Hemodynamics

1. Hemodynamics

2. Hemodynamics - the study of blood circulation and the forces and motion of blood flow • Flow - the ability to move from one point to another when a force is applied

3. Matter is classified into 3 categories: • Gas • Liquid • Solid

4. Fluid • Substances that flow & assume the shape of their container Gases & liquids are fluids

5. Blood • A liquid • Supplies nutrients & oxygen to the cells • Removes waste products • Average human has 5 liters of blood

6. Blood Comprised of plasma, erythrocytes (red cells), leukocytes (white cells) & platelets • Plasma - approx. 90% H2O; remainder is proteins • Blood cells - 40% of blood’s volume; known as the hematocrit • Erythrocytes - about 99% of all the blood cells • Leukocytes - larger than erythrocytes & function to protect the body against disease organisms • Platelets - smaller than erythrocytes & are important in blood clotting

7. Blood -is a fluid because it is a liquid - it follows the properties of a fluid

8. Fluid Fluids have 2 important characteristics: • Density • Viscosity

9. Density • Mass per unit volume; grams/milliliter (g/mL) • Mass - a measure of an object’s inertia (resistance to acceleration) • The greater the mass - the greater the force must be to accelerate it • Blood’s density (1.05 g/mL) > water’s (1 g/mL) because of the proteins & cells

10. Viscosity (ת) • fluid’s ability to resist a change in flow • Unit - poise or kg/m-sOne poise is 1 g/cm-s • Viscosity varies with flow speed

11. Viscosity Blood is a viscous fluid containing cells & plasma • Blood viscosity - 0.035 poise at 370C - approx. 5X that of water • Blood viscosity varies from 0.02 (anemia) to 0.10 (polycythemia) • Since erythrocytes are the major cellular component of blood, if the number of RBCs increases, the viscosity of the blood increases (directly related) • Anemia (low # of RBCs) has low viscosity • Polycythemia (high # of RBCs) has high viscosity

12. Friction • A force that occurs when two bodies in contact are in motion relative to one another (rub together) • Frictional force acts in a direction opposite to the direction of motion • One cause of friction is the roughness of the two surfaces in contact

13. Pressure • force per unit area • is equally distributed throughout a static fluid & exerts its force in all directions A pressure difference is required for flow to occur

14. Fluids • substances that flow & conform to the shape of their containers, such as gases & liquids • Pressure applied to a fluid acts different than pressure applied to a solid. • The difference is that not only is there pressure pushing down at a given point, but there is also the same pressure pushing up and to the sides. • The pressure is the same in all directions in a fluid at a given point. This is true because liquids and gases to take the shape of their container.

15. PRESSURE’S EFFECT ON FLUIDS FORCE When force is applied to a fluid, pressure , even though the force is directional, the pressure is omnidirectional. Fluid pressure at a given point is the same in all directions Pressure is applied equally throughout a liquid

16. Pressure • Equal pressures applied at both ends of a liquid-filled tube results in no flow • If the pressure is greater at one end, the liquid will flow from the higher-pressure end to the lower-pressure end • This pressure difference can be generated by a pump or by the force of gravity • The greater the pressure difference, the greater the flow rate will be

17. Pressure The difference in pressure (between the high pressure end & the low pressure end) is called a pressure gradient or energy gradient Pressure gradient is calculated by taking the pressure difference & dividing it by the distance between the 2 pressure locations

18. Pressure Gradient Pressure Gradient (∆P) = P1-P2 Distance (length) between P1 and P2

19. Volume Flow Rate (Q) = volume of blood passing a point per unit time • expressed in milliliters (mL)/minute or /second or cc/second (this is not a measurement of speed) • Total adult blood flow rate (cardiac output) is about 5,000 mL/min (our total blood volume circulates in about 1 minute) • Q in a long straight tube is determined by the pressure difference & the resistance to flow

20. Volume Flow Rate (Q) Volume flow rate (mL/s) = ∆P (dyne/cm2) Flow resistance (g/cm4-s) • Pressure gradient & volume flow rate are directly related (in a straight tube). • Therefore, the greater the pressure difference, the greater the flow rate

21. Flow Resistance Flow resistance (in a long, straight tube) depends on: • Fluid’s viscosity • Tube’s length & radius

22. Flow Resistance Flow Resistance = 8 X length x viscosity (ת)  X radius4

23. Poiseuille’s Equation Substituting the flow resistance equation into the flow-rate equation & using tube diameter rather than radius yields Poiseuille’s equation for volume flow rate

24. Poiseuille’s Equation Poiseuille’s equation predicts steady volume flow in long straight tubes Thus, it serves only as a rough approximation to the conditions in blood circulation

25. Flow Resistance = 8 X length X viscosity (ת)  X radius4 Volume flow rate (Q) = ∆P Flow resistance

26. Q = ∆P Flow resistance Q = ∆P 8 X length x viscosity  X radius4 Q= ∆P X  X radius4 8 X length X ת

27. Poiseuille’s Equation Q = ∆P  r4 8 תL Q = ∆P  d4 128 תL

28. Extra Credit – 10 points Prove to me on paper why both of the equations given are equal. Due next class meeting

29. Using this formula, what relationships can be seen? Q = ∆P  r4 8 תL Q = ∆P  d4 128 תL

30. Imagine you are drinking through a straw & you want to get more flow through the straw: •  pressure gradient (suck harder) •  radius of the vessel (use a bigger straw) •  viscosity of the fluid (dilute or heat it) •  length of the vessel (cut the straw in ½)

31. Which factor affects flow, resistance & velocity the most? The one raised to the fourth power!!! A small change in vessel diameter will create a large difference. Resistance to flow depends on viscosity of blood, the radius of the blood vessel’s lumen & the length of the vessel.

32. Increases in: Causes increased Causes decreased Flow resistance Volume flow rate Pressure difference Volume flow rate Length Flow resistance Volume flow rate Radius Volume flow rate Flow resistance Diameter Volume flow rate Flow resistance Viscosity Flow resistance Volume flow rate

33. Types of Flow Profiles Flow is divided into 5 spatial categories: 1) plug 2) laminar 3) parabolic 4) disturbed 5) turbulent

34. Flow Profiles • Velocity (speed & direction RBCs are traveling) is not constant or uniform across the vessel lumen • Various flow profiles are seen in normal flow in: • various vessels • at different points in the cardiac cycle • Velocity profile across a vessel depends on: • curvature of a vessel • branching to a smaller vessel • obstruction in a vessel • diverging cross section

35. Plug Flow - is constant flow velocity across the vessel It occurs in large vessels such as the aorta

36. Laminar Flow - is flow that occurs when straight, parallel layers of fluid slide over each other • In a normal vessel, friction produces lowest velocities along the vessel wall & highest velocities occur in the center of the vessel • Laminar flow is commonly seen; its absence often indicates abnormal flow conditions at a site where there is vascular or cardiac-valvular disease

37. Laminar Flow • flow that occurs when straight, parallel layers of fluid slide over each other

38. Parabolic Flow - flow is steady laminar flow whose varying flow speeds across the tube are described by a parabola - ave. flow speed = ½ max. flow speed (at the center)

39. Parabolic Flow - not commonly seen in blood circulation because the vessels generally are not long & straight

40. Disturbed Flow - a form of laminar flow but the parallel streamlines are altered from their straight form Disturbed flow fluid flows in a forward direction

41. Disturbed Flow This occurs in the region of stenosis or at a bifurcation (the point at which a vessel splits into two)

42. Turbulent Flow - (turbulence) is nonlaminar flow with random & chaotic speeds • Turbulence with multiple velocity components is known as chaotic flow • Eddies currents (flow particles moving in circles) occur creating regions of reverse flow • As flow speed , turbulence will ultimately occur

43. Turbulent Flow

44. Turbulent Flow • Flow speed for turbulent flow depends on the fluid’s density & viscosity and the vessel’s diameter • Reynolds number predicts the onset of turbulent flow. • If the Reynolds number (Re) exceeds about 2000 to 2500 (depending on tube geometry), flow becomes turbulent • This is called the critical Reynolds number

45. Reynolds number = Average flow speed x tube diameter X density Viscosity

46. If there is an increase in: • Flow speed • Diameter • Density Reynolds number  If viscosity , Reynolds number 

47. With the exception of the heart and proximal aorta, turbulent flow does not typically occur in normal circulation Turbulent flow most commonly occurs beyond an obstruction, such as a stenosis, particularly in systole but will also occur in abnormal arterial geometry (kinked, bent, or tortuous vessels)

48. Steady Flow - is nonpulsatile hemodynamics that occurs in the venous system because the veins offer little resistance to flow and can accommodates a large change in volume with little change in pressure • volume flow rate is simply related to pressure difference & flow resistance

49. Venous pressure & flow are affected by: • Respiration • Hydrostatic pressure (p)

50. Respiration Inspiration: diaphragm moves inferiorly, causing an increase in abdominal pressure, that slows down the flow of blood in the lower extremities. • This causes a decrease in the thoracic pressure increasing the blood flow in the veins of the thorax & upper extremities • The opposite happens on expiration This is called phasicity (waxes and wanes)