Can someone define "venous compliance" and the factors that influence it and how "venous compliance" is related to "venous return"
thanks
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venous compliance
- Thread starter QST
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This is my understanding so if this is wrong, someone please correct me.
Compliance for any organ/vessels is defined is a change in volume over a change in pressure (C = delta V/delta P). For vessels, how easy it is for them to expand. For lungs, how easy it is for them to inflate and so on. Vasodilation of the vessels will make it more compliant and constriction will do the reverse. When SNS acts on the veins it causes the vessels to constrict (making them less compliant) and this increases venous return to the heart.
I hope that helps.
Compliance for any organ/vessels is defined is a change in volume over a change in pressure (C = delta V/delta P). For vessels, how easy it is for them to expand. For lungs, how easy it is for them to inflate and so on. Vasodilation of the vessels will make it more compliant and constriction will do the reverse. When SNS acts on the veins it causes the vessels to constrict (making them less compliant) and this increases venous return to the heart.
I hope that helps.
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This is my understanding so if this is wrong, someone please correct me.
Compliance for any organ/vessels is defined is a change in volume over a change in pressure (C = delta V/delta P). For vessels, how easy it is for them to expand. For lungs, how easy it is for them to inflate and so on. Vasodilation of the vessels will make it more compliant and constriction will do the reverse. When SNS acts on the veins it causes the vessels to constrict (making them less compliant) and this increases venous return to the heart.
I hope that helps.
Sorry for reviving the old topic, but I am having the same problem understanding this concept. If SNS acts on the veins and causes the vessels to constrict, that means the diameter of the vessel decreases and flow would also decrease.
Blood flow = pressure / resistance
and the resistance increases if diameter decreases.
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i would just add to the concept of the previous post...since there is an increase in resistance and decrease in diameter of the veins, it would result in an increase in blood pressure (though there is decreased blood flow) that would therefore increase the venous return to the heart as initially there was pooling of blood in the veins in the legs..this is usually seen during hypovolemic shock....that is basically the sympathetic part that is activated which also increases the heart rate...the other part of compensation for loss of blood in hypovolemic shock is the hormonal renin Ag aspect..i hope it clears the concept..gudluck...
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i would just add to the concept of the previous post...since there is an increase in resistance and decrease in diameter of the veins, it would result in an increase in blood pressure (though there is decreased blood flow) that would therefore increase the venous return to the heart as initially there was pooling of blood in the veins in the legs..this is usually seen during hypovolemic shock....that is basically the sympathetic part that is activated which also increases the heart rate...the other part of compensation for loss of blood in hypovolemic shock is the hormonal renin Ag aspect..i hope it clears the concept..gudluck...
decreased diameter = inc resistance = less blood flowing = less venous return
decreased diameter = inc pressure = more blood flowing = more venous return
are these correct? If so, does inc pressure overcome the inc resistance and make blood flow more?
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according to what i understood, normally it doesnt increase the blood flow when there is increase in contraction of the blood vessels ( as we see in tahcycardia there is decreased blood flow through the coronary artery to the heart) but in the case of leg veins there is pooling of blood in the veins normally due to gravity and when there is increase in contraction of the veins this blood is directed towards the heart and thereby increases the venous return...hopefully this would clear ur concept as that is what i think about the whole process...
This is my understanding so if this is wrong, someone please correct me.
Compliance for any organ/vessels is defined is a change in volume over a change in pressure (C = delta V/delta P). For vessels, how easy it is for them to expand. For lungs, how easy it is for them to inflate and so on. Vasodilation of the vessels will make it more compliant and constriction will do the reverse. When SNS acts on the veins it causes the vessels to constrict (making them less compliant) and this increases venous return to the heart.
I hope that helps.
I think I agreed with everythign here up until s/he says "Vasodilation of the vessles will make it more compliant..." I understand vasodilation to decrease resistance, but not necessarily increase compliance at that new diameter. Think of when you blow air into a balloon... It is harder to blow air as the balloon gets bigger (compare a very large balloon to a dilated blood vessel). It is the properties of the vessel wall that contribute to compliance. That is why, loose, flabby vein walls have higher compliance than the thick, muscular/elastic arterial walls.
Also, I never thought that the Sympathetic Nervous System (SNS) had any effect on veins. SNS acts on muscles in arterioles. Veins/venules have very little muscle in their tunica media. Correct me if I'm wrong please.
decreased diameter = inc resistance = less blood flowing = less venous return
decreased diameter = inc pressure = more blood flowing = more venous return
are these correct? If so, does inc pressure overcome the inc resistance and make blood flow more?
I think your first two equations, are sort of the same thing... Increased resistance leads to the blood flowing through it to have an increased pressure.
According to Flow x Resistance = Change in Pressure. If you increase resistance, your pressure automatically increases. Furthermore, if you decrease the diameter of a tube, the fluid flowing thru it will exert more pressure on the walls of the tube to try and compensate for the decreased cross-sectional area. The net flow may actually be less, as a result. But of course, if you look at it step-by-step which is what you are trying to do, yes increased pressure will increase blood flow. But in the larger picture, I don't think this is the case.
Can someone define "venous compliance" and the factors that influence it and how "venous compliance" is related to "venous return"
thanks
decreased venous compliance (i.e., sympathetic stimuli) increase venous pressure and venous return.
increased venous compliance ( wide veins ) decrease venous pressure and venous return
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Compliance is ability of the vessel wall to stretch in response to high blood volume ( increase blood volume - increased diameter - decreased blood pressure )
Resistance is ability of the vessel wall NOT stretch but keep its shape and diameter in response to high blood volume ( increased blood volume - same diameter - increased blood pressure ).
Arteries - high resistance, low compliance
Veins - low resistance, high compliance.
Resistance is ability of the vessel wall NOT stretch but keep its shape and diameter in response to high blood volume ( increased blood volume - same diameter - increased blood pressure ).
Arteries - high resistance, low compliance
Veins - low resistance, high compliance.
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I know this thread is quite old so this reply is probably of no use to the OP but for anyone looking on this thread, some of the ideas above are slightly flawed. The first replier was right in that vasodilation does increase compliance as there is less muscle tone so the walls can stretch more easily (the person with the analogy about the balloon - it does actually get easier somewhere in the middle to blow it up due to decreased compliance). The person who questioned why blood flow increases even though diameter is reduced during venoconstriction, you make a very good point. However, you have to remember that the limiting point in the circulation (the "tap") is in the arterioles, they have a much smaller overall diameter than an other vessel (capillaries have smaller individual diameters but there are a lot more of them). Therefore a decrease in diameter of venules (which is where venoconstriction takes place), will have negligible impact on flow.
A lot of posts on this thread allude to the fact that increases sympathetic venoconstriction increases venous return. However, although this may impact venous return transiently, it will not make a huge impact in the longer term. The main reason for venoconstriction upon exercise etc. is that it increases MSFP (mean systemic filling pressure - ie the average pressure your blood is under). This allows the difference between the arterial and venous pressures to be greater upon the heart pumping (without venous pressure reducing to below 0) and therefore increases flow: blood flow=pressure difference/resistance.
A lot of posts on this thread allude to the fact that increases sympathetic venoconstriction increases venous return. However, although this may impact venous return transiently, it will not make a huge impact in the longer term. The main reason for venoconstriction upon exercise etc. is that it increases MSFP (mean systemic filling pressure - ie the average pressure your blood is under). This allows the difference between the arterial and venous pressures to be greater upon the heart pumping (without venous pressure reducing to below 0) and therefore increases flow: blood flow=pressure difference/resistance.