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Hemodynamics

Hemodynamics. Hemodynamics …. The study of the movement of blood and of the forces concerned. Physical Characteristics of Circulation. Functional parts of the circulation…. Arteries: Transport blood. Strong vascular wall. Rapid flow of blood. Arterioles: Control valves.

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Hemodynamics

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  1. Hemodynamics

  2. Hemodynamics… • The study of the movement of blood and of the forces concerned.

  3. Physical Characteristics of Circulation

  4. Functional parts of the circulation… • Arteries: • Transport blood. • Strong vascular wall. • Rapid flow of blood. • Arterioles: • Control valves. • Strong muscular wall.

  5. Cont… • Capillaries: • Exchange fluid, nutrients, electrolytes & other substances. • Very thin wall. • Venules: • Collect blood from capillaries. • Coalesce into larger veins.

  6. Cont… • Veins: • Transport blood from tissues back to heart. • Major reservoir of blood. • Thin muscular wall. • Low pressure.

  7. Volumes of blood in different parts of circulation

  8. Cross sectional areas & velocities of blood flow

  9. Cont… • Velocity of blood flow is inversely proportional to cross sectional area. • Under resting conditions: • 33 cm/sec in aorta. • 0.3 mm/sec in capillaries.

  10. Pressure in various portions of circulation

  11. Biophysical consideration:interrelationships among pressure, flow and resistance

  12. Relations among P,R & Q Pressure gradient. Vascular resistance.

  13. Cont… • Ohm’s law:

  14. Blood flow • Quantity of blood that passes a given point in the circulation in a given period. • 5000 ml/min

  15. Laminar flow of blood Parabolic velocity profile during laminar flow

  16. Cont… • Critical velocity: • Laminar flow occurs up to a certain critical velocity. • At or above this critical velocity, flow is turbulent.

  17. Cont… • Turbulent flow: • Blood flow in all directions and continually mixing with the vessel. • Reynold’s number: • The measure of the tendency for turbulence to occur.

  18. Reynold’s Number Reynold’s Number = vpD/u p is the weight-density of the fluid u is the dynamic viscosity of the fluid v is the velocity of the fluid flow D is the diameter of the tube

  19. Cont… If Re < 2000  flow is not turbulent. If Re > 3000  turbulence is almost always present.

  20. Conditions where turbulent flow may occur • When rate of blood flow becomes too great. • Obstruction in a vessel. • Sharp turn. • Viscosity of blood is low.

  21. Measuring blood flow • Electromagnetic flowmeter. • Ultrasonic Doppler flowmeter

  22. Electromagnetic flowmeter

  23. Blood pressure • The force exerted by the blood against any unit area of the vessel wall. • Measured in: • mmHg (using mercury manometer). • Cm H2o. 1 mmHg = 1.36 cm H2o

  24. Methods for measuring blood pressure • Mercury manometer. • High fidelity method.

  25. Mercury manometer

  26. High fidelity method:electronic pressure transducers

  27. Resistance • The impediment to blood flow in a vessel. • Can’t be measured. • Instead calculated from measurement of blood flow & pressure difference.

  28. Cont… Unit of resistance: peripheral resistance unit.

  29. Total peripheral resistance & total pulmonary resistance Strong constriction 4 PRU Strong dilation 0.2 PRU Mean Rt arterial P= 16 mmHg Mean Lt atrial P= 2 mmHg

  30. Conductance of blood & resistance • Conductance: a measure of blood flow through a vessel for a given change in pressure. • ml/ sec/ mmHg. • The reciprocal of resistance.

  31. Changes in diameter & conductance Conductance of a vessel increase in proportion to the forth power of the diameter.

  32. Cont… • Poiseuille’s law: Note: Q is directly proportional to the forth power of radius, demonstrating thatdiameterof a blood vessel plays the greatest role.

  33. Importance of forth power law in determining arteriolar resistance • Arteriolar respond to nervous signals or local tissue signals. • Turning off blood flow or increase flow.

  34. Effect of blood hematocrit & viscosity on vascular R & Q • Viscosity: the friction developed between RBCs as they slide over each other during flow of blood. • An important factor in Poiseuill’s law.

  35. Cont…

  36. The percentage of blood volume occupied by erythrocytes as they are packed down in a centrifuged blood sample. Hematocrit

  37. Effect of hematocrit on blood viscosity

  38. Effects of pressure on vascular resistance & tissue Q

  39. Vascular Distensibility & Function of the Arterial & Venous System

  40. Vascular distensibility • Normally expressed as the fractional increase in volume for each mmHg rise in pressure. • When a vessel being distensible it means it can swell out by pressure from within.

  41. E.g…. Distensibility = 0.1 per mmHG or 10 %/ mmHg

  42. Difference in distensibility between arteries & veins • Walls of arteries are thicker than veins. • Veins are 8 х as distensible as arteries.

  43. Vascular compliance • The total quantity of blood that can be stored in a given portion of the circulation for each mmHg rise in pressure. • It’s a measure of how easily the structure can be stretched.

  44. Cont… Compliance = distensibility x volume e.g: veins are 24x more compliant than arteries (8x as distensible x 3x as great volume) = 24

  45. Compliance and distensibility are quit different

  46. Volume- pressure curve of the arterial and venous circulation

  47. Delayed compliance ( stress- relaxation) of vessels

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