Measurement of Flow and Volume of Blood
Topic One: Introduction to Flow and Volume of Blood
Characteristics of Blood Flow
Blood flow is the volume of blood passing through a specific point in the circulatory system per unit
of time.
The highest blood flow occurs in the pulmonary artery and aorta, known as cardiac output, which
ranges from 3.5 to 5 liters per minute in a normal resting adult.
In capillaries, blood flow can be slow enough to observe individual blood cells under a microscope.
The stroke volume is the amount of blood ejected during each heart contraction and can be calculated
by dividing the cardiac output by the number of heartbeats per minute.
Mean circulation time is obtained by dividing the total blood volume by the cardiac output.
Mean velocity of blood at a specific point can be calculated by dividing the blood flow through a
vessel by the vessel's cross-sectional area.
Blood flow in arteries is pulsatile, but the elasticity of vessel walls helps to smooth out the pulsations.
Blood flow depends on blood pressure and flow resistance in blood vessels, similar to how electrical
current flow depends on voltage and resistance.
The flow resistance of capillary beds can vary widely due to factors like temperature, drugs,
vasoconstriction, and vasodilation.
Blood velocity varies across the cross-section of a vessel, with the highest velocity at the center and
zero velocity near the vessel walls.
Some blood flow meters measure mean velocity, which can be calibrated to determine blood flow if
the vessel's cross-sectional area is known.
Reduced blood supply to an organ can severely limit its function, while complete obstruction can lead
to tissue death.
Cerebrovascular accidents (strokes), myocardial infarctions (heart attacks), angina pectoris (chest
pain), embolisms, and thrombosis are examples of circulatory system disorders caused by reduced or
obstructed blood flow.
Determining blood flow in such cases is crucial for early diagnosis and prompt treatment to prevent
irreparable tissue damage.
Figure 1.1Laminar flow in a blood vessel
Mechanical malfunctions, such as heart valve issues, or conditions like shock can significantly
reduce cardiac output and affect the entire circulatory system.
Understanding blood flow and its measurement is important for diagnosing and treating various
circulatory system disorders and ensuring proper organ function.
FLOW and Volume of Blood Mechanism
, The prime guarantor of tissue oxygenation is tissue blood flow. Therefore, on the premise of intact
macrohemodynamics, the microcirculation has three major responsibilities:
Providing access for oxygenated blood to the tissues and appropriate return of volume;
Maintaining global tissue flood flow, even in the face of changes in central blood pressure;
Linking local blood flow to local metabolic needs.
It is an intriguing concept of nature to do this mainly by local regulatory mechanisms, impacting
primarily on flow resistance, be this via endothelial or direct smooth muscle actions. The final goal of
microvascular blood flow per unit of time is to ensure the needed exchange of substances between tissue
and blood compartments. The two principle means of accomplishing this are diffusion and filtration.
Blood flow is one important physiological parameter and also one of the most difficult to measure
accurately because the instruments for measuring the flow through blood vessels within the body have to
meet certain specifications:
sensitivity
stability
Such requirements depend upon the magnitude of flow, location, and diameter of the individual vessels.
The average velocities of blood flow vary over a wide range in vessels with diameters ranging from 2 cm
to a few millimeters.
The measuring system shall meet the specific clinical requirement of being the least traumatic making
Blood flow measurement a difficult engineering and clinical problem. However, a variety of techniques
have been developed to meet the requirements of an ideal flow metering system.
There are techniques for measuring blood flow and velocity which are categorized into;
Invasive (surgical)
Non-invasive (through the skin).
The blood flow and volume measurements are:
Instantaneous Flow Measurement
Mean Flow Measurement
INSTANTANEOUS FLOW MEASUREMENT:
1. Electromagnetic flowmeter
2. Ultrasonic flowmeter
3. NMR Blood Flowmeter
The electromagnetic flowmeter
The electromagnetic flowmeter operates based on the generation of an induced current when a conductor
moves within a magnetic field. The direction of this current is determined using Fleming's right-hand rule.
In this rule, the thumb points in the direction of the conductor's motion, the index finger points in the
Topic One: Introduction to Flow and Volume of Blood
Characteristics of Blood Flow
Blood flow is the volume of blood passing through a specific point in the circulatory system per unit
of time.
The highest blood flow occurs in the pulmonary artery and aorta, known as cardiac output, which
ranges from 3.5 to 5 liters per minute in a normal resting adult.
In capillaries, blood flow can be slow enough to observe individual blood cells under a microscope.
The stroke volume is the amount of blood ejected during each heart contraction and can be calculated
by dividing the cardiac output by the number of heartbeats per minute.
Mean circulation time is obtained by dividing the total blood volume by the cardiac output.
Mean velocity of blood at a specific point can be calculated by dividing the blood flow through a
vessel by the vessel's cross-sectional area.
Blood flow in arteries is pulsatile, but the elasticity of vessel walls helps to smooth out the pulsations.
Blood flow depends on blood pressure and flow resistance in blood vessels, similar to how electrical
current flow depends on voltage and resistance.
The flow resistance of capillary beds can vary widely due to factors like temperature, drugs,
vasoconstriction, and vasodilation.
Blood velocity varies across the cross-section of a vessel, with the highest velocity at the center and
zero velocity near the vessel walls.
Some blood flow meters measure mean velocity, which can be calibrated to determine blood flow if
the vessel's cross-sectional area is known.
Reduced blood supply to an organ can severely limit its function, while complete obstruction can lead
to tissue death.
Cerebrovascular accidents (strokes), myocardial infarctions (heart attacks), angina pectoris (chest
pain), embolisms, and thrombosis are examples of circulatory system disorders caused by reduced or
obstructed blood flow.
Determining blood flow in such cases is crucial for early diagnosis and prompt treatment to prevent
irreparable tissue damage.
Figure 1.1Laminar flow in a blood vessel
Mechanical malfunctions, such as heart valve issues, or conditions like shock can significantly
reduce cardiac output and affect the entire circulatory system.
Understanding blood flow and its measurement is important for diagnosing and treating various
circulatory system disorders and ensuring proper organ function.
FLOW and Volume of Blood Mechanism
, The prime guarantor of tissue oxygenation is tissue blood flow. Therefore, on the premise of intact
macrohemodynamics, the microcirculation has three major responsibilities:
Providing access for oxygenated blood to the tissues and appropriate return of volume;
Maintaining global tissue flood flow, even in the face of changes in central blood pressure;
Linking local blood flow to local metabolic needs.
It is an intriguing concept of nature to do this mainly by local regulatory mechanisms, impacting
primarily on flow resistance, be this via endothelial or direct smooth muscle actions. The final goal of
microvascular blood flow per unit of time is to ensure the needed exchange of substances between tissue
and blood compartments. The two principle means of accomplishing this are diffusion and filtration.
Blood flow is one important physiological parameter and also one of the most difficult to measure
accurately because the instruments for measuring the flow through blood vessels within the body have to
meet certain specifications:
sensitivity
stability
Such requirements depend upon the magnitude of flow, location, and diameter of the individual vessels.
The average velocities of blood flow vary over a wide range in vessels with diameters ranging from 2 cm
to a few millimeters.
The measuring system shall meet the specific clinical requirement of being the least traumatic making
Blood flow measurement a difficult engineering and clinical problem. However, a variety of techniques
have been developed to meet the requirements of an ideal flow metering system.
There are techniques for measuring blood flow and velocity which are categorized into;
Invasive (surgical)
Non-invasive (through the skin).
The blood flow and volume measurements are:
Instantaneous Flow Measurement
Mean Flow Measurement
INSTANTANEOUS FLOW MEASUREMENT:
1. Electromagnetic flowmeter
2. Ultrasonic flowmeter
3. NMR Blood Flowmeter
The electromagnetic flowmeter
The electromagnetic flowmeter operates based on the generation of an induced current when a conductor
moves within a magnetic field. The direction of this current is determined using Fleming's right-hand rule.
In this rule, the thumb points in the direction of the conductor's motion, the index finger points in the
