Vascular resistance
Encyclopedia
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 (SVR), while the resistance offered by the vasculature of the lungs is known as the pulmonary vascular resistance (PVR). The systemic vascular resistance may also be referred to as the total peripheral resistance
. Vasoconstriction
(i.e., decrease in blood vessel diameter) increases SVR, whereas vasodilation
(increase in diameter) decreases SVR.
Units for measuring vascular resistance are dyn
·s·cm−5, pascal seconds per cubic metre (Pa·s/m³) or, for ease of deriving it by pressure (measured in mmHg) and cardiac output
(measured in l/min), it can be given in mmHg·min/l. This is numerically equivalent to hybrid reference units (HRU), also known as Wood units, frequently used by pediatric cardiologists. To convert from Wood units to MPa·s/m3 you must multiply by 8, or to dyn·s·cm−5 you must multiply by 80 .
The systemic vascular resistance can therefore be calculated in units of dyn·s·cm−5 as
where mean arterial pressure is 2/3 of diastolic blood pressure plus 1/3 of systolic blood pressure.
The pulmonary vascular resistance can therefore be calculated in units of dyn·s·cm−5 as
where the pressures are measured in units of millimetres of mercury (mmHg) and the cardiac output is measured in units of litre
s per minute
(L/min). The pulmonary artery wedged pressure (also called pulmonary artery occlusion pressure or PAOP) is a measurement in which one of the pulmonary arteries is occluded, and the pressure downstream from the occlusion is measured in order to approximately sample the left atrial pressure. Therefore the numerator of the above equation is the pressure difference between the input to the pulmonary blood circuit (where the heart's right ventricle connects to the pulmonary trunk) and the output of the circuit (which is the input to the left atrium of the heart). The above equation contains a numerical constant to compensate for the units used, but is conceptually equivalent to the following:
where R is the pulmonary vascular resistance (fluid resistance), ΔP is the pressure difference across the pulmonary circuit, and Q is the rate of blood flow through it.
As an example:
If
Systolic pressure: 120 mmHg,
Diastolic pressure: 80 mmHg,
Right atrial mean pressure: 3 mmHg,
Cardiac output: 5 l/min,
Then
Mean Arterial Pressure would be : (Systolic pressure + 2 Diastolic Pressure)/3 = 93.3 mmHg, and
Peripheral vascular resistance: (93 - 3) / 5 =18 Wood Units. or
Peripheral vascular resistance: 18 x 80 = 1440 dyn·s/cm5
These values are in the normal limits.
s) tone. These vessels are from 450 µm
down to 100 µm in diameter. (As a comparison, the diameter of a capillary
is about 3 to 4 µm.)
Another determinant of vascular resistance is the pre-capillary arterioles. These arterioles are less than 100 µm in diameter. They are sometimes known as autoregulatory vessels since they can dynamically change in diameter to increase or reduce blood flow.
Any change in the viscosity
of blood (such as due to a change in hematocrit
) would also affect the measured vascular resistance.
-derived substances, including serotonin
, are vasodilatory when the endothelium
is intact and are vasoconstrictive when the endothelium is damaged.
Cholinergic stimulation causes release of endothelium-derived relaxing factor
(EDRF) (later it was discovered that EDRF was nitric oxide
) from intact endothelium, causing vasodilation. If the endothelium is damaged, cholinergic stimulation causes vasoconstriction.
, AMP). Most of the adenosine that is produced leaves the cell and acts as a direct vasodilator on the vascular wall. Because adenosine acts as a direct vasodilator, it is not dependent on an intact endothelium to cause vasodilation.
Adenosine causes vasodilation in the small and medium sized resistance arterioles (less than 100 µm in diameter). When adenosine is administered it can cause a coronary steal
phenomenon, where the vessels in healthy tissue dilate as much as the ischemic tissue and more blood is shunted away from the ischemic tissue that needs it most. This is the principle behind adenosine stress testing
.
Adenosine is quickly broken down by adenosine deaminase
, which is present in red cell
s and the vessel wall.
).
Local metabolic control (based on metabolic demand) is the most important mechanism of control of coronary flow. Decreased tissue oxygen content and increased tissue CO2 content act as vasodilators. Acidosis acts as a direct coronary vasodilator and also potentiates the actions of adenosine on the coronary vasculature.
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
through the circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
. The resistance offered by the peripheral circulation is known as the systemic vascular resistance (SVR), while the resistance offered by the vasculature of the lungs is known as the pulmonary vascular resistance (PVR). The systemic vascular resistance may also be referred to as the total peripheral resistance
Total peripheral resistance
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 is the sum of the resistance of all peripheral vasculature in the systemic circulation...
. 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...
(i.e., decrease in blood vessel diameter) increases SVR, whereas vasodilation
Vasodilation
Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...
(increase in diameter) decreases SVR.
Units for measuring vascular resistance are dyn
Dyne
In physics, the dyne is a unit of force specified in the centimetre-gram-second system of units, a predecessor of the modern SI. One dyne is equal to exactly 10 µN...
·s·cm−5, pascal seconds per cubic metre (Pa·s/m³) or, for ease of deriving it by pressure (measured in mmHg) and cardiac output
Cardiac output
Cardiac 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...
(measured in l/min), it can be given in mmHg·min/l. This is numerically equivalent to hybrid reference units (HRU), also known as Wood units, frequently used by pediatric cardiologists. To convert from Wood units to MPa·s/m3 you must multiply by 8, or to dyn·s·cm−5 you must multiply by 80 .
| Measurement | Reference Range | ||
|---|---|---|---|
| dyn·s/cm5 | MPa·s/m3 | mmHg·min/l or HRU/Woods units |
|
| Systemic vascular resistance | 700–1600 | 70–160 | 9–20 |
| Pulmonary vascular resistance | 20–130 | 2–13 | 0.25–1.6 |
Calculation of resistance
The basic tenet of calculating resistance is that flow is equal to driving pressure divided by resistance.The systemic vascular resistance can therefore be calculated in units of dyn·s·cm−5 as
where mean arterial pressure is 2/3 of diastolic blood pressure plus 1/3 of systolic blood pressure.
The pulmonary vascular resistance can therefore be calculated in units of dyn·s·cm−5 as
where the pressures are measured in units of millimetres of mercury (mmHg) and the cardiac output is measured in units of litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
s per minute
Minute
A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...
(L/min). The pulmonary artery wedged pressure (also called pulmonary artery occlusion pressure or PAOP) is a measurement in which one of the pulmonary arteries is occluded, and the pressure downstream from the occlusion is measured in order to approximately sample the left atrial pressure. Therefore the numerator of the above equation is the pressure difference between the input to the pulmonary blood circuit (where the heart's right ventricle connects to the pulmonary trunk) and the output of the circuit (which is the input to the left atrium of the heart). The above equation contains a numerical constant to compensate for the units used, but is conceptually equivalent to the following:
where R is the pulmonary vascular resistance (fluid resistance), ΔP is the pressure difference across the pulmonary circuit, and Q is the rate of blood flow through it.
As an example:
If
Systolic pressure: 120 mmHg,
Diastolic pressure: 80 mmHg,
Right atrial mean pressure: 3 mmHg,
Cardiac output: 5 l/min,
Then
Mean Arterial Pressure would be : (Systolic pressure + 2 Diastolic Pressure)/3 = 93.3 mmHg, and
Peripheral vascular resistance: (93 - 3) / 5 =18 Wood Units. or
Peripheral vascular resistance: 18 x 80 = 1440 dyn·s/cm5
These values are in the normal limits.
Determinants of vascular resistance
The major determinant of vascular resistance is small arteriolar (known as resistance arterioleArteriole
An arteriole is a small diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries.Arterioles have muscular walls and are the primary site of vascular resistance...
s) tone. These vessels are from 450 µm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
down to 100 µm in diameter. (As a comparison, the diameter of a capillary
Capillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
is about 3 to 4 µm.)
Another determinant of vascular resistance is the pre-capillary arterioles. These arterioles are less than 100 µm in diameter. They are sometimes known as autoregulatory vessels since they can dynamically change in diameter to increase or reduce blood flow.
Any change in the viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
of blood (such as due to a change in hematocrit
Hematocrit
The hematocrit or packed cell volume or erythrocyte volume fraction is the percentage of the concentration of red blood cells in blood. It is normally about 45% for men and 40% for women...
) would also affect the measured vascular resistance.
Regulation of vascular resistance
There are many factors that alter the vascular resistance. Many of the plateletPlatelet
Platelets, or thrombocytes , are small,irregularly shaped clear cell fragments , 2–3 µm in diameter, which are derived from fragmentation of precursor megakaryocytes. The average lifespan of a platelet is normally just 5 to 9 days...
-derived substances, including serotonin
Serotonin
Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...
, are vasodilatory when the endothelium
Endothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...
is intact and are vasoconstrictive when the endothelium is damaged.
Cholinergic stimulation causes release of endothelium-derived relaxing factor
Endothelium-derived relaxing factor
Endothelium-derived relaxing factor is produced and released by the endothelium to promote smooth muscle relaxation. The best-characterized is nitric oxide . Some sources equate EDRF and nitric oxide....
(EDRF) (later it was discovered that EDRF was nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
) from intact endothelium, causing vasodilation. If the endothelium is damaged, cholinergic stimulation causes vasoconstriction.
Role of adenosine
Adenosine probably doesn't play a role in maintaining the vascular resistance in the resting state. However, it causes vasodilation and decreased vascular resistance during hypoxia. Adenosine is formed in the myocardial cells during hypoxia, ischemia, or vigorous work, due to the breakdown of high-energy phosphate compounds (e.g., adenosine monophosphateAdenosine monophosphate
Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...
, AMP). Most of the adenosine that is produced leaves the cell and acts as a direct vasodilator on the vascular wall. Because adenosine acts as a direct vasodilator, it is not dependent on an intact endothelium to cause vasodilation.
Adenosine causes vasodilation in the small and medium sized resistance arterioles (less than 100 µm in diameter). When adenosine is administered it can cause a coronary steal
Coronary steal
Coronary steal is a phenomenon where an alteration of circulation patterns lead to a reduction in the blood directed to the coronary circulation. It is caused when there is narrowing of the coronary arteries and a coronary vasodilator is used - "stealing" blood away from those parts of the heart...
phenomenon, where the vessels in healthy tissue dilate as much as the ischemic tissue and more blood is shunted away from the ischemic tissue that needs it most. This is the principle behind adenosine stress testing
Cardiac stress test
Cardiac stress test is a test used in medicine and cardiology to measure the heart's ability to respond to external stress in a controlled clinical environment....
.
Adenosine is quickly broken down by adenosine deaminase
Adenosine deaminase
Adenosine deaminase is an enzyme involved in purine metabolism. It is needed for the breakdown of adenosine from food and for the turnover of nucleic acids in tissues.-Reactions:...
, which is present in red cell
Red cell
"Red cell" may refer to* Red blood cell, a type of cell in the blood that transports oxygen* Red Cell, a term in US Government parlance for teams that test the effectiveness of tactics* Red Cell , an episode in the TV series NCIS...
s and the vessel wall.
Coronary vascular resistance
The regulation of tone in the coronary arteries is a complex subject. There are a number of mechanisms for regulating coronary vascular tone, including metabolic demands (i.e.: hypoxia), neurologic control, and endothelial factors (i.e.: EDRF, endothelinEndothelin
Endothelins are proteins that constrict blood vessels and raise blood pressure. They are normally kept in balance by other mechanisms, but when they are over-expressed, they contribute to high blood pressure and heart disease....
).
Local metabolic control (based on metabolic demand) is the most important mechanism of control of coronary flow. Decreased tissue oxygen content and increased tissue CO2 content act as vasodilators. Acidosis acts as a direct coronary vasodilator and also potentiates the actions of adenosine on the coronary vasculature.
See also
- Adenosine
- PerfusionPerfusionIn physiology, perfusion is the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue. The word is derived from the French verb "perfuser" meaning to "pour over or through."...
- 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...
- VasoconstrictionVasoconstrictionVasoconstriction 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...
- VasodilationVasodilationVasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...