Vasodilation and Vasoconstriction. This is so contradictory?

[scilearner]

Hello everyone,

 

I find this so contradictory so please help to understand this.

 

In vasoconstriction,

 

1. The flow of blood should increase but this would also mean higher resistance so does the flow increase or not. Normally when we constrict something we expect higher speed don't we.

2. In vasoconstriction pressure should be higher but again does the resistance decrease the pressure.

 

My questions for vasodilations are exactly the same but vise versa.

 

Thanks for anyone who is going to help

[John Cuthber]

"The flow of blood should increase but this would also mean higher resistance so does the flow increase or not. Normally when we constrict something we expect higher speed don't we."

 

You turn the tap off and expect the water to come out faster?

[scilearner]

"The flow of blood should increase but this would also mean higher resistance so does the flow increase or not. Normally when we constrict something we expect higher speed don't we."

 

You turn the tap off and expect the water to come out faster?

 

But then why does Bernoulli effect state otherwise. Why is Bernoulli effect useful when it is not true!!

[Mokele]

The Bernoulli effect applies to changes in *one* pipe.

 

Vasoconstriction decreases blood flow because, due to the increased resistance, more blood flows into *other* arteries.

[John Cuthber]

Imagine I have 3 bits of pipe connected in series A thin piece between two wide pieces.

If I force water through them the flow rate in ml/m must be very nearly the same because water is practically incompressible.

In the thin bit of pipe the flow velocity must be higher but the flow trates are the same througout.

With a given pump (or head of water) you would get more flow if you took out the thin bit and replaced it by (another) wide bit.

It's not velocity of blood that carries stuff about the body, it's the flow rate.

[J.C.MacSwell]

Imagine I have 3 bits of pipe connected in series A thin piece between two wide pieces.

If I force water through them the flow rate in ml/m must be very nearly the same because water is practically incompressible.

In the thin bit of pipe the flow velocity must be higher but the flow trates are the same througout.

With a given pump (or head of water) you would get more flow if you took out the thin bit and replaced it by (another) wide bit.

It's not velocity of blood that carries stuff about the body, it's the flow rate.

 

Everyone keep that in mind next time they wish to order a 3 egg cheese omelette.

[John Cuthber]

Everyone keep that in mind next time they wish to order a 3 egg cheese omelette.

 

With beans?

[scilearner]

Imagine I have 3 bits of pipe connected in series A thin piece between two wide pieces.

If I force water through them the flow rate in ml/m must be very nearly the same because water is practically incompressible.

In the thin bit of pipe the flow velocity must be higher but the flow trates are the same througout.

With a given pump (or head of water) you would get more flow if you took out the thin bit and replaced it by (another) wide bit.

It's not velocity of blood that carries stuff about the body, it's the flow rate.

 

Thanks for the answer. Do you mean that the thin pipe has less water molecules and when velocity is increased the flow rate is maintained?

[Bignose]

For a constant volumetric flow rate, it is indeed true that as you make the pipe smaller, the average velocity of the output would have to increase. This is necessary to maintain the constant rate of volume.

 

However, most real-life situations are not going to be constant volume. I real-life, if you introduce constrictions downstream, the pump isn't going to be able to output the exact same volume as before.

 

Bernoulli's equation, usually written as:

[math]0.5 \rho v^2 + \rho g z + p = constant[/math] where

[math]\rho[/math] is the fluid density

v is the fluid velocity

g is acceleration due to gravity

z is horizontal distance

and p is pressure

 

A lot of times you will see this written as:

[math]0.5 \rho (v_2^2-v_1^2) + \rho g (z_2-z_1) + (p_2-p_1) = 0[/math] where the subscripts indicate different locations along the same line.

 

The above equation is derived from an integration of Euler's equations. One huge thing is missing in Euler's equations: viscosity. Euler's equations assume a perfectly inviscid fluid.

 

There is an "engineering" Bernoulli's equation that tries to remedy some of these short comings. Typically it may look like:

 

[math]0.5 \rho (v_2^2-v_1^2) + \rho g (z_2-z_1) + (p_2-p_1) + \Sigma F + W= 0[/math]

where [math]\Sigma F[/math] stands for the sum of all the frictional energy loss and

W stands for any work done by or added to the fluid.

 

There are good correlations out there to estimate F based on the type of pipe material and average fluid velocity. But, I think you can see now that as you constrict the flow, you will increase the F term, and thereby causing the output velocity to decrease. Consult a good undergraduate-level engineering fluid mechanics text for a lot more information. This is an equation that is used quite a lot still because it is usually good enough (with the right correlations).

[Mokele]

I think the mistake isn't departures from the underlying assumptions of Bernoulli's equation, it's that we're considering blood flow across numerous, branching pipes.

 

For instance, imagine I jump out at you with a knife and try to kill you.

 

Among the many effects of the resulting adrenaline surge are that you get vasoconstriction in the gut, in order to route blood flow elsewhere in the body. This is a useful example, because of the layout of the circulatory system.

 

Here's normal:

 

Descending Aorta

| |

| |

| |

| -------

| -------Foregut (Stomach etc.)

| |

| |

| |

| -------

| -------Midgut (small intestines, mostly)

| |

| |

| |

| -------

| -------Hindgut (most of the large intestines)

| |

| |

| |

(blood to legs and lower body)

 

So, in a normal situation, blood goes down the descending aorta, and along the way, some is pushed off to the sides to the various sections of the gut.

 

 

 

Now, here's what happens when I try to kill you:

 

Descending Aorta

| |

| |

| |

| ======Foregut (Stomach etc.)

| |

| |

| |

| =====Midgut (small intestines, mostly)

| |

| |

| |

| =====Hindgut (most of the large intestines)

| |

| |

| |

(blood to legs and lower body)

 

Notice how the arteries going to the gut have all constricted. This causes increased resistance to flow, so more blood will bypass the arteries and go straight to your legs to help you run away.

 

 

You can try it yourself. Get a garden hose and poke a bunch of holes it it. Water will come out the holes and the end of the hose. Now, cover those holes loosely with your hand, reducing the flow but not shutting it off. What happens to the flow out of the end of the hose?

[scilearner]

I think the mistake isn't departures from the underlying assumptions of Bernoulli's equation, it's that we're considering blood flow across numerous, branching pipes.

 

For instance, imagine I jump out at you with a knife and try to kill you.

 

Among the many effects of the resulting adrenaline surge are that you get vasoconstriction in the gut, in order to route blood flow elsewhere in the body. This is a useful example, because of the layout of the circulatory system.

 

Here's normal:

 

Descending Aorta

| |

| |

| |

| -------

| -------Foregut (Stomach etc.)

| |

| |

| |

| -------

| -------Midgut (small intestines, mostly)

| |

| |

| |

| -------

| -------Hindgut (most of the large intestines)

| |

| |

| |

(blood to legs and lower body)

 

So, in a normal situation, blood goes down the descending aorta, and along the way, some is pushed off to the sides to the various sections of the gut.

 

 

 

Now, here's what happens when I try to kill you:

 

Descending Aorta

| |

| |

| |

| ======Foregut (Stomach etc.)

| |

| |

| |

| =====Midgut (small intestines, mostly)

| |

| |

| |

| =====Hindgut (most of the large intestines)

| |

| |

| |

(blood to legs and lower body)

 

Notice how the arteries going to the gut have all constricted. This causes increased resistance to flow, so more blood will bypass the arteries and go straight to your legs to help you run away.

 

 

You can try it yourself. Get a garden hose and poke a bunch of holes it it. Water will come out the holes and the end of the hose. Now, cover those holes loosely with your hand, reducing the flow but not shutting it off. What happens to the flow out of the end of the hose?

 

lol what an analogy!! Thanks a lot I did understand everything you said so it worked. Now my question is how is this related to Bernoulli's equation. Do you mean the reason velocity increases is that unrestricted branches get more flow. That is true but isn't bernoulli referring to one pipe!!

 

Basically in real life is bernoulli's equation useless because resistance is too great!!