Hemodynamics of Blood Flow in The Circulatory System PDF

# Hemodynamics of Blood Flow in The Circulatory System ## Prof. Sahar El Agaty | Professor of Physiology ## Hemodynamics 2-Physiology Lecture 8 ### Intended Learning Outcomes - Differentiate laminar and turbulent blood flow - Identify transmural (distending) pressure of blood vessels and critical...

# Hemodynamics of Blood Flow in The Circulatory System ## Prof. Sahar El Agaty | Professor of Physiology ## Hemodynamics 2-Physiology Lecture 8 ### Intended Learning Outcomes - Differentiate laminar and turbulent blood flow - Identify transmural (distending) pressure of blood vessels and critical closing pressure. - Recognize pressure-volume relationship in blood vessels. - Give an account on velocity of blood flow in different types of blood vessels. - Discuss Bernoulli's principle. - Identify Laplace law and state its significance in heart, lung and capillaries. ## 2. Blood flow ### 2.6. Types of blood flow #### 1. Laminar flow: - The flow of blood in straight blood vessels, like the flow of liquids in narrow rigid tubes, is normally laminar, smooth and silent. - The outer layer of blood stream in contact with the wall of the vessel does not move. - Then the velocity of blood increases gradually from outer to inner layers; till reach the greatest velocity in the center of the stream. - Laminar flow occurs at velocities up to a certain critical velocity. At or above this velocity, flow is turbulent. #### 2. Turbulent flow: - It is a disturbed blood flow, forming eddies in different directions. - It has a sound (bruits, or murmurs) that can be heard by stethoscope. - It may produce shedding of the endothelium which leads to endothelial damage and atherosclerosis. ### The probability of turbulence (Reynolds number): - Laminar flow may be changed into turbulent flow. - The probability of turbulence is called Reynolds number. - It is directly related to the density of blood, velocity of blood flow, and diameter of the vessel and is inversely related to the viscosity of the blood. - This probability can be expressed as follows: - $Re = \frac{ρDV}{η}$ - Re is the Reynolds number - p is the density of the fluid - D is the diameter of the tube - V is the velocity of the flow - n is the viscosity of the fluid. - Flow is usually not turbulent if Re is less than 2000. - If Re > 2000, turbulent flow may occur (transitional flow). - When Re is > 3000, turbulence is almost always present. ### Examples of turbulent flow: - Normally turbulence occurs at the branching of vessels and the root of aorta during ejection of blood (systole), because of high velocity. - The sounds of Korotkoff heard when measuring blood pressure by auscultatory method are bruits of turbulent flow. - Pathologically turbulence occurs: 1. Beyond points of constriction e.g., atherosclerotic plaques. 2. Severe anemia due to increased blood velocity (as a result of decreased blood viscosity) 3. Cardiac valve stenosis or regurgitation (incompetence). ## 3. Transmural (distending) pressure of blood vessels and critical closing pressure. - The blood vessel is kept opened because the internal pressure (Pi) is greater than the external (tissue) pressure (Pt) the vessel. In other word the transmural (or distending pressure) is positive. ### Transmural (Distending) pressure (∆Ρ)= Pi-Pt - If the transmural pressure is positive the blood vessel expands. - If the transmural pressure is negative the blood vessel collapse (compressed by the high outside pressure) and the flow stops although the internal pressure is not zero. - The pressure at which flow stops, and vessel collapse is called the critical closing pressure. ## 4. Pressure-Volume relationship in blood vessels - Vascular elasticity is the resistance to stretch and recoil after removing the stretching force. An elastic vessel has the capacity to resume its normal shape after stretching and compressing. - Vascular compliance (distensibility) is the amount by which a vessel will increase in volume for a given increase in distending pressure. It is measured by the following equation: - $Compliance= \frac{ΔΡ}{ΔV}$ - AP= the change in transmural pressure - ∆ V= The change in volume - Compliance is inversely related to elasticity. The higher the elastic tissue in a vessel wall, the less compliant it is. - A compliant vessel will accommodate a large volume of blood at low pressure and show little rise in pressure when a large volume of blood is injected into it. - The compliance of the large veins enables them to accommodate large volume of blood and returning it to the heart which maintains normal venous return and cardiac output. - In the systemic venous system, the volume normally ranges from 2000 to 3500 ml, and a large increase in blood volume results in minimal increase in venous pressure. This explains why one-half liter of blood can be transfused into a healthy person in a few minutes without greatly changing the venous pressure and the function of the circulation. - The compliance and elasticity of the aorta and large arteries enable them to accommodate blood ejected from the left ventricle. - The aorta, the pulmonary artery, and their major branches have a large amount of elastin in their walls, which makes these vessels distensible (i.e., compliant). - This distensibility reduces the pulsatile nature of blood flow that results from the heart pumping blood intermittently. - When blood is ejected from the ventricles during systole, these vessels distend, and during diastole, they recoil back and push the blood forward. - Importance: 1. Because of aortic compliance (distensibility), a portion of energy of the ejected blood is used to stretch the wall of the aorta, thus preventing an excessive rise in systolic blood pressure. 2. During diastole the distending pressure decreases and the elastic recoil of the aorta (Wind Kessel function) acts as a second pump (second heart) which: - Prevents marked decrease in diastolic blood pressure. - Maintains the blood flow to the peripheral circulation during diastole - Maintains blood flow to the coronary circulation. ## 3. The elastic nature of the large arteries also reduces the work of the heart. - If these arteries were rigid, the pressure would rise markedly during systole, and the ventricles pump blood against a high pressure and thus increase the work of the heart. - Decreased aortic elasticity with aging or atherosclerosis results in: - Increased systolic blood pressure - Decreased diastolic blood pressure - Increased pulse pressure. - Increased work of the heart. ## 5. Velocity of flow - The velocity of flow is the speed, or distance per unit of time, that blood flows forward through the circulatory system. - It is important to differentiate between velocity, which is displacement per unit time (e.g., cm/s), and flow, which is volume per unit time (e.g., cm³/s). - Velocity can be measured by this equation: - $Velocity= Flow (Q)/Total cross-sectional area (TCA).$ - Thus, Flow = Velocity X TСА - If flow stays constant, velocity increases in direct proportion to any decrease in total cross-sectional area - Because the circulatory system is a closed system, the volume of blood flowing through any level of the system must equal the C.O. (5 L/min). - Thus, 5 L per minute must flow through the arteries, arterioles, capillaries, and veins. So, the flow rate is the same at all levels of the circulatory system. However, the velocity with which blood flows through the different segments of the vascular tree varies because velocity of flow is inversely proportional to the total cross-sectional area (TCA) of all vessels at any given level. | | Aorta | Capillaries | |-----------------------|----------------|-------------------| | TCA | Small | Large | | | | (750 times that of aorta) | | Velocity of blood flow | High | Slow | | Flow rate | 5 L/min | 5 L/min | - The slow velocity in capillaries allows adequate time for exchange of nutrients and metabolic end products between blood and tissue cells—the main purpose of the circulatory system. - As capillaries rejoin to form veins, the total cross-sectional area is again reduced, and the velocity of blood flow increases as blood returns to the heart. ## 6. Law of Laplace - This law states that tension in the wall of a cylinder or hollow viscus (T) is equal to the product of the transmural pressure (P) and the radius (r) divided by the wall thickness (w): - $T= \frac{PXr}{W}$ - T=wall tension - P= transmural pressure - r= radius - w= thickness - $P=\frac{TXW}r$ - The small radius of a capillary decreases its wall tension which allow it to withstand high pressures (e.g., 65 to 70 mm Hg in the kidneys) without being ruptured. - The law of Laplace also explains a disadvantage faced by dilated hearts. When the radius of a cardiac chamber is increased, a greater tension must be developed in the myocardium to produce any given pressure; consequently, a dilated heart must do more work than a nondilated heart. ## 7. Bernoulli principle - Bernoulli principle states that in a tube or a blood vessel the total energy (the sum of the kinetic energy of flow and the potential energy) is constant. - Total energy= Kinetic energy + Potential energy - The pressure drop in any segment of the arterial system is due to: - Resistance; in this case the energy is dissipated as heat [lost energy]. - Conversion of potential into kinetic energy. When a vessel narrows the kinetic energy is increased and the potential energy is decreased. This change is reversed when the vessel widens out again. - When a vessel is narrowed in atherosclerosis the velocity of flow in the narrowed portion increases and the distending pressure decreases. Therefore, the lateral pressure at the constriction is decreased and the narrowing tends to maintain itself.. ## Student Activity ### True and false - Laminar flow has a sound. **F** - Velocity in Aorta is greater than that of capillaries. **T** - In atherosclerosis systolic pressure is decreased. **F** ## References - Barrett KE, Barman SM, Brooks HL, and Yuan JX. (2019). Ganong's Review of Medical Physiology. 26th ed. ebook by McGraw-Hill Education. - Hall JE, and Hall ME. (2021). Guyton and Hall Textbook of Medical Physiology. 14th ed. eBook by Elsevier, Inc. - Sherwood L, (2016). Human Physiology From Cells to Systems. 9th ed. eBook by Nelson Education, Ltd.