Dynamics of Blood Flow Dynamics of Blood Flow and Blood Pressureand Blood Pressure

2.4.122.4.12

Alignment of erythrocytes at high Alignment of erythrocytes at high shear ratesshear rates

• At low shear rates, the tendency of red blood cells to occupy the axis of the stream is minimal

• At high shear rat)es, erythrocytes occupy the central axis of the tube and they move with long axis parallel to the direction of flow. Here the flow rate is fastest

• Under normal conditions, endothelial cells continuously release endothelial derived relaxing factors (EDRF) or prostacyclin (PGl2)

• With increased blood velocity, shear stress of blood stream acting on the endothelial cells of blood vessels rises and causes greater release of EDRF or prostacyclin (PGl2)

• This leads to vessel dilation and partial return of the

shear stress towards the control state

Effect of velocity of blood on blood Effect of velocity of blood on blood vesselsvessels

Entrance EffectEntrance Effect

Conditions which increase the Conditions which increase the tendency of turbulent flowtendency of turbulent flow

• When the rate of blood flow becomes too great• When blood passes through an obstruction in a vessel• When blood takes sharp turns during flow• When blood passes over a rough surface

• When there is abrupt change in the caliber of vessel or chamber across which blood is flowing. This type of turbulence is artificially induced during recording of blood pressure with sphygmomanometer. Sound produced due

to turbulence is heard as a korotkow sound

Critical velocityCritical velocity• Critical velocity in a tube for a liquid is that velocity above

which flow is turbulent and when equal to or below it is streamline or laminar flow

• Transition from laminar to turbulent flow characterised by Reynold’s Number, K

• Critical velocity is directly proportional to the Raynold’s number

Critical velocityCritical velocity

• The model system shows that near the branching of vessels special properties of flow behavior occur

• The critical Reynolds number in these regions are lower than in an unbranched vessel. This depends on the angle of bifurcation (α in Fig)

• For α=180°, the laminar flow becomes critical at K=350. If the angle is only 165°, K=1500

• Additionally the critical K number depends on the relation

of the radius of the branches

Critical velocityCritical velocity