Total peripheral resistance
Encyclopedia
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. Total peripheral resistance (TPR) is the sum of the resistance of all peripheral vasculature in the systemic circulation. This should not be confused with Pulmonary Vascular Resistance
(PVR), which is the resistance in the pulmonary circulation.
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 =
diastolic + 
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
.
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.
Systemic circulation
Systemic circulation is the part of the cardiovascular system which carries oxygenated blood away from the heart to the body, and returns deoxygenated blood back to the heart. This physiologic theory of circulation was first described by William Harvey...
, while the other is the pulmonary circulation. Total peripheral resistance (TPR) is the sum of the resistance of all peripheral vasculature in the systemic circulation. This should not be confused with Pulmonary Vascular Resistance
Vascular resistance
Vascular resistance is a term used to define the resistance to flow that must be overcome to push blood through the circulatory system. The resistance offered by the peripheral circulation is known as the systemic vascular resistance , while the resistance offered by the vasculature of the lungs...
(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 outputCardiac outputCardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...
)
In other words:
- Total Peripheral Resistance = (Mean Arterial PressureMean arterial pressureThe mean arterial pressure is a term used in medicine to describe an average blood pressure in an individual. It is defined as the average arterial pressure during a single cardiac cycle.-Calculation:...
- Mean Venous Pressure) / Cardiac OutputCardiac outputCardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...
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 =
diastolic + 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
Central venous pressure
Central venous pressure describes the pressure of blood in the thoracic vena cava, near the right atrium of the heart...
.
Causes of Change in TPR
Peripheral resistance increases due to binding of norepinephrineNorepinephrine
Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...
and epinephrine
Epinephrine
Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...
to the α1 receptor on vascular smooth muscles. These hormones cause vasoconstriction
Vasoconstriction
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...
, 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.