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: