Hemodynamics
Encyclopedia
Hemodynamics, meaning literally "blood movement" is the study of blood
flow or the circulation.
All animal cells require oxygen
(O2) for the conversion of carbohydrates, fats and proteins into carbon dioxide
(CO2), water and energy in a process known as aerobic respiration. The circulatory system functions to transport the blood
to deliver O2, nutrients and chemicals to the cells
of the body to ensure their health and proper function, and to remove the cellular waste product
s.
The circulatory system
is a connected series of tubes, which includes the heart
, the arteries, the microcirculation
, and the vein
s.
The heart is the driver of the circulatory system generating cardiac output (CO) by rhythmically contracting and relaxing. This creates changes in regional pressures, and, combined with a complex valvular system in the heart and the veins, ensures that the blood moves around the circulatory system in one direction. The “beating” of the heart generates pulsatile blood flow which is conducted into the arteries, across the micro-circulation and eventually, back via the venous system to the heart. The aorta
, the main artery, leaves the left heart
and proceeds to divide into smaller and smaller arteries until they become arteriole
s, and eventually capillaries, where oxygen transfer occurs. The capillaries connect to venule
s, into which the deoxygenated blood passes from the cells back into the blood, and the blood then travels back through the network of veins to the right heart
. The micro-circulation—the arterioles, capillaries, and venules—constitutes most of the area of the vascular system and is the site of the transfer of O2, glucose
, and enzyme substrates into the cells. The venous system returns the de-oxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO2 and other gaseous wastes exchanged and expelled during breathing. Blood then returns to the left side of the heart where it begins the process again. Clearly the heart, vessels and lungs are all actively involved in maintaining healthy cells and organs, and all influence hemodynamics.
The factors influencing hemodynamics are complex and extensive but include CO, circulating fluid volume, respiration, vascular diameter and resistance, and blood viscosity
. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs or alcohol, obesity and excess weight.
Our understanding of hemodynamics depends on measuring the blood flow at different points in the circulation. A basic approach to understanding hemodynamics is by “feeling the pulse”. This gives simple information regarding the strength of the circulation via the systolic stroke
and the heart rate
, both important components of the circulation which may be altered in disease. The blood pressure can be simply measured using a plethysmograph
or cuff connected to a pressure sensor (mercury or aneroid manometer
). This is the most common clinical measure of circulation and provides a peak systolic pressure and a diastolic
pressure, often quoted as a normal 115/75. Sometimes the mean arterial pressure is calculated.
where:
The arterial pulse pressure can be measured by placing a tonometer or pressure sensor on the skin surface above an artery. This provides a continuous pressure trace or arterial pulse pressure waveform which reflects cardiovascular performance (Fig1). A non-invasive Doppler can also be used to measure blood flow at any point in the circulation, including within the heart, the CO, and can be converted to a pressure difference using the modified Bernoulli equation, ΔP=4V2. An invasive manometer (pressure sensor) can be inserted into an artery on the end of a catheter to measure intra-arterial pulse pressures providing information on cardiovascular performance. Importantly all of these measures should be accompanied by a measure of CO so that the function of the heart and vessels can be distinguished. This allows for more effective understanding and treatment of the cardiovascular system.
The heart and the vascular beds are a dynamic and connected part of the circulatory system and combine to effect efficient transportation of the blood. Circulation is influenced by the resistance of the vascular bed against which the heart is pumping. For the right heart this is the pulmonary vascular bed, creating Pulmonary Vascular Resistance (PVR), while for the systemic circulation this is the systemic vascular bed, creating Systemic Vascular Resistance (SVR). The vessels actively change diameter under the influence of physiology or therapy, vasoconstrictors decrease vessel diameter and increase resistance, while vasodilators increase vessel diameter and decrease resistance. Put simply increasing resistance (narrowing the vessel) decreases CO, and conversely decreased resistance (widening the vessel) increases CO.
This can be explained mathematically:
By simplifying Darcy's Law
, we get the equation that:
When applied to the circulatory system, we get:
However, as MAP >> RAP, and RAP is approximately 0, this can be simplified to:
Physiologists will often re-arrange this equation, making MAP the subject, to study the body's responses.
Hemodynamics both as a clinical medical and as a discipline in physiology and bioengineering has many layers of complexity, beginning in the modern sense in the 1950s. For further information see the references at the end of the article on the Windkessel effect
, and also the 1954 reference to the dynamics of pulsatile blood flow below.
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....
flow or the circulation.
All animal cells require oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2) for the conversion of carbohydrates, fats and proteins into carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2), water and energy in a process known as aerobic respiration. The circulatory system functions to transport the blood
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....
to deliver O2, nutrients and chemicals to the cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
of the body to ensure their health and proper function, and to remove the cellular waste product
Cellular waste product
Cellular waste products are formed as a byproduct of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP...
s.
The circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
is a connected series of tubes, which includes the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
, the arteries, the microcirculation
Microcirculation
The microcirculation is a term used to describe the small vessels in the vasculature which are embedded within organs and are responsible for the distribution of blood within tissues; as opposed to larger vessels in the macrocirculation which transport blood to and from the organs...
, and the vein
Vein
In the circulatory system, veins are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart...
s.
The heart is the driver of the circulatory system generating cardiac output (CO) by rhythmically contracting and relaxing. This creates changes in regional pressures, and, combined with a complex valvular system in the heart and the veins, ensures that the blood moves around the circulatory system in one direction. The “beating” of the heart generates pulsatile blood flow which is conducted into the arteries, across the micro-circulation and eventually, back via the venous system to the heart. The aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
, the main artery, leaves the left heart
Left heart
Left heart is a term used to refer collectively to the left atrium and left ventricle of the heart; occasionally, this term is intended to reference the left atrium, left ventricle, and the aorta collectively....
and proceeds to divide into smaller and smaller arteries until they become arteriole
Arteriole
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, and eventually capillaries, where oxygen transfer occurs. The capillaries connect to venule
Venule
A venule is a very small blood vessel in the microcirculation that allows deoxygenated blood to return from the capillary beds to the larger blood vessels called veins. Venules range from 8 to 100μm in diameter and are formed when capillaries unite .Venules are blood vessels that drain blood...
s, into which the deoxygenated blood passes from the cells back into the blood, and the blood then travels back through the network of veins to the right heart
Right heart
Right heart is a term used to refer collectively to the right atrium and right ventricle of the heart; occasionally, this term is intended to reference the right atrium, right ventricle, and the pulmonary trunk collectively....
. The micro-circulation—the arterioles, capillaries, and venules—constitutes most of the area of the vascular system and is the site of the transfer of O2, glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, and enzyme substrates into the cells. The venous system returns the de-oxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO2 and other gaseous wastes exchanged and expelled during breathing. Blood then returns to the left side of the heart where it begins the process again. Clearly the heart, vessels and lungs are all actively involved in maintaining healthy cells and organs, and all influence hemodynamics.
The factors influencing hemodynamics are complex and extensive but include CO, circulating fluid volume, respiration, vascular diameter and resistance, and blood viscosity
Blood viscosity
Blood viscosity is a measure of the resistance of blood to flow, which is being deformed by either shear or extensional strain.Blood is a liquid that consists of plasma and particles, such as the red blood cells. The viscosity of blood thus depends on the viscosity of the plasma, in combination...
. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs or alcohol, obesity and excess weight.
Our understanding of hemodynamics depends on measuring the blood flow at different points in the circulation. A basic approach to understanding hemodynamics is by “feeling the pulse”. This gives simple information regarding the strength of the circulation via the systolic stroke
Systole (medicine)
Systole is the contraction of the heart. Used alone, it usually means the contraction of the left ventricle.In all mammals, the heart has 4 chambers. The left and right ventricles pump together. The atria and ventricles pump in sequence...
and the heart rate
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....
, both important components of the circulation which may be altered in disease. The blood pressure can be simply measured using a plethysmograph
Plethysmograph
A plethysmograph is an instrument for measuring changes in volume within an organ or whole body .-Lungs:...
or cuff connected to a pressure sensor (mercury or aneroid manometer
Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....
). This is the most common clinical measure of circulation and provides a peak systolic pressure and a diastolic
Diastole
Diastole is the period of time when the heart fills with blood after systole . Ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing...
pressure, often quoted as a normal 115/75. Sometimes the mean arterial pressure is calculated.
- MAP ≈ ((BPdia × 2) + BPsys)/3 mmHg (or torrTorrThe torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
)
-
- BPdia is counted twice since the heart spends two thirds of the heart beat cycle in the diastolic.
where:
- MAP = Mean Arterial Pressure
- BPdia = Diastolic blood pressure
- BPsys = Systolic blood pressure.
The arterial pulse pressure can be measured by placing a tonometer or pressure sensor on the skin surface above an artery. This provides a continuous pressure trace or arterial pulse pressure waveform which reflects cardiovascular performance (Fig1). A non-invasive Doppler can also be used to measure blood flow at any point in the circulation, including within the heart, the CO, and can be converted to a pressure difference using the modified Bernoulli equation, ΔP=4V2. An invasive manometer (pressure sensor) can be inserted into an artery on the end of a catheter to measure intra-arterial pulse pressures providing information on cardiovascular performance. Importantly all of these measures should be accompanied by a measure of CO so that the function of the heart and vessels can be distinguished. This allows for more effective understanding and treatment of the cardiovascular system.
The heart and the vascular beds are a dynamic and connected part of the circulatory system and combine to effect efficient transportation of the blood. Circulation is influenced by the resistance of the vascular bed against which the heart is pumping. For the right heart this is the pulmonary vascular bed, creating Pulmonary Vascular Resistance (PVR), while for the systemic circulation this is the systemic vascular bed, creating Systemic Vascular Resistance (SVR). The vessels actively change diameter under the influence of physiology or therapy, vasoconstrictors decrease vessel diameter and increase resistance, while vasodilators increase vessel diameter and decrease resistance. Put simply increasing resistance (narrowing the vessel) decreases CO, and conversely decreased resistance (widening the vessel) increases CO.
This can be explained mathematically:
By simplifying Darcy's Law
Darcy's law
Darcy's law is a phenomenologically derived constitutive equation that describes the flow of a fluid through a porous medium. The law was formulated by Henry Darcy based on the results of experiments on the flow of water through beds of sand...
, we get the equation that:
- Flow = Pressure/Resistance
When applied to the circulatory system, we get:
- CO = 80 x (MAP – RAP)/TPR
-
- RAP = Mean Right Atrial Pressure in mmHg and
- TPR = Total Peripheral Resistance in dynes-sec-cm-5.
However, as MAP >> RAP, and RAP is approximately 0, this can be simplified to:
- CO ~= 80 x MAP/TPR
-
- For right heart CO ~= MAP/PVR
- For left heart CO ~= MAP/SVR
Physiologists will often re-arrange this equation, making MAP the subject, to study the body's responses.
- 80 x MAP ~= CO x TPR
Hemodynamics both as a clinical medical and as a discipline in physiology and bioengineering has many layers of complexity, beginning in the modern sense in the 1950s. For further information see the references at the end of the article on the Windkessel effect
Windkessel effect
Windkessel effect is a term used in medicine to account for the shape of the arterial pressure waveform in terms of the interaction between the stroke volume and the compliance of the aorta and large elastic arteries . Windkessel in German literally means 'air chamber', but is generally taken to...
, and also the 1954 reference to the dynamics of pulsatile blood flow below.
See also
- 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...
- Blood pressureBlood pressureBlood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
- Blood flowBlood flowBlood flow is the continuous running of blood in the cardiovascular system.The human body is made up of several processes all carrying out various functions. We have the gastrointestinal system which aids the digestion and the absorption of food...
- Electrical CardiometryElectrical CardiometryElectrical Cardiometry is a method based on the model of Electrical Velocimetry, and non-invasively measures stroke volume , cardiac output , and other hemodynamic parameters through the use of 4 surface ECG electrodes. Electrical Cardiometry is a method trademarked by Cardiotronic, Inc., and is U.S...
- PhotoplethysmographPhotoplethysmographA photoplethysmogram is an optically obtained plethysmogram, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption...
- Impedance cardiographyImpedance cardiographyImpedance cardiography is a plethysmography technique of using sensors to detect the properties of the blood flow in the thorax.-Introduction:...
- Blood hammerBlood hammerThe blood hammer phenomenon is a sudden increase of the upstream blood pressure in a blood vessel when the bloodstream is abruptly blocked by vessel obstruction...
- WindkesselWindkesselWindkessel literally means air chamber and is a German translation of a concept described previousy by Stephen Hales, who likened the effect of the elastic arteries in dampening the arterial pulse to that of an air chamber in some 18th century fire engines...