Hemodynamics of Blood Flow in The Circulatory System PDF

# Hemodynamics of Blood Flow in The Circulatory System ## Introduction **Basic organization of the cardiovascular system.** * All blood pumped by the right side of the heart passes through the pulmonary circulation to the lungs for O2 pickup and CO2 removal. * The left side of the heart pumps blo...

# Hemodynamics of Blood Flow in The Circulatory System ## Introduction **Basic organization of the cardiovascular system.** * All blood pumped by the right side of the heart passes through the pulmonary circulation to the lungs for O2 pickup and CO2 removal. * The left side of the heart pumps blood through the systemic circulation that provides blood flow to the other organs and tissues of the body. **1.2. Division of the circulation** * The pulmonary circulation: It's aim is gas exchange (O2 pickup and CO2 elimination). * The systemic circulation: It's aim is capillary exchange in all tissues for nutrition. **1.3. Parallel arrangement of the circulation** * The blood pumped by the left side of the heart into the systemic circulation is distributed in different proportions to the systemic organs through a parallel arrangement of vessels that branch from the aorta. * Advantages of parallel arrangement of the systemic circulation: * Ensures that all organs receive blood of the same composition e.g., O2, CO2, and nutrients. * Blood flow through each systemic organ can be independently adjusted as needed. * Lower total peripheral resistance (TPR). **1.4. The vascular system** * Blood vessels are divided into: * **Arteries:** * They are blood vessels that carry blood away from the heart. * They are divided into: * **Elastic vessels:** Aorta, pulmonary artery and big arteries. * **Low resistance vessels:** Medium sized arteries. * **High Resistance vessels:** Arterioles. The volume of blood flowing through an organ can be controlled by regulating the diameter of the arteriole: * Arteriole vasoconstriction → ↓ blood flow * Arteriole vasodilatation → ↑ blood flow * **Capillaries:** (exchange vessels) They are the smallest of vessels, that have thin porous wall across which all exchanges are made with surrounding cells. Capillary exchange is the main purpose of the circulatory system. * **Veins:** (capacitance vessels ): * They carry the blood toward the heart. * Capillaries rejoin to form small venules → small veins → large veins → inferior and superior vena cava → Heart ## Blood flow **2.1. Flow, Pressure and Resistance** * Blood always flows from areas of high pressure to areas of low pressure. * The relationship between flow, pressure, and resistance in the blood vessels can be expressed by the formula: $Flow (F) = \dfrac{Pressure \ gradient (\Delta P)}{Resistance (R)}$ **2.2. Pressure** * Blood pressure: is the force exerted by the blood against the vessel wall. * It is measured in mmHg, or cm H2O. * 1 mmHg= 1.36 cmH2O. * Arterial blood pressure (ABP): is the pressure exerted by circulating blood upon the walls of arteries. * Because heart pumping is pulsatile (beating of the heart during systole and diastole) the arterial pressure normally alternates between a systolic pressure level of average 120 mm Hg and a diastolic pressure level of average 80 mm Hg under resting conditions. * Systolic blood pressure (SBP): Is the maximum pressure exerted by circulating blood upon the walls of blood vessels during systole. (120 mmHg) * Diastolic blood pressure (DBP): Is the minimum pressure exerted by circulating blood upon the walls of blood vessels during diastole. (80 mmHg) * Pulse pressure: Is the difference between systolic and diastolic blood pressure. Its normal value is 40 mmHg. * Mean arterial blood pressure (MAP): Is the average pressure in the circulation during the cardiac cycle. It differs in aorta and peripheral vessels. **2.3. Pressure Gradient** * The pressure gradient is the difference in pressure between the beginning and the end of a vessel. * The greater the pressure gradient forcing blood through a vessel, the greater the flow rate through that vessel. * Blood flows from an area of higher pressure to an area of lower pressure down a pressure gradient. * Contraction of the heart provides pressure to the blood, which is the main driving force for flow through a vessel. * Because of frictional resistance, the pressure drops as blood flows throughout the vessel's length. Thus, pressure is higher at the beginning than at the end of the vessel. * Flow rate is determined by the difference in pressure between the two ends of a vessel, not the magnitude of the pressures at each end. **Comparison between pressure gradient in systemic and pulmonary circulation** * Systemic circulation: * Pressure gradient (ΔP) = MAP- CVP * MAP: Mean arterial pressure = =90 mmHg * CVP: Central venous pressure (pressure of great veins at their entrance into the right atrium)= =0 mmHg * ΔP= ΜΑΡ- CVP= 90-0= 90 mmHg * F = AP/R * 5 L/min= 90/R * Pulmonary circulation: * ΔP= ΜΡΑP-MPVP * MPAP: Mean pressure in pulmonary artery during cardiac cycle = =15mmHg * MPVP: Mean pressure in pulmonary veins at their entrance into the left atrium= =0 mmHg * ΔP= ΜΡΑΡ- MPVP= 15-0= 15 mmHg * F = AP/R * 5 L/min= 15/R **2.4. Resistance** * Definition: It is a measure of the hindrance or opposition to blood flow through the vessel, caused by friction between the moving fluid and the stationary vascular walls. * As resistance to flow increases, the flow rate decreases (at constant ΔP). * F = AP/R * When resistance increases, the pressure gradient must increase to maintain the same flow rate. * Thus, when the resistance increases, the heart must work harder to maintain adequate flow rate (C.O.). * The units of resistance in the cardiovascular system is sometimes expressed in R units, which are obtained by dividing pressure in mm Hg by flow in mL/s. * F= AP/R * R= AP/F * Resistance to blood is: * Directly proportional to viscosity of the blood. * Directly proportional to vessel length. * Inversely proportional to vessel radius, which is the most important: * 𝝆∝ηL/r4 * Resistance can be measured by the following formula: * $R = \dfrac{8ηL}{πr^4}$ * π= 3.14 * Factors affecting the resistance to blood flow (determinants): * **Blood viscosity:** * Definition: It is the friction developed between the molecules of a fluid as they slide over each other during flow of the fluid. * The thicker a liquid is, the greater its viscosity, the greater the resistance to flow. * For example, honey flows more slowly than water because honey has greater viscosity. * Factors affecting blood viscosity: * **Hematocrit (HCT):** It is the principal determinant of blood viscosity. Increase in HCT increases resistance e.g., severe polycythemia (increased RBCs number). * **Plasma composition:** Plasma viscosity is 2 times that of water because it contains plasma proteins e.g., albumin, globulin and fibrinogen. Markedly increased plasma proteins as in case of hypergammaglobulinemia, increases blood viscosity. * **Resistance of RBC to deformity** also increases blood viscosity e.g., hereditary spherocytosis. * **Blood vessel (B.V.) diameter:** small B.V. contains low numbers of RBCs, and has lower blood viscosity than larger one because of plasma skimming. * **Blood flow velocity:** decreased velocity of blood flow, increases blood viscosity. * **Blood temperature:** cooling increases blood viscosity and vice versa. * **Blood vessel length:** The greater the vessel surface area in contact with the blood, the greater the resistance to flow. Surface area is determined by both the length and the radius of the vessel. Because vessel length remains constant in the body, it is not a variable factor in the control of vascular resistance. Therefore, the major determinant of resistance to flow is the vessel's radius. * **Vessel radius:** * Blood faces more resistance in small vessels than large ones. * So, blood flow is greater in large vessel than small vessel. * Resistance is inversely proportionate to the fourth power of radius. * Doubling the radius, decreases the resistance by 1/16 (r4= 2x2x2x2= 16). * Importantly, the radius of arterioles can be regulated and has an important role in controlling resistance to blood flow in circulation. * Total peripheral resistance (TPR) in systemic circulation is the resistance provided by all blood vessels but mainly arterioles. * Percentage of TPR offered by various vessels is as follows: Arteries 15%, arterioles 50%, Capillaries 25% and veins 10% * At the level of arterioles there is a sharp increase in the resistance which results in a marked drop in mean arterial blood pressure. **2.5. Poiseuille-Hagen Formula** * Because flow is equal to pressure difference divided by resistance (R) * F= ∆ P/R * $R = \dfrac{8ηL}{πr^4}$ * $F= \Delta P \times (\dfrac{π}{8}) \times (\dfrac{1}{η}) \times (\dfrac{r^4}{L})$ * $\dfrac{πΔPr^4}{8ηL}$ * The relationship between the flow in a long narrow tube, the viscosity of the fluid, and the radius of the tube is expressed mathematically in the Poiseuille-Hagen formula: * $Flow \ rate = \dfrac{πΔPr^4}{8ηL}$ * F = flow * ΔP= pressure gradient (pressure difference between two ends of the tube) * η = viscosity * r = radius of tube * L = length of tube * π= 3.14 * The factors that affect flow rate through a vessel are integrated in Poiseuille's law as follows * Blood flow is directly related to pressure gradient and to the fourth power of radius. * Blood flow is inversely related to blood viscosity and length of blood vessel. * Doubling the radius of the blood vessel results in 16-fold increase in blood flow. * Conversely, when the radius of the blood vessel is halved, 1/16 of blood will flow through a vessel. * Importantly, the radius of arterioles can be regulated and is the key factor in controlling resistance to blood flow throughout the vascular circuit. ## References 1. Barrett KE, Barman SM, Brooks HL, and Yuan JX. (2019). Ganong's Review of Medical Physiology. 26th ed. ebook by McGraw-Hill Education. 2. Hall JE, and Hall ME. (2021). Guyton and Hall Textbook of Medical Physiology. 14th ed. eBook by Elsevier, Inc. 3. Sherwood L, (2016). Human Physiology From Cells to Systems. 9th ed. eBook by Nelson Education, Ltd.