Chapter 13 Hemodynamics

1. Chapter 13Hemodynamics

2. Hemodynamics is the study of the physical principles of blood circulation. Viscosity is the physical parameter that characterizes a fluid’s ability to resist a change in its shape.

3. Resistance to flow depends on the viscosity of the blood and The radius of the vessel lumen. Frictional forces caused by the viscosity of the blood produce variations in velocity across the vessel lumen.

4. The major cellular component of blood is the erythrocyte or red blood cell (RBC). The viscosity of blood at normal hematocrit is 0.03 poise or dynes-seconds per cm², a value approximately four times the viscosity of water.

5. Velocity profile • The velocity of blood movement is not uniform across the vessel lumen. • The distribution of flow velocities into layers is called laminar flow. • The highest velocities are in the central portion of the vessel, and lowest velocities are along the wall.

6. Velocity profile Velocity profile of blood exhibiting laminar flow in a vessel.

7. Pressure / Flow relationship • Poiseuille’s equation Q = πpr⁴/8lη • Q volume flow rate is the quantity of blood moving through the vessel per unit of time • l is the length of the vessel • η is the viscosity • p is the pressure difference between the ends of the vessel • r is the radius of the vessel • π is a constant (value 3.14)

8. Intravascular pressure • Pressure produced by contraction of the heart, static filling pressure, and hydrostatic pressure each contribute to the overall intravascular pressure.

9. Static filling pressure • Filling of the vessel causes additional force, and hence pressure, to be applied to the blood within the vessel by the elastic walls. • Filling pressure is typically much lower (about 7 mm Hg) than hydrostatic pressure and pressure from heart contractions.

10. Hydrostatic pressure • Hydrostatic pressure (P) is generated when a fluid is positioned vertically in a gravitational field. P = -ρgh • ρ is the density of blood • g the acceleration due to gravity • h is the height of the blood

11. Bernoulli’s principle • Types of fluid energy • Energy conversion

12. Types of fluid energy • Total fluid energy consists of kinetic energy from the blood’s moving at a certain velocity, potential energy due to its elevation in a gravitational field, and the work done when pressure (force) is applied to move it.

13. Mathematically, Bernoulli’s equation expresses this energy relationship as P+ ρgh+1/2ρv²=Constant A comparison between any two points can be obtained by P1+ ρgh1+1/2ρv1²=P2+ ρgh2+1/2ρv2²

14. Energy conversion • An important consequent of Bernoulli’s principle is the conversion of potential energy to kinetic energy or increased pressure.

15. Energy loss • Last equations are valid for a frictionless fluid system. • Since blood is a viscous fluid, energy losses occur as it moves through the circulation. • The dissipated energy appears in the form of heat. P1+ ρgh1+1/2ρv1²=P2+ ρgh2+1/2ρv2²+Heat

16. Arterial hemodynamics • Cardiac output • Arterial pressure • Compliance • Pulsatile flow

17. Cardiac output • The volume of blood per minute pumped is the cardiac output. • The normal cardiac output for a healthy adult man is about 5 liters per minute. • Woman have ,on average, 10% less than men of the same size.

18. Arterial pressure • High but fluctuating arterial pressure is maintained by the heart pumping. • Peak pressure, usually about 120 mm Hg, occurs during systole.

19. Arterial and venous pressure Pressure variations throughout the circulatory system caused by heart contractions.

20. Compliance • As blood is ejected from the heart, the arteries become distended and store large quantities of blood. This property is called compliance.

21. Pulsatile flow • In normal peripheral arteries the flow velocity increases rapidly to a peak during early systole, follow by an end-systolic flow reversal of short duration, and then a resumption of forward flow at lower velocity during diastole.

22. Pulsatile flow Time course of flow in an artery during one heart cycle.

23. Venous hemodynamics • Venous function • Venous pressure • Control of flow

24. Venous function • The major function of veins is to act as s conduit for the flow of blood back to the heart, but venous system also participates in regulation of the circulation. • Veins can constrict or enlarge to change peripheral resistance and alter flow.

25. Venous pressure • The pressure pulses in the arteries are damped out before they reach the veins. • The constant pressure difference along the vessel tend to give rise to continuous flow.

26. Arterial and venous pressure Pressure variations throughout the circulatory system caused by heart contractions.

27. Control of flow • Constriction of the veins increases the peripheral resistance and raises arterial pressure. • Also stored blood in the veins is released to increase the blood volume and enhance venous return.

28. Peak velocity • Peak velocity is the maximum velocity within the lumen of the vessel. • It varies with anatomic location, being highest in vessels near the heart. • Peak velocity tend to decrease distal to the heart, because the total cross-sectional area of all vessels increases.

29. Modifications of velocity profile • Velocity profile can be affected by accelerated flow, curvature of a vessel, branching to smaller vessels, obstruction in a vessel and diverging cross section.

30. Modifications of velocity profile Laminar flow is changed to plug flow when blood undergoes acceleration.

31. Modifications of velocity profile A narrowed lumen alters laminar flow to a flat velocity profile.

32. Modifications of velocity profile Vessel curvature skews the blood flow velocities so higher velocity components are present at the outer edge of the vessel.

33. Eddy flow • Eddy flow is the localized slow rotation of concentric blood layers. • The rotation creates regions of reversed flow. • A zone of stagnant flow, called flow, called flow separation, divides the circular motion of eddy flow from the central region of high-velocity flow.

34. Modifications of velocity profile A sudden increase in vessel diameter creates multiple flow patterns. Region1, uniform high velocity flow Region2, stagnant flow Region3, eddy flow

35. Eddy flow Regional flow patterns at the carotid bifurcation.

36. Turbulence • Turbulence is chaotic flow in which the coherence of flow velocities across the vessel lumen is lost. • Turbulence is more likely to occur at high velocities within large vessels, distal to an obstruction, along a rough surface, and within the sharp turn of a vessel.

37. Reynolds' number • The likelihood of turbulence is expressed in terms of the Reynolds' number (Re): Re=ρvd/η • v is the velocity (cm/s) • Ρ is the density of blood (g/cm³) • d is the diameter of the vessel (cm) • η is the viscosity (poise)

38. Obstruction • Arterial obstruction is usually in the form of a plaque. • Plaques are associated with degenerative changes in the arterial wall accompanied by lipid deposits and often, calcium deposits as well.