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Hemodynamics

Hemodynamics. Conwin Vanterpool, Ph.D. Requirements for Proper Function of the Cardiovascular System. A Pump A tubing system Transport media (blood) Exchange areas for pickup (lungs, GI tract, glands, fat, ect) Drop off sites (metabolic tissue, kidneys, liver ect).

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Hemodynamics

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  1. Hemodynamics Conwin Vanterpool, Ph.D

  2. Requirements for Proper Function of the Cardiovascular System • A Pump • A tubing system • Transport media (blood) • Exchange areas for pickup (lungs, GI tract, glands, fat, ect) • Drop off sites (metabolic tissue, kidneys, liver ect)

  3. Unfortunately, the body cannot maintain a high enough cardiac output to supply every tissue with any conceivable need. Therefore, control systems are needed such as: • Baroreceptors – blood pressure control • Chemoreceptors – oxygen and pH control • Flow control – to direct the correct flow to the appropriate vascular beds.

  4. Hemodynamics • Hemodynamics is the study of the forces that cause flow to occur in the vascular system and the forces that impede flow. • In general, in the human cardiovascular system we describe flow through any vascular bed by the following equation: Q = ΔP / R • Q – denotes flow • ΔP – is the pressure difference – the driving force for flow • R – is resistance – the force which impedes flow.

  5. For the Following System Po Pi Flow Rtotal Q = ΔP / R Qtotal = (Pi – Po)/ Rtotal

  6. Pressure • Pressure is defined as force / unit area • Smaller areas give larger pressures • Larger areas give smaller pressures • In the cardiovascular system, pressure is created when the heart pumps a volume of blood (stroke volume) into the aorta. • The stretched walls trying to compress the volume of blood in the aorta results in pressure. • Pressure in the aorta – Pressure in the right atrium = pressure in the cardiovascular system.

  7. Measurement of Pressure • Arterial pressure was first measured by inserting a long glass tube in the carotid artery of an anesthetized horse. • Blood in the tube rose 8 feet 3 inches high – that was the blood pressure. • Eventually tubes were filled with mercury and pressure was reported as millimeters of mercury (mmHg).

  8. Two factors are very important to remember when measuring blood pressure. • The pressure that is measured is the difference between pressure in the arteries and atmospheric pressure. Thus, if you measure the pressure of someone standing on Mount Everest, vs. someone at sea level, the pressures should be approximately the same. • Pressure must be referenced to some point – usually this is the pressure in the right atrium. That is why pressure is measured in the arm with the patients arm held at the same level as the heart. If pressure is measured in the leg, the patient must be reclining so that the leg is at the same level as the heart.

  9. Flow • Fluid can flow through vessels in two forms – laminar flow or turbulent flow.

  10. The following diagram illustrates the two types of flow. Laminar Flow Turbulent Flow

  11. Laminar Flow – is characterized by all fluic molecules moving in straight line in the direction of flow. Actually, if you looked at a cross sectional area of a blood vessel the molecules adjacent to the wall of the vessel would not be flowing ( they are basically stuck to the walls).

  12. Turbulent Flow – is characterized by fluid molecules in concentric rings with the highest velocities in the middle of the vessel. Different directions of travel, and velocity may be observed throughout the diameter of the vessel. However, the integration of all directions of travel results in bulk flow of fluid from left to right in the previous diagram.

  13. Resistance • Resistance is the force that impedes flow. There are a number of factors that contribute to overall resistance to flow of fluid in a tube. • Viscosity (η) • Length of the tube (l) • Radius of the tube (r) • These are related to resistance in the following equation. • R = 8lη /πr4

  14. In the previous equation, 8 and π are constants having to do with the facts that we are dealing with a round tube. • If you dissect the equation, resistance to flow is directly related to viscosity of the fluid and the length of the tube, and are inversely related to the fourth power of the radius of the tube. • Physiologically the cardiovascular system does not change in length, except in growth. • The viscosity of the blood is about 3 times that of water. Viscosity of the blood does change with changes in hematocrit.

  15. Because resistance is related inversely to the fourth power of radius, a small change in radius results in a large change in resistance. • For example, if radius of a vessel were decreased by one half, resistance would increase to 16 times greater than its original value.

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