Hemodynamics.
Hemodynamics refers to the study of movement of blood through circulatory system. Major function of cardiovascular system is to pump the blood and to 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. Circulatory system is designed for carrying out all these actions.
Mean volume of blood flow.
Mean volume of blood flow is the volume of blood which flows into the region of circulatory system in a given unit of time. It is the product of mean velocity and the cross- sectional area of the vascular bed.
Importance of Mean Volume of Blood Flow.
Blood transport of foodstuffs and oxygen to the tissues and waste products away from the tissues, mean volume of blood flow is of greater physiological importance than linear velocity.
Types of Blood Flow.
Blood flow through a blood vessel is of two types:
1. Streamline or laminar flow.
2. 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 moves very slowly. Next layer within the vessel has a low momentum. Next layer of blood 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 . Streamline 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 critical level, the flow becomes turbulent. Turbulent flow creates sounds.
Factor Determines Volume of Blood Flow.
Volume of blood flow is determined by five factors:
1. Pressure gradient.
2. Resistance to blood flow.
3. Viscosity of blood.
4. Diameter of blood vessels.
5. Velocity of blood flow.
1. Pressure Gradient.
Volume of blood flowing through any blood vessel is directly proportional to the pressure gradient. Pressure gradient is the pressure difference between the two ends of the blood vessel.
Formula to determine pressure gradient .
Pressure gradient = P1– P2
Where, P1 = Pressure at proximal end of the vessel .
P2 = Pressure at distal end of the vessel.
Maximum pressure gradient exists between the aorta and the inferior vena cava. The pressure in aorta is 120 mm Hg and the pressure in inferior vena cava is 0 mm Hg. So, the pressure gradient is 120 – 0 = 120 mm Hg.
2. Resistance to Blood Flow (Peripheral Resistance).
Volume of blood flow is inversely proportional to the 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. Though resistance exists in all the blood vessels to some extent, it is remarkable in the peripheral vessels, particularly the arterioles.
Determinants of peripheral resistance.
a. Radius of blood vessels
b. Pressure gradient
c. Viscosity of blood.
Peripheral resistance is inversely related to radius of the blood vessel, i.e. lesser the radius, more will be the resistance. Radius of the arterioles is very less. It is because the arterioles remain partially constricted all the time due to sympathetic tone. So, the resistance is more. Hence, the arterioles are called resistant vessels.
3. Viscosity of Blood .
Volume of blood flow is inversely proportional to the 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.
Factors determining viscosity.
RBC count 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 case of burns or in polycythemia, the viscosity increases and the velocity of blood flow decreases, so the volume of blood reaching the organ is decreased.
4. Diameter of Blood Vessels.
Volume of blood flow is directly proportional to the diameter of the blood vessels. When the diameter of a segment of blood vessel is considered, the aorta has the maximum diameter and capillary has got 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. Cross-sectional area is progressively increased as the arteries ramify and as the distance from the heart is increased.
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 2,500 cm2 . But, the cross-sectional area is subjected to variations under physiological and pathological conditions. Diameter of the aorta depends upon the elasticity of the wall and its recoiling tendency helps in maintaining the flow and pressure. Diameter of the arterioles depends upon the sympathetic tone.
5. Velocity of Blood Flow.
Volume of blood flow is directly proportional to the velocity of blood flow. Velocity of blood flow is the rate at which blood flows through a particular region.
Windkessel effect.
Windkessel effect is the recoiling effect of blood vessels that converts the pulsatile flow of blood into a continuous flow. Blood vessels showing the windkessel effect are known as windkessel vessels. 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 during diastole, it becomes almost negative. This
variation is noticed even in the larger arteries.
During systole, velocity of blood flow reaches maximum, because of the force created by contraction of the heart. Therefore, the maximum volume of blood is pumped into the aorta. During diastole, this force is absent and the volume of blood entering the aorta is zero. Thus, the flow of blood into the aorta is not continuous. This type of flow is called pulsatile flow. However, the flow of blood through other blood vessels is continuous. It is because of the behavioral pattern of aorta and to a little extent, the behavioral pattern of 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.
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 blood vessel.
Mean Velocity of Blood Flow in different vessels.
Mean velocity (cm/second) of blood flow in different blood vessels:
1. Large arteries : 50.00
2. Small arteries : 5.00
3. Arterioles : 0.50
4. Capillaries : 0.05
5. Venules : 0.10
6. Small veins : 1.00
7. Large veins : 2.00 .
Factor Maintaining Velocity.
Three factors are responsible for the maintenance of the velocity of blood flow:
1. Cardiac output
2. Cross-sectional area of the blood vessel
3. Viscosity of the blood.
1. Cardiac Output.
Velocity of blood flow is directly proportional to cardiac output. Increase in cardiac output leads to increase in the velocity of blood flow in all parts of the circulation.
2. Cross-sectional Area of Blood Vessels.
Velocity of blood flow is inversely proportional to the total cross-sectional area of the vascular bed, through which the blood circulates. Cross-sectional area increases progressively as the arteries ramify. 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, velocity of blood flow is decreased as the distance from the heart is increased.
3. Viscosity of Blood.
Velocity of blood flow is inversely proportional to the viscosity of blood. If viscosity is more, the velocity of blood flow is reduced . It is because of the friction of blood against arterial wall, which is more when viscosity of blood is increased.
Phase Changes in the Velocity of Blood Flow.
Velocity of blood flow is altered according to the phases of cardiac cycle. Blood flows in the large arteries at a greater speed during systole than during diastole. In common carotid artery, the velocity reaches 50 cm/sec during systole and it is only 30 cm/sec during diastole.
Circulation Time Definition.
Circulation time is the time taken by blood to travel through a part or whole of the circulatory system. If a substance is injected into a vein, the time taken by it to appear in the blood of the same vein or in the corresponding vein on the opposite side shows the total circulation time.
Similarly, if the transit is from vein to the lungs, it shows the circulation time through pulmonary circuit and if it is from vein to capillaries, it shows the time for flow through 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 bloodstream and determining the time when the substance appears at a given point (end point) in the circulation. The injected substance must produce some characteristic response at its end point, so that its appearance could be easily recognized. Introduction of the substance into circulation is done by injecting through median cubital vein or directly into the heart.
Substances used for Measuring Circulation Time.
1. Histamine: Causes flushing of face due to vasodilatation.
2. Dehydrocholine (20%): Gives a bitter taste when it reaches the tongue.
3. Ether or acetone: Detectable in breath by smell.
4. Sodium cyanide (small dose): Causes hyperpnea when it reaches the carotid artery (by acting on baroreceptors).
5. Dye fluorescein: Identified at the end point by yellow color; it is used for total circulation time.
6. Radioactive substances: Detected at various points of the body by using an ionization chamber.
Typical Circulation Time.
1. Arm vein to arm vein (total circulation time): 25 seconds (22 to 28), determined by using dye fluorescein.
2. Arm vein to face: 24 seconds, determined by using histamine.
3. Arm vein to tongue: 11 seconds (8 to 16), determined by using dehydrocholine .
4. Arm vein to lung (pulmonary circulatory time): 6 seconds (4 to 6), determined by using ether or acetone.
5. Arm vein to heart (shortest circulation time): 4 seconds, determined by using radioactive substances.
6. Arm vein to carotid artery: 14 seconds (12 to 15), determined by using sodium cyanide.
Total Circulation Time & Heartbeat.
Number of heartbeat/total circulation time, however, remains the same for human beings and all the animals, i.e. about 30/total circulation time.
Conditions alter Circulation Time.
Circulation time is decreased when the velocity of blood flow is increased and the circulation time is more when the velocity is less.
Conditions when Circulation Time is Prolonged (Sluggish Blood Flow).
1. Myxedema: Due to decreased metabolic activity.
2. Polycythemia: Due to increased viscosity of blood.
3. Cardiac failure: Due to inability of the heart to pump blood.
Conditions when Circulation Time is Shortened (Rapid Blood Flow).
1. Exercise: Due to increased cardiac activity and vasodilatation.
2. Adrenaline administration: Due to increased cardiac activity.
3. Hyperthyroidism: Due to increased metabolic activity.
4. Anemia: Due to decreased blood volume and less viscosity.
5. Decrease in peripheral resistance: Due to vasodilatation.
Local regulation of Blood Flow by autoregulation.
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. Local mechanisms start functioning and the blood flow is brought to relatively normal level within few minutes. Autoregulatory response is independent of neural and hormonal influences. So, it is the intrinsic capacity of the organ.
Role of Pressure in Autoregulation.
Perfusion Pressure and Effective Perfusion Pressure.
Generally, the term perfusion pressure refers to balance between the pressure in blood vessels on either side of the organ, i.e. arterial pressure minus venous pressure (PA – PV) across the organ. However, the blood flow to any organ or region of the body depends up on the effective perfusion pressure. Effective perfusion pressure is the perfusion pressure divided by resistance in the blood vessels.
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, blood flow through an organ is kept constant when the mean arterial pressure increases up to 170 mm Hg or when it falls till 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.
Autoregulation is explained by two theories:
1. Myogenic theory.
2. 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 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.
Stretching of blood vessels due to increased blood pressure increases the flow of calcium ions into the cells from ECF. Calcium influx causes 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 increase in blood flow.
2. Metabolic Theory.
According to metabolic theory the 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 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.
Common vasodilators of metabolic origin:
a. Adenosine
b. Carbon dioxide
c. Lactate
d. Hydrogen.
Autoregulation in some Vitals Organs.
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 kidney ,heart and brain . Mechanism of autoregulation also varies slightly in these organs.
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