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2013/4/26 Learning objectives 2013 Sp. BME-- Introduction to BME 1. Understanding the circulatory system consists of a circulating fluid, a system of vessels, and a pump 2. The composition of blood and the role of cells in determining blood’s physical properties Circulation 3. The general structure of the vascular system-• the relationship between vessel radius, resistance to flow, and pressure drop • the function of capillaries in flow distribution and transport of molecules Patrick C.H. Hsieh (謝清河), M.D., Ph.D. Institute of Clinical Medicine, Dept. of Surgery & Dept. of Biomedical Engineering National Cheng Kung University & Hospital, Tainan, Taiwan Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan Department of Bioengineering, University of Washington, Seattle, WA, USA 4. The anatomy of the heart and the electrical system-• the events in the cardiac cycle • the generation of pressure 國立成功大學 暨 成大醫院 臨醫所,外科 暨 醫工系 中央研究院 生醫所 美國 華盛頓大學 生物工程系 Why circulation is important? Composition of circulating fluid Whole blood-- plasma and blood cells 1. Transporting O2 and other nutrients 2. Removing waste products 3. Maintaining homeostasis (body temperature, blood pressure and pH) Heart-- pump; Vasculature– plumbing; Blood-- circulated fluid • Plasma-- 55% Electrolytes < 1% Proteins < 7% (albumin, globulins etc.) Water 93% • Blood cells-- 45% Erythrocytes Leukocytes Platelets Circulating stem/ progenitor cells The blood vessels vein Allergic Acute inflammation diseases Immediate -type hypersensitivity artery Chronic inflammation capillary 1 2013/4/26 Pressure drop in circulatory system Blood flow through a cylindrical vessel The Navier-Stokes equations (Text: Box 8.2) Why? dramatic pressure decrease in arterioles Q: blood flow rate Δp: pressure drop Flow rate, pressure and resistance rv: radius of the vessel μ: fluid viscosity : fluid density L: length of the vessel Homeostasis– control of local organ flow simplifying R ∝ rv−4 1/30 the greatest overall resistance is provided by arterioles Arterioles have muscular walls to adjust their diameter local blood flow to a tissue is controlled by constriction and dilation of the arterioles delivering blood to that tissue Turbulent flow is atherogenesis Laminar flow & turbulent flow Vessel compliance and elasticity Atherogenesis ∝ E-1 high compliant low elastic C: blood vessel compliance E: blood vessel elasticity ΔV: change in vessel volume for a given Δp Δp: change in hydrostatic pressure Mechanotransduction: Studies of mechanisms by which cells convert mechanical stimulus into biochemical activity • An elastic artery allows it to serve as a “pressure reservoir” - Without vessel elasticity, the pressure of aorta will change dramatically during systole and diastole Shu Chien (錢煦院士) Fellow of National Medal of Science, American Academy of Arts and Sciences, National Academy of Engineering and National Academy of Sciences low compliant high elastic Nat. Rev. Mol. Cell Biol. 2009 • A compliant vein allows it to serve as a “volume reservoir” - high compliance enables veins to expand greatly in volume with a small increase in pressure Figure 22.5 2 2013/4/26 Structure and function of capillaries Anatomy of the heart • exchange of molecules, e.g. O2, CO2, glucose, nutrients • fenestrated monolayer • every metabolically active cell is within 100 μm of the nearest capillary for diffusion of molecules • diameter of capillaries ~ size of red blood cell (6–8 μm) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Figure 22.5 Path of blood through the heart Excitability-contraction coupling in cardiac cells Electrical activity of the heart The cardiac cycle LV pressure-volume loop (pacemaker) A, mitral valve open B, late diastole C, mitral valve close D, aortic valve open E, LV eject F, aortic valve close 3 2013/4/26 Summary 1. 2. 3. 4. 5. 6. 7. The cardiovascular system is composed of a pump, the heart, which circulates a specialized fluid, blood, through an elaborate system of branched vessels. The blood is a special fluid composed of cells dispersed in a protein-rich fluid called plasma. White blood cells are involved in the inflammatory response and immune function; RBCs transport oxygen to tissues. The blood circulates in the body through a network of vessels including arteries, veins, and capillaries. Many of the biophysical properties of the circulation can be deduced using a simple engineering model: fluid flow in a straight cylindrical tube. The heart is equipped with a muscular wall that contracts to pump blood from its chambers (atria and ventricles) to other parts of the body. The heart’s muscular wall is made up of self-excitable cardiac cells that contract in response to electrical stimulation. The heart contracts rhythmically to create blood pressure, which drives blood flow. Questions 1. Veins and capillaries are both low-pressure vessels. Why do veins typically have thicker, stronger walls than capillaries? 2. How is compliance related to wall tension in the wall of a vessel? Which type of vessel is more compliant: veins or capillaries? What property allows these vessels to be more compliant? What function does this higher compliance serve? 3. Using library resources, investigate what a pacemaker (the medical device, not the natural pacemaker tissues of the heart) is and how it works. Why is it important to control a patient’s heart rate? 4