Total peripheral resistance
In physiology, total peripheral resistance (TPR) is the total resistance opposing blood flow in the systemic circulation. Total peripheral resistance is used in calculations of blood pressure, blood flow, and cardiac function.
In contrast, resistance in the pulmonary circulation is represented by the pulmonary vascular resistance (PVR), and is not included in calculation of the TPR.
TPR is calculated mathematically using the following formula:
- R is TPR
- ΔP is the change in pressure across the systemic circulation from its beginning (immediately after exiting the left heart) to its end (entering the right atrium)
- Q is the flow through the vasculature (equal to cardiac output)
In other words:
Mean arterial pressure is most commonly measured using a sphygmomanometer, and calculating a specialized average between systolic and diastolic blood pressures. Venous pressure, also known as central venous pressure, is measured at the right atrium and is usually very low (normally around 4mm Hg). As a result, it is sometimes disregarded.
Causes of change in TPR
Factors that influence TPR are represented in an adapted form of the Hagen–Poiseuille equation:
- R = resistance to blood flow
- L = length of the vessel
- η = viscosity of blood
- r = radius of the blood vessel
Vessel length is generally not subject to change in the body.
Blood viscosity increases as blood is more hemoconcentrated, and decreases as blood is more dilute. The greater the viscosity of blood, the larger the resistance will be. In the body, blood viscosity increases as red blood cell concentration increases, thus more hemodilute blood will flow more readily, while more hemoconcentrated blood will flow more slowly.
The major regulator of TPR in the body is regulation of vessel radius. In humans, there is very little pressure change as blood flows from the aorta to the large arteries, but the small arteries and arterioles are the site of about 70% of the pressure drop, and are the main regulators of TPR. When environmental changes occur (e.g. exercise, immersion in water), neuronal and hormonal signals, including binding of norepinephrine and epinephrine to the α1 receptor on vascular smooth muscles, cause either vasoconstriction or vasodilation. Because resistance increases proportionally to the fourth power of vessel radius, changes to arteriole diameter can result in large increases or decreases in peripheral resistance.
Effects of TPR on the body
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.
- "Cardiac Output and Blood Pressure". biosbcc. Retrieved 7 April 2011.