer to the diastolic blood pressure value. MAP is calculated from the following formula: MAP = diastolic + (systolic - diastolic) / 3. Another way, and perhaps easier way, to write this is MAP = $(2/3)$ diastolic + $(1/3)$ systolic.

Mean Venous Pressure is measured at the right atrium and is usually very low (normally around 4mm Hg). As a result, it is sometimes disregarded. A synonymous term is central venous pressure.

## Causes of Change in TPR

Peripheral resistance increases due to binding of norepinephrine and epinephrine to the α1 receptor on vascular smooth muscles. These hormones cause vasoconstriction, thus decreasing the radius of the vessels in the periphery. Resistance is inversely proportional to radius to the power of four. Thus, a decreased radius will greatly increase the resistance.

Peripheral Resistance is also affected by the viscosity of the blood flowing through it. The viscosity of blood is normally associated with its osmolarity.

Peripheral Resistance is also dependent upon the capacitance of the blood vessel through which the blood travels.

A decrease in TPR (e.g., during exercising) will result in an increased flow to tissues and an increased venous flow back to the heart. An increased TPR will decrease flow to tissues and decrease venous flow back to the heart.

TPR also decreases in immersion of water.