Introduction+to+the+Cardiovascular+System+and+Hemodynamics+2024.pdf

Introduction to the Cardiovascular System and Hemodynamics Irving H. Zucker, Ph.D. Dept of Cellular and Integrative Physiology [email protected] 402 559 7161 September 23, 2024 Learning objectives 1. Compare the n...

Introduction to the Cardiovascular System and Hemodynamics Irving H. Zucker, Ph.D. Dept of Cellular and Integrative Physiology [email protected] 402 559 7161 September 23, 2024 Learning objectives 1. Compare the normal O2 content and saturation in the arterial and venous compartments of the cardiovascular system, and the normal ranges for pressures within the chambers of the heart and the vasculature 2. List the normal values for hematocrit, hemoglobin, and plasma electrolytes (K+, Na+, Ca2+) 3. Describe how blood volume is distributed within the cardiovascular system and be able to estimate total blood volume in a patient. 3. Describe the hemodynamic relationship between pressure, flow, and resistance, and what determines wall tension in blood vessels and the heart (law of Laplace) 4. Describe how flow is related to velocity and cross-sectional area 5. Explain Poiseuille's law and the factors that determine resistance in the cardiovascular system 6. Distinguish between laminar and turbulent blood flow, and describe those factors that favor turbulent flow 7. Describe the factors that influence the viscosity of blood and explain the Farhraeus-Linqvist effect Factoids: The heart The heart beats 115,000 times per day. The heart pumps 7,230 liters of blood per day at rest. The peak blood pressure in the heart is about one-sixth of an atmosphere, and the heart develops about two watts of mechanical power.  Basics of the cardiovascular system The cardiovascular system is a closed-loop system consisting of a parallel arrangement of organ blood supply William Harvey 1578-1657 Essential functions of the cardiovascular system Delivers O2 and nutrients Removes CO2 and other metabolic waste products Transports hormones Part of the immune system Involved in temperature regulation Involved in regulation of fluid volume Oxygen content/saturation and pressures in the CV system O2 content: ml O2/100 ml blood O2 saturation: % saturation of hemoglobin with O2 Pressure: mmHg Normal hemoglobin is ~ 14 g/100 ml blood. Hemoglobin has an oxygen-binding capacity of 1.34 ml O2 per gram of hemoglobin 9 g/dl of hemoglobin binds 12.1 ml of O2 /100 ml blood. Normal O2 content; 14 g/dl x 1.34 ml O2/g hemoglobin = 18.8 ml O2/100 ml blood.  Blood composition Hematocrit (Hct) is the % by volume of RBCs, WBCs, and platelets  The solutes of plasma consist of electrolytes, proteins, and non-protein components  General hemodynamic features of the cardiovascular system Blood volume is greatest in the venous vessels. Blood volume in humans: normally 7 - 8% of body weight. Estimated blood volume - Body weight (kg) x 0.08 = Weight of blood Assuming that 1 kg = 1 liter, 70 kg x.08 = 5.6 kg or ~ 5.6 liters.  Blood velocity is lowest in the capillaries where cross-sectional area is greatest  Blood flow is redistributed under different conditions; e.g. exercise, postprandial Pressure  Pressure, flow, and resistance For hemodynamics, P = Q x R; P, pressure; Q, flow; R, resistance - P = (P1-P2) or ΔP - From the standpoint of tissue viability, blood flow (Q) is most important physiological variable - The inverse of resistance (R) is conductance (g): g = 1/R In the CV system, Q is generated by the cardiac output (CO), which is equal to stroke volume (SV) x heart rate (HR): CO = SV x HR Total intravascular pressure: sum of static and dynamic pressures, Ptotal = Pstatic + Pdynamic Static (lateral) pressure, Pstatic, is potential energy; hydrostatic pressure. Dynamic pressure, Pdynamic, is due to the movement of blood; Pdynamic is kinetic energy - In most arterial locations the dynamic component is a small fraction of the total pressure. - Pdynamic makes up a greater fraction of Ptotal in places where blood velocity is high. Transmural pressure and wall tension; the law of Laplace The difference between the internal (Pi) and external (Pt) pressure across the wall of a blood vessel or of a cardiac chamber is termed transmural pressure: ΔP = Pi-Pt Wall tension (T; stress) is equal to transmural pressure x radius/wall thickness (law of Laplace) - For a cylinder (e.g. blood vessel), T = ΔPr/h, where h is wall thickness - For a sphere (e.g. heart), T = ΔPr/2h  The Law of Laplace Pierre-Simon Laplace (March 23, 1749- March 5, 1827) The law of LaPlace explains how tension (T) on the surface of a hollow viscus is increased when there is an increase in the radius even when the pressure (P) remains the same; T=Pr.  Velocity, flow, and area Q = v x A; Q, flow; V, velocity (distance/time); A, cross-sectional area v = Q/A  Poiseuille’s law: resistance in blood vessels Poiseuille’s experiments used glass tubes and reservoirs to determine the effects of pressure gradient (ΔP), tube length (L), tube radius (r), and fluid viscosity (η) on flow Poiseuille’s conclusions - Q is proportional to ΔP - Q is inversely proportional to tube length, L - Q is proportional to r4 - Q is inversely proportional to viscosity, η  Poiseuille’s Law: Q = ΔPr4π / ηL8  Demonstration https://youtu.be/u2np3cwuP8k  Total resistance in the CV system is the sum of parallel and series resistances ↑ Number of resistances in parallel → ↓ total resistance  Laminar and turbulent flow Laminar flow is flow that occurs in layers; i.e. streamlined flow Lamina with the fastest velocity are at the center of the vessel; lamina in contact with the vessel wall has zero velocity - The velocity profile across a cylindrical tube is a parabola  With turbulence (breakdown of lamina), vascular conductance (ΔQ/ΔP; inverse of resistance) of fluid through vessels decreases when flow becomes turbulent; i.e. turbulence →↑resistance to flow  Reynold's number (Re) describes the conditions under which orderly, laminar flow becomes disordered, turbulent flow Re = (2r x v x d) / η - v: velocity - r: vessel radius - d: density of the fluid - η: viscosity of the fluid For Re < 2000 flow is laminar; for Re > 2000 flow becomes turbulent Turbulent flow can occur at vessel branch points, at bends, and across stenotic valves  Viscosity of blood Blood is a non-Newtonian fluid - The viscosity of a Newtonian fluid is constant regardless of any external stress that is placed upon it. - Blood viscosity changes during flow conditions (i.e. non-Newtonian). Under flow conditions, the reduction in apparent viscosity associated with a decrease in tube (vessel) diameter < 0.2 mm is known as the Fahraeus-Lindqvist effect. (Wide bore tube)  The F-L effect involves the axial accumulation of RBCs where the fastest lamina reside - Velocity of RBC's toward the center of the vessel > the velocity of plasma toward the vessel walls - A cell-free layer exists at the inner wall of the vessel F-L effect effectively decreases the resistance to flow in smaller vessels  Because viscosity of blood can change markedly, the term ‘apparent viscosity’ is used to indicate the viscosity of blood under particular conditions of measurement. - More common, ‘relative viscosity’ is used to express measured viscosity relative to pure water. Blood viscosity is dominated by the hematocrit (Hct) - ↑↑Hct (polycythemia) → ↑η (viscosity) → ↑R (R = η x L x 8 / r4 x π) - ↓↓Hct (anemia) → ↓η The relative viscosity of water is 1 and for plasma 1.5 - Plasma viscosity is determined largely by the plasma proteins. 1. Pressure in the cardiovascular system depends on flow and resistance according to the hydraulic equivalent of Ohm’s law (P =Q xR 2. Vessel and cardiac wall tension depends on transmural pressure and wall thickness (Law of LaPlace) 3. Flow velocity is inversely proportional to total cross-sectional area 4. Flow is governed by Poiseulle’s law (Q = ΔPr4π / ηL8) 5. Series resistances summate and parallel resistances inversely summate 6. Flow is laminar and turbulent. The Reynolds number defines turbulent flow 7. Blood is a non-Newtonian fluid so that viscosity decreases as shear increases 8. The Fahreaus-Linqvist effect predicts a decrease in viscosity at vessels smaller than 200 um