Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Hemodynamics of the Vasculature OBJECTIVES: • Distribution of blood volume, flow, pressure, vessel resistance throughout the circulatory system. • Discuss Poiseuille's Law and the effects of radius, length, viscosity and resistance on blood flow. • Limitations of applying classical hemodynamics to blood. HEMODYNAMICS The Physical properties of blood, blood vessels and the heart and their interactions Consists of : Pressure = Mean Arterial Pressure (MAP) Flow = Cardiac Output (CO) Resistance = Total peripheral resistance (TPR) Flow = Pressure Difference Resistance (Ohm’s Law) Effect of Pressure Difference on Blood Flow Flow P Flow is inversely proportional to vessel length (L) Q= 10 ml/s Q= 5 ml/s Q 1/L Q= 10 ml/s Q= 160 ml/s Q r4 Flow is dependent on 4th power of the radius (r4) Effect of Radius on Flow Q r4 Flow is Inversely Proportional to Viscosity Q ή Poiseuille’s Law Poiseuille’s Law - Assumptions • Flow is steady (constant) – The pump (heart) is pulsatile – Arterial vessels dampen changes, but not steady • Flow is laminar – Generally true except at bifurcations • Fluid is Newtonian – Newtonian fluid is homogeneous, fixed viscosity – Is suspension, non-homogeneous – Viscosity increases with increasing hematocrit Poiseuille’s Law Q = ΔP π ήL 8 r4 Q = ΔP/R R = ΔP/Q R=8ήL 4 πr Where: R = Resistance ή = Viscosity of Blood L = length of blood vessel R4 = radius of blood vessel raised to the 4th power Effect of the diameter of the blood vessel on the velocity of blood flow. Downloaded from: StudentConsult (on 9 March 2007 10:18 PM) Cardiovascular Dynamics Simulation based on 3D noninvasive imaging Based on contrast-enhanced magnetic resonance angiogram of the abdominal aorta Coarctation of the Aorta Significant morbidity (hypertension, aneurysms, stroke) may be attributed to abnormal hemodynamics in the aorta and its branches Laminar Flow Parabolic velocity profile Laminar Flow Parabolic velocity profile Axial and Radial Flow Turbulent Flow Comparison of laminar flow to turbulent blood flow.. Laminar Flow– all points in fluid move parallel to walls of tube – Each layer of blood stays at same distance from wall – Blood cells forces to center of vessel Turbulent Flow– – – – At bifurcations of blood vessels Pressure drop greater than with laminar (square) Makes heart work harder Blood clots and thrombi much more likely to develop Effect of turbulence on pressureflow relationship Turbulence decreases flow at any given perfusion pressure Pressure-Flow Relationship Reynolds's Number Reynolds number above 2000 associated with turbulent flow Dimensionless number, relates inertial forces to viscous forces Reynold’s Number = density * diameter * mean velocity Figure 4-4 Effect of the diameter of the blood vessel on the velocity of blood flow. Downloaded from: StudentConsult (on 9 March 2007 10:18 PM) © 2005 Elsevier Systemic CirculationComprised of Parallel and Series Circuits Parallel and Series Circuits Arrangements of blood vessels in series and in parallel. Arrows show direction of blood flow. R=Resistance Figure 4-9 Systemic arterial pressure during the cardiac cycle. Systolic pressure is the highest pressure measured during systole. Diastolic pressure is the lowest pressure measured during diastole. Pulse pressure is the difference between systolic pressure and diastolic pressure. (See the text for a discussion of mean arterial pressure.) Downloaded from: StudentConsult (on 9 March 2007 10:18 PM) © 2005 Elsevier Figure 4-1 A schematic diagram showing the circuitry of the cardiovascular system. The arrows show the direction of blood flow. Percentages represent the percent (%) of cardiac output. See the text for an explanation of the circled numbers. Downloaded from: StudentConsult (on 9 March 2007 10:18 PM) © 2005 Elsevier Law of LaPlace Vessels are “built to withstand the wall tensions they normally “see” If intravascular pressure increases will increase vessel wall tension (T) In response, vascular smooth muscle contracts and T returns to normal Law of LaPlace T = (∆P*r) / µm Where T = tension in the vessel wall ∆P = Transmural pressure r = radius of the vessel µm = wall thickness May explain critical closing pressure Law of LaPlace Law of LaPlace- Relevance • For given BP, increasing the radius of the vessel leads to a increase in tension. • Arteries must have thicker walls than veins because they carry much higher BP. • Capillaries also carry significant BP, but unlike arteries, capillary walls are thin. Small size leads to reduced level of tension so thick walls not needed. • Conclusions: Properties of this relationship helps us understand the variable thickness of arteries, veins, and capillaries. LaPlace’s Law Explains … • Aneurysms • Blood vessel distensibility • Effects of ventricular dilatation on contraction End of lecture