HEMODYNAMICS

[sathyasai baba]

---

Members don't see this ad.

QUOTE FROM LINDA
"SHEAR
Shear is a consequence of the fact that blood travels at different velocities within a blood vessel (see Fig. 4-6). Shear occurs if adjacent layers of blood travel at different velocities; when adjacent layers travel at the same velocity, there is no shear. Thus, shear is highest at the blood vessel wall, according to the following reasoning. Right at the wall, there is a motionless layer of blood (i.e., velocity is zero); the adjacent layer of blood is moving and therefore has a velocity. The greatest relative difference in velocity of blood is between the motionless layer of blood right at the wall and the next layer in. Shear is lowest at the center of the blood vessel, where the velocity of blood is highest, but where the adjacent layers of blood are essentially moving at the same velocity. One consequence of shear is that it breaks up aggregates of red blood cells and decreases blood viscosity. Therefore, at the wall, where shear rate is normally highest, red blood cell aggregation and viscosity are lowest."

iam confused,(according to my understanding)
viscosity is friction between molecules as they slide each other
so when particles travel at higher relative velocities dont the have more friction ?
please clarify

 

[SBR249]

You may be looking at this the wrong way. Friction is what prevents or opposes relative motion. If you have higher velocity, that means you must have lower friction or else the former would not be possible.

 

[sathyasai baba]

thanks sir

 

[sathyasai baba]

sir what is the relationship between viscosity and velocity?

 

[sathyasai baba]

sir,what is the relationship between viscosity & velocity?

 

[SBR249]

sir,what is the relationship between viscosity & velocity?

Inverse. Just think about real life examples. Which is faster? Pouring water or pouring honey? Which one is more viscous?

 

[sathyasai baba]

Sir ,in the layer nearer to the blood vessel wall the velocity with which it moves is lowest when compared to the velocity of the lamina of blood in the centre.
but shear is highest at the vessel wall moving rbc aggregates & decreases viscosity.
iam still confused as the lamina of blood near vessel wall seems to have low velosity as well as low viscosity.
please help me .

 

[SBR249]

This is getting way too deep into the weeds.

But it can be mathematically shown that velocity is inverse linearly related to both viscosity and the surface area of the lamina. While axial accumulation of RBCs will cause local viscosity to decrease in the vicinity of the vessel wall, the bigger influence is the dependence of the surface area of the lamina on the radius. Whatever small decrease in viscosity due to RBC depletion is more than offset by the fact that the surface area of the cylindrical laminae increase dramatically as you approach the wall. It is this influence that drives the decrease in velocity.

 

[sathyasai baba]

Thank you very much,sir .I got it ,many thanks for sharing your knowledge and helping me understand.🙂🙂🙂