Hemodynamics Introduction
- Dynamics means the study of motion. The term hemodynamics refers to the study of the movement of blood through the circulatory system.
- The major function of the cardiovascular system is to pump blood and circulate it through different parts of the body.
- It is essential for the maintenance of pressure and other physical factors within the blood vessels so that, the volume of blood supplied to different parts of the body is adequate.
- The circulatory system is designed for carrying out all these actions.
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Table of Contents
Mean Volume Of Blood Flow
Definition And Formula
The mean volume of blood flow is the volume of blood, which flows into the region of the circulatory system in a given unit of time.
It is the product of mean velocity and the cross-sectional area of the vascular bed.
Q = V x A
Where,
- Q = Quantity of blood
- V = Velocity of blood flow
- A = Cross-sectional area of the blood vessel.
Blood Flow Importance
In terms of the transport of foodstuffs and oxygen to the tissues and waste products away from the tissues, the mean volume of blood flow is of greater physiological importance than the linear velocity.
Methods Of Study
1. By Using Flowmeters
The different types of flowmeters.
2. By Using Plethysmograph
A plethysmograph is an instrument used for measuring the volume of an enclosed organ
3. By Venous Occlusion Plethysmography
In this, the venous outflow from an organ is stopped by clamping the vein without disturbing the artery.
The blood flowing into the organ causes a corresponding increase in its volume for the first few seconds.
This increase in volume is recorded graphically. The amount of flow is determined by the proper calibration of the graph.
4. By Fick’s Principle
Fick’s principle is explained in the measurement of cardiac output.
Types Of Blood Flow
The flow of blood through a blood vessel is of two types:
- Streamline or laminar flow
- Turbulent flow.
1. Streamline Flow
- Streamline flow is a silent flow. Within the blood vessel, a very thin layer of blood is in contact with the vessel wall. It does not move or move very slowly.
- The next layer within the vessel has a low momentum. The next layer has a slightly higher momentum.
- Gradually, the momentum increases in the inner layers so that the momentum is greatest in the center of the stream.
- This type of flow is known as streamline flow and it does not produce any sound within the vessel.
- The streamlined flow occurs only at velocities up to a critical level.
2. Turbulent Flow
Turbulent flow is the noisy flow. When the velocity of blood flow increases above a critical level, the flow becomes turbulent.
Turbulent flow creates sounds. The critical velocity at which the flow becomes turbulent is known as Reynold’s number.
The formula to determine Reynold’s number is:
- NR= PDV/η
- Nr = Reynold’s number
- P = Density of the blood
- D = Diameter of the vessel
- V = Velocity of the flow
- η = Viscosity of the blood
Factors Maintaining Volume Of Blood Flow
Bleed flows are determined by five factors:
- Pressure gradient
- Resistance to blood flow
- Viscosity of blood
- Diameter of blood vessels
- The velocity of blood flow
1. Pressure Gradient
The pressure gradient is the pressure difference between the two ends of the blood vessel. The volume of blood flowing through any blood vessel is directly proportional to the pressure difference.
The pressure gradient is expressed as follows:
Pressure gradient = P., – p2.
Where,
P-1 = Pressure at the proximal end of the vessel
P2 = Pressure at a distal end of the vessel.
The maximum pressure gradient exists between the aorta and the inferior vena cava. The pressure in the aorta is 120 mm Hg and the pressure in the inferior vena cava is 0 mm Hg.
So, the pressure gradient is 120 – 0 = 120 mm Hg. The pressure gradient in different areas of the vascular bed.
2. Resistance to the Blood Flow (Peripheral Resistance)
- Resistance is the friction, tension, or hindrance against which the blood has to flow.
- Peripheral resistance means the resistance offered to blood flow in peripheral blood vessels.
- The volume of blood flow is inversely proportional to the resistance.
- Though resistance exists in all the blood vessels to some extent, it is remarkable in the peripheral vessels, particularly the arterioles.
Three important factors determine peripheral resistance:
- The radius of blood vessels
- Pressure gradient
- Viscosity of blood.
Peripheral resistance is inversely related to the radius of the blood vessel, i.e.
- The lesser the radius,-the more will be the resistance. The radius of the arterioles is very less because of the sympathetic tone.
- The arterioles remain partially constricted all the time because of the sympathetic tone. So, the resistance is more here.
- The arterioles are known as resistant vessels because of this reason.
The simple formula to determine resistance is as follows:
Resistance = Pressure gradient/The volume of blood flow =P1 -P2/Q
3. Viscosity of Blood
- Viscosity is the friction of blood against the wall of the blood vessel. Isaac Newton described viscosity as the internal friction or lack of slipperiness. Viscosity influences the blood flow through resistance.
- The volume of blood flow is inversely proportional to the viscosity of blood.
- The number of red blood cells is the main factor, which determines the viscosity of the blood.
- Another factor determining viscosity is plasma protein, mainly albumin.
- When hemoconcentration occurs as in the case of burns or /Cynthia, the viscosity increases and the veio -blood flow decreases, so the volume of blood returned to me organ is decreased.
4. Diameter of Blood Vessels
- The volume of blood flow is directly proportional to the diameter. When the diameter of a segment of a blood vessel is considered, the aorta has the maximum diameter and the capillary has the minimum diameter.
- But, in circulation, the diameter of the vessel is considered in relation to the cross-sectional area through which the blood flows.
- The cross-sectional area is progressively increased as the arteries ramify and as the distance from the heart is increased.
- The cross-sectional area of each branch is smaller, but the sum of the cross-sectional areas of all the branches is always greater than that of the parent vessel.
- In this way, the aorta has got less cross-sectional area of 4 cm2 compared to that of capillaries, which is 2500 cm2.
- However, the cross-sectional area is subjected to variations under physiological and pathological conditions.
- The diameter of the aorta depends upon the elasticity of the wall. The recoiling tendency helps in maintaining the flow and pressure.
- The diameter of the arterioles depends upon the sympathetic tone.
5. Velocity of Blood Flow
- The velocity of blood flow is the rate at which blood flows through a particular region.
- The mean volume of blood flow is directly proportional to the velocity of blood flow.
Hagen-Poiseuille Equation
Hagen and Poiseuille have worked on dynamics extensively. The equation, which explains the relationship between different variables of dynamics, is named after them.
The variables of dynamics are applied to hemodynamics also.
According to the Hagen-Poiseuille equation, the volume- (Q) of any fluid flowing through a rigid tube is:
- Directly proportional to a pressure gradient (P1 – P2)
- Directly proportional to the fourth power of radius {A4)
- Inversely proportional to the length of the tube (L)
K is the constant for fluid flowing at a temperature. It is directly proportional to the temperature of the fluid.
The viscosity of the fluid is also affected by the temperature. The viscosity is inversely proportional to the temperature of the fluid.
Therefore, in the equation, the constant ‘K’ is expressed as the reciprocal of viscosity.
The volume of flow of fluid is always expressed in a given unit of time, p/8 is the arithmetic value derived while determining the volume of fluid flowing in a given unit of time.
So, the equation has to be rewritten as:
Windkessel Effect
- The Windkessel effect is the recoiling effect of blood vessels that converts the pulsatile flow of blood into a continuous flow.
- The blood vessels showing the windkessel effect are known as windkessel vessels.
- The mean velocity of the blood that flows through the aorta is more than 50 cm/second, but it is not constant.
- During systole, it increases up to 120 cm/second, and article, it becomes almost negative. This variation in the larger arteries.
- The velocity of blood flow reaches the use of the force created by the contract. Therefore, the maximum volume of blood -to the aorta.
- During diastole, this force is ended ihe volume of blood entering the aorta is zero, in us, ihe flow of blood into the aorta is not continuous. This type of flow is called pulsatile flow.
- However, the flow of blood through the other blood vessels is continuous.
- It is because of the behavioral pattern of the aorta and to a lesser extent, the behavioral pattern of the larger arteries.
- During systole, the aorta is completely dilated and, during diastole, it recoils.
- The elastic recoiling of this vessel creates the continuous momentum of blood. So, the pulsatile flow of blood is converted into a continuous flow.
- This effect was named as windkessel effect by Otto Frank in 1899. Windkessel is a German word used for an elastic reservoir.
- Thus, the windkessel vessels play an important role in maintaining the continuous flow of blood through the circulatory tree by acting as a second pump, the first pump being the heart.
Velocity Of Blood Flow Definition
- The velocity of blood flow is the rate at which blood flows through a particular region of the body.
- It mainly depends upon the diameter or cross-sectional area of the blood vessel.
Mean Velocity Of Blood Flow In Different Vessels
The following is the mean velocity (cm/second) of blood flow in different blood vessels:
- Large arteries: 50.00
- Small arteries: 5.00
- Arterioles: 0.50
- Capillaries: 0.05
- Venules: 0.10
- Small veins: 1.00
- Large veins: 2.00
Methods Of Study
1. By Using Flo waters
The flowmeters.
2. By Hemodromography
Hemodromography is a technique by which the velocity of blood is continuously recorded.
Factors Maintaining Velocity
Three factors are responsible for the maintenance of the velocity of blood flow:
- Cardiac output
- The cross-sectional area of the blood vessel
- The viscosity of the blood.
1. Cardiac Output
The velocity of blood flow is directly proportional to cardiac output. An increase in cardiac output leads to an increase in the velocity of blood flow in all parts of the circulation.
2. Cross-sectional Area of Blood Vessels
Velocity varies inversely with the total cross-sectional area of the vascular bed through which the blood circulates.
The cross-sectional area increases progressively as the arteries ramify. The cross-sectional area of each branch is smaller, but the sum of the cross-sectional areas of all the branches is always greater than that of the parent vessel.
So, the velocity of blood flow is decreased as the distance from the heart is increased.
3. Viscosity of Blood
- The velocity of blood flow is inversely proportional to the viscosity of blood.
- If viscosity is more, the velocity of blood flow is reduced (see in factors maintaining the volume of blood flow).
- It is because of the friction of blood against the arterial wall, which is more when the viscosity of blood is increased.
Phasic Changes In The Velocity Of Blood Flow
- The velocity of blood flow is altered according to the phases of the cardiac cycle.
- Blood flows in the large arteries at a greater speed during systole than during diastole.
- In the common carotid artery, the velocity reaches 50 cm/ second during systole and it is only 30 cm/second during diastole.
Circulation Time Definition
- Circulation time is the time taken by the blood to travel through a part or whole of the circulatory system.
- If a substance is injected into a vein, the time taken by if K appears in the blood of the same vein or in the corresponding vein on the opposite side shows the total Arison lime.
- shady, if the transit is from vein to the lungs circulation time through the pulmonary circuit a from vein to capillaries, it shows the time for iUw through the pulmonary circuit, left heart, and arteries to capillaries, i.e. the total circulation time minus the time for venous return.
Measurement Of Circulation Time
- Circulation time is measured by introducing some easily recognized substance into the bloodstream and determining the time when the substance appears at a given point (endpoint) in the circulation.
- The injected substance must produce some characteristic response at its end point so that, its appearance could be easily recognized.
- The introduction of the substance into circulation is done by injecting it through a median cubital vein or directly into the heart.
Substances Used for Measuring Circulation Time
- Histamine – causes flushing of the face due to vasodilatation.
- Dehydrocholine (20%) – gives a bitter taste when it reaches the tongue.
- Ether or acetone – is detectable in breath by smell.
- Sodium cyanide (small dose) – causes hyperpnea when it reaches the carotid artery (by acting on baro- receptors).
- Dye fluorescein – identified at the endpoint by yellow color. It is used for total circulation time.
- Radioactive substances – which are detected at various points of the body by the use of an ionization chamber.
Typical Circulation Times
- Arm vein to arm vein – total circulation time – 25 seconds (22 to 28): (Dye fluorescein)
- Arm vein to face – 24 seconds: Histamine
- Arm vein to tongue – 11 seconds (8 to 16): Dehydrocholine
- Arm vein to lung – Pulmonary circulatory time – 6 seconds (4 to 6): Ether or acetone
- Arm vein to heart – 4 seconds: Radioactive substance-shortest circulation time
- Arm vein to carotid artery – 14 seconds (12 to 15): By sodium cyanide.
Total Circulation Time And Heartbeat
The number of heartbeats/total circulation time, however, remains the same for human beings and all animals, i.e. about 30/total circulation time.
Conditions Altering Circulation
The circulation time is decreased when the velocity si blood flow is increased and, the circulation time is more when the velocity is less.
Conditions in which Circulation Time is Prolonged (Sluggish Blood Flow)
- Myxedema – due to decreased metabolic activity
- Polycythemia – due to increased viscosity of blood
- Cardiac failure – due to the inability of the heart to pump blood.
- Conditions in which Circulation Time is Shortened (Rapid Blood Flow)
- Exercise – due to increased cardiac activity and vasodilatation
- Adrenaline administration – due to increased cardiac activity
- Hyperthyroidism – due to increased metabolic activity
- Anemia – due to decreased blood volume and less viscosity
- Decrease in peripheral resistance – due to vasodilatation.
Local Regulation Of Blood Flow – Autoregulation
Blood Flow Introduction
- Autoregulation means the regulation of blood flow to an organ by the organ itself.
- It is defined as the intrinsic ability of an organ to regulate a constant blood flow in spite of changes in the perfusion pressure (arterial pressure – venous pressure).
- Normally, a sudden increase or decrease in arterial blood pressure momentarily increases or decreases the blood flow.
- The local mechanisms start functioning and the blood flow is brought to a relatively normal level within a few minutes.
- The autoregulatory response is independent of neural and hormonal influences. So it is the intrinsic capacity of the organ.
Role Of Pressures In Autoregulation
Perfusion Pressure and Effective Perfusion Pressure
- Generally, the term perfusion pressure refers to the balance between the pressure in blood vessels on either side of -can i.e. arterial pressure minus venous pressure organ (PA-PV).
- The blood flow (F) to any organ or region, of Disney defends up on the effective perfusion pressure.
- Tba effective perfusion pressure is the perfusion pressure divided by resistance (R) in the blood vessels.
- It is expressed as But basically, the major factor that determines the perfusion pressure and effective perfusion pressure is the mean arterial pressure. The normal mean arterial blood pressure is about 93 mm Hg.
- Usually, flood flows through an organ are kept constant when the mean arterial pressure increases up to 170 mm Hg or when it falls to 60 mm Hg (the range varies slightly in different organs).
- However, beyond this range, the autoregulation fails and the blood flow is altered in relation to rise or fall in pressure.
Theories Of Autoregulation
Two theories are available to explain the autoregulation:
- Myogenic theory
- Metabolic theory
1. Myogenic Theory
- According to this theory, the intrinsic contractile property of the smooth muscle fibers present in the blood vessels is responsible for autoregulation.
- It is known that the sudden stretching of blood vessels causes the contraction of smooth muscle fibers present in the wall of the vessels, particularly, small arteries and arterioles.
- So, when the arterial blood pressure increases suddenly, the stretching of the blood vessels immediately causes vasoconstriction and thereby the blood flow is controlled.
- The stretching of blood vessels due to increased blood pressure increases the flow of calcium ions into the cells from ECF.
- The influx of calcium ions causes the contraction of smooth muscles in the blood vessels leading to vasoconstriction.
- On the other hand, when the blood pressure is less, the stretching of blood vessels is less causing vasodilatation and an increase in blood flow.
2. Metabolic Theory
- According to metabolic theory, normal blood flow is maintained by the metabolic end products.
- Normally, the flow of blood washes away the metabolic end products. When the blood flow is reduced, there is an accumulation of metabolites.
- These metabolites dilate the blood vessels and bring the blood flow back to normal.
- Conversely, when blood flow increases, the vasodilator metabolites are washed out of the tissues quickly.
- It leads to vasoconstriction and the volume of blood flow becomes normal.
- The common vasodilators of metabolic origin are adenosine, carbon dioxide, lactate, and hydrogen ions.
Autoregulation In Some Vital Organs
- The volume of blood flow is regulated by local mechanisms in almost all the tissues of the body. However, autoregulation is more effective in some of the vital organs like the kidneys, heart, and brain.
- The mechanism of autoregulation varies slightly in these organs.
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