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Chapter 5.9 Vascular Function Hemodynamics
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Chapter 5.9: VASCULAR FUNCTION: HEMODYNAMICS
The circulatory system uses four major physical principles:
Flow is driven by a pressure difference.
The total mechanical energy is:
The total equivalent pressure is:
Fig. 5.9.1 Difference between lateral and end pressure
Fig. 5.9.2 The Bernoulli principle
Compliance describes the relation between pressure and volume
The compliance of the veins is much greater than the compliance of the arteries.
Ejection of blood into the arterial tree cause the arterial pressure pulse.
The pulse pressure depends on the stroke volume and arterial compliance.
Fig. 5.9.3 Proximal arterial pressure pulse
Fig. 5.9.4 Relation between pressure pulse and stroke volume
Diastolic pressure plus one-third pulse pressure estimates mean arterial pressure.
Fig. 5.9.5 Effect of decreased compliance on pressure pulse
Fig. 5.9.6 Changes in the pressure pulse from proximal to distal arteries
Fig. 5.9.7 Estimationof blood pressure bysphygmomanometry
Fig. 5.9.8 Pressure profiles in the systemic and pulmonary circulation
Poiseuille’s Law approximately describes flow in the vasculature.
Assumptions for Poiseuille Flow:
• the fluid is Newtonian (viscosity is independent of shear rate)
• flow is laminar
• no “slippage” at the walls
• tube is cylindrical with circular cross section, parallel walls
• the walls of the tube are rigid
• the tube is long compared to the entrance length
Fig. 5.9.9 Entrance effects in establishing Poiseuille flow
Poiseuille flow has a parabolic velocity profile:
The ratio of P to Q defines the vascular resistance:
This is analogous to Ohm’s Law:
Resistances in series add:
Resistances in parallel add inversely: