resistance capacitance

[lemonade90]

In terms of hemodynamics, how exactly are resistance and capacitance related? I understand the equations for both but am having trouble visualizing what actually goes on in the body.

Right now I am sort of viewing resistance as a component of the resistance formula that slows flow down and thereby causes some blood to pool up behind the point of resistance (sort of like a dam with a little pipe of water or really viscous fluid, etc)...when resistance goes down flow increases and that pooling of blood you get is eliminated (i.e. arterial portions of CV system have less blood). However I am having some difficulty in terms of visualizing capacitance. Clearly the body doesn't just take blood out of circulation and store it away.

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[123165]

In terms of hemodynamics, how exactly are resistance and capacitance related? I understand the equations for both but am having trouble visualizing what actually goes on in the body.

Right now I am sort of viewing resistance as a component of the resistance formula that slows flow down and thereby causes some blood to pool up behind the point of resistance (sort of like a dam with a little pipe of water or really viscous fluid, etc)...when resistance goes down flow increases and that pooling of blood you get is eliminated (i.e. arterial portions of CV system have less blood). However I am having some difficulty in terms of visualizing capacitance. Clearly the body doesn't just take blood out of circulation and store it away.

It is the ability to increase volume without increasing pressure. It is more related to elasticity than resistance

Capacitance is change in volume/change in pressure. Higher capacitance means more volume increase with less pressure increase.

Resistance has more to do with the friction between the blood and the vessel

 

[Valadi]

It is the ability to increase volume without increasing pressure. It is more related to elasticity than resistance

Capacitance is change in volume/change in pressure. Higher capacitance means more volume increase with less pressure increase.

Resistance has more to do with the friction between the blood and the vessel

True. And that friction is a function of length, viscosity, and radius.

Resistance = (8*length*coefficient of viscosity) / (pi*radius^4)
Flow = (Pressure difference*pi*radius^4) / (8*length*viscosity)

Can't believe I actually remembered these equations. Hanging myself for being a nerd now..

 

[123165]

Almost true. Resistance is not to do with friction but with length, viscosity, and radius, which all translate into flow/pressure relationships.

Resistance = (8*length*coefficient of viscosity) / (pi*radius^4)
Flow = (Pressure difference*pi*radius^4) / (8*length*viscosity)

Can't believe I actually remembered these equations. Hanging myself for being a nerd now..

Length, viscosity and radius are the contributors of the friction, I don't think it is an incorrect term to use. It is friction between the blood and vessel walls

I think this is just semantics, in my opinion resistance in this sense is from friction.

 

[notarealname]

In terms of hemodynamics, how exactly are resistance and capacitance related? I understand the equations for both but am having trouble visualizing what actually goes on in the body.

Right now I am sort of viewing resistance as a component of the resistance formula that slows flow down and thereby causes some blood to pool up behind the point of resistance (sort of like a dam with a little pipe of water or really viscous fluid, etc)...when resistance goes down flow increases and that pooling of blood you get is eliminated (i.e. arterial portions of CV system have less blood). However I am having some difficulty in terms of visualizing capacitance. Clearly the body doesn't just take blood out of circulation and store it away.

Correct me if I am wrong, I look at capacitance like a storage sort like your veins. In a circuitry, capacity is like the amount of charges can be stored on one surface of the plate.

 

[Valadi]

Length, viscosity and radius are the contributors of the friction, I don't think it is an incorrect term to use. It is friction between the blood and vessel walls

I think this is just semantics, in my opinion resistance in this sense is from friction.

I had to run through physics websites to remind myself about all this stuff, but you're exactly right. Editing my post to reflect that. Thanks!

 

[lemonade90]

I thought about this some more and realized the difficulty I had was understanding why the capacitance didn't change when resistance changed (i.e. when the radius became smaller in particular). I then realized that changing the resistance has no bearing on capacitance because if the volume for the fluid became less, the pressure would rise so as to maintain the constant slope of the compliance curve. Until there was any inherent change in the composition of the wall of the vasculature which altered the slope of the compliance curve, compliance wouldn't change. A comparable argument can be made as to why resistance wouldn't change when capacitance changes.

Thanks for your help everyone.

 

[lemonade90]

Became confused again.

I read this: http://www.cvpharmacology.com/vasodilator/vasodilators.htm

It discusses how constricting veins decreases compliance while dilating them increases compliance. So from this it seems that by altering the degree of contraction in smooth muscles, one can alter compliance.

I am left pondering why altering smooth muscle contraction in arterioles wont lead to decreased compliance and why contracting smooth muscles in veins will lead to increased resistance? The mechanism, at least in terms of contraction of smooth muscles, is similar.

 

[notarealname]

Became confused again.

I read this: http://www.cvpharmacology.com/vasodilator/vasodilators.htm

It discusses how constricting veins decreases compliance while dilating them increases compliance. So from this it seems that by altering the degree of contraction in smooth muscles, one can alter compliance.

I am left pondering why altering smooth muscle contraction in arterioles wont lead to decreased compliance and why contracting smooth muscles in veins will lead to increased resistance? The mechanism, at least in terms of contraction of smooth muscles, is similar.

When smooth muscles contracts the vessel itself is less likely to stretch, thus decrease in compliance (which is the vessel's ability to stretch or recoil). Arteries has thicker wall thus stretch less in the first place. Compliance = change of Volume/ change of pressure. Take veins you increase the volume of that thing, the pressure in the vessel wall changes very little, thus a big number divided by a small number you get a large compliance. Just take a trip to the wiki land ... http://en.wikipedia.org/wiki/Compliance_(physiology)