Hemodynamics Part II

1. Hemodynamics Part II

2. P =Total energy – kinetic energy P =Total energy = gρv2 /2 A forward facing tube stops the moving molecules converting their kinetic energy into potential energy (pressure) so that its pressure is equal to the total energy. The kinetic energy of a fluid in motion is = ρv2 /2 Total energy = pressure + kinetic energy

3. 100 cm/sec 20 cm/sec 200 cm/sec 200 ml/sec (12L/min) The kinetic energy of a fluid in motion is = ρv2 /2 The total energy = pressure + kinetic energy Pressure = total energy – kinetic energy

4. 100 cm/sec 20 cm/sec 200 cm/sec The total energy = pressure + kinetic energy Therefore if the fluid is caused to speed up the pressure must fall (Bernoulli effect) Note that total energy is unchanged along the tube.

5. 100 cm/sec 20 cm/sec 200 cm/sec 5 mmHg 75 mmHg 20 mmHg In a real vessel there would be both resistive and kinetic effects. The resistive effects are seen in the forward pressures and both are seen in the side pressures.

6. Kinetic components in the cardiovascular system are usually less than a mmHg. • The highest flow velocity is in the root of the aorta where a kinetic component of only 5-10 mmHg might be encountered. • Thus the physician can usually ignore kinetic components. • However, with pathological shunts or exercise, kinetic components of up to 20 or 30 mmHg have been seen.

7. There is a limit to how fast blood can be forced through a blood vessel. When that occurs laminar flow becomes turbulent.

8. The properties of turbulent flow are: • Movement between lamina. • Flow no longer proportional to pressure • Noise

9. The transition from laminar to turbulent can be predicted by the Reynolds number (Re).   Re = D v ρ η 2000 = critical

10. Factors that lead to turbulence in the body Low Viscosity: anemia Gurgling sounds heard in normal vessels Re = D v ρ η

11. Factors that lead to turbulence in the body Arterial narrowing: atherosclerotic plaques coarctation Rushing sounds in the affected arteries: Bruit Re = D v ρ η

12. Flow = 3.14 ml/sec Radius = 1 Area = 3.14 v = F/A = 3.14/3.14 = 1 D v = 2 · 1 = 2 Radius = 2 Area = 3.14 · 4= 12.56 v = F/A = 3.14/12.56 = 0.25 D v = 4 · 0.25 = 1 Note that the D·v increases as the diameter decreases Re = D·v·ρ η

13. Factors that lead to turbulence in the body Valves that do not completely open or close cause murmurs Rushing sounds around the affected valve Re = D v ρ η

14. Water or plasma is a Newtonian fluid whose viscosity is constant regardless of the sheer force on it.

15. Water or plasma is a Newtonian fluid whose viscosity is constant regardless of the sheer force on it. Whole blood is a plastic where its viscosity decreases with increasing sheer force

16. This anomalous viscosity is because the blood cells try to stick to each other when the blood is moving slowly This causes the resistance to flow to increase when the cardiac output is low and worsens the situation. This is often referred to as blood sludging.

17. Relative to water Plasma Water The hematocrit refers to percentage of the blood volume that is made up of red blood cells. Oxygen delivery = oxygen content x blood flow

18. Increasing the hematocrit increases blood viscosity in a non-linear way

19. At a low hematocrit (anemia) oxygen delivery is poor because the fall in blood oxygen content is not offset by the increase in blood flow.

20. At an abnormally high hematocrit (polycythemia) oxygen delivery is again low, this time because the nonlinear curve causes flow to fall at a faster rate than oxygen content increases

21. Oxygen delivery A normal hematocrit optimizes oxygen delivery

22. Blood seems to get less viscous as the tube diameter decreases. This is called the Fahraeus-Lindquist effect That facilitates flow through the micro vessels

23. Axial streaming. Bernoulli forces suck red cells toward the center of the vessel. Reduced number of lamina in small vessels actually makes the blood flow more efficiently. The Fahraeus-Lindquist effect is due to:

24. Plasma Viscosity is independent of vessel radius for a Newtonian fluid like plasma.

25. Axial streaming reduces the intra-organ hematocrit.

26. Axial streaming reduces the intra-organ hematocrit.

27. Hematocrit in the central circulation is greater than the whole-body hematocrit.

28. LaPlace’s Law for wall tension states T = ΔP · r

29. Both tires have the same wall tension 100 PSI 15 PSI

30. T = ΔP · r Capillaries have low wall tension because of their small radius. Because their wall tension is so low they need no connective tissue. Capillaries are composed only of endothelial cells.

31. T = ΔP · r Wall tension is greatest in the aorta where both pressure and radius are large. The wall is very thick with a high collagen content to support the high wall tension. The aorta is most prone to failure (aneurysm) of any vessel in the body.

32. Aneurysms are unstable because once a vessel starts to balloon out it increases its radius and thus its wall tension. A positive feedback situation.

33. The volume of a compartment is related to its pressure (P) times its capacitance (C) C = ΔV/ΔP V = P · C + unstressed volume The unstressed volume is the volume at zero pressure

34. Capacitance is determined by the structure of the vessel wall Veins have a large capacitance Arteries have a small capacitance C = ΔV/ΔP

35. The Pulmonary Arterial Wedge Pressure (PAWP) Measured with a Swan-Ganz balloon catheter Used to estimate Pulmonary venous pressure (left atrial pressure)

36. The Pulmonary Arterial Wedge Pressure (PAWP) LAP is often elevated in left heart failure and that can cause pulmonary edema

37. The Pulmonary Arterial Wedge Pressure (PAWP) ΔP = 0 If the flow across the bed is zero then the pressure drop across it is zero ΔP = F · R

38. The Pulmonary Arterial Wedge Pressure (PAWP) ΔP = 0 The reason it works is because there are virtually no collateral connection between the parallel circulations in the lung.

40. End of part II