Vascular function/ venous return curve help!

[mossyfiber12]

Okay so I am really confused about this vascular function curve concept. I don't understand how, when you have negative pressure in the RA, the veins collapse YET you still get the maximum venous return to the RA.

If the veins have collapsed, i.e. resistance has gone up infinitely, how are you pumping any blood through them.

Is it because the big veins (IVC, SVC) don't collapse and it's the venules that collapse? If this is true then your pressure gradient goes up, right? It also means that you have a greater flow through your veins (assuming the bigger veins did not collapse due to the negative pressue).

Someone please clarify this for me! Thanks

P.S. When we are talking about venous return curve, it is assumed that CO is zero, yes?

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[Dirt]

Okay so I am really confused about this vascular function curve concept. I don't understand how, when you have negative pressure in the RA, the veins collapse YET you still get the maximum venous return to the RA.


Is it because the big veins (IVC, SVC) don't collapse and it's the venules that collapse? If this is true then your pressure gradient goes up, right? It also means that you have a greater flow through your veins (assuming the bigger veins did not collapse due to the negative pressue).



P.S. When we are talking about venous return curve, it is assumed that CO is zero, yes?

The veins don't collapse when the right atrial pressure is negative.

Flow=change in pressure/resistance. If you have a negative pressure, you have a high change in pressure, and thus a high flow.

When we talk about a venous return curve, CO is not zero, CO=venous return.

 

[EMCC]

The pressure in the venous system never reaches zero--not even during death. So, to say that the negative RA pressure will collapse the vessels is incorrect.

The atria do not really contribute a significant amount of 'negative' pressure at any given time. What determines the amount of blood volume that will return to the right side of the heart is the central venous pressure--which is determined by the capacitance of the entire venous system, but primarily the great venous vessels (IJ, SC, IVC).

Remember that CO=HR*SV. Heart rate is easy. The stroke volume is determined by afterload, preload and contractility. The most important of these variables is the Preload. CO is typically 4-8, assuming a healthy "pumping" heart.

 

[NRAI2001]

Okay so I am really confused about this vascular function curve concept. I don't understand how, when you have negative pressure in the RA, the veins collapse YET you still get the maximum venous return to the RA.

If the veins have collapsed, i.e. resistance has gone up infinitely, how are you pumping any blood through them.

Is it because the big veins (IVC, SVC) don't collapse and it's the venules that collapse? If this is true then your pressure gradient goes up, right? It also means that you have a greater flow through your veins (assuming the bigger veins did not collapse due to the negative pressue).

Someone please clarify this for me! Thanks

P.S. When we are talking about venous return curve, it is assumed that CO is zero, yes?

The RA never reaches negative pressure neither do the veins at any point. The veins must always have a greater mean pressure than does the RA or no movement of blood would occur. Why do you assume CO is zero? This is never the case accept in death.

The point of the venous return curve is to demonstrate that as the pressure difference between the veins and the RA increases the VR (venous return) will increase this is primarily accomplished by droping RA pressure by speeding up the heart such that more blood is sucked into the ventricles at a faster rate = effectively lowering the pressure in the RA. So as RA pressure drops Cardiac Output is increased bc VR has increased and more blood is available to be pumped out to the rest of the body.

 

[coolioyo]

The vena cava collapses below a pressure of 0. Even if you decrease the pressure lower than 0, the effective driving pressure will be the same because it will be P of arteries - P of atmospheric or intrapleural. This is the same case as a starling resistor or what West describes as zone 2 in the lungs. When I talk of pressure I mean right atrial or central venous, because they are effectively the same thing. Let me know if you don't understand something I said.

 

[NRAI2001]

The vena cava collapses below a pressure of 0. Even if you decrease the pressure lower than 0, the effective driving pressure will be the same because it will be P of arteries - P of atmospheric or intrapleural. This is the same case as a starling resistor or what West describes as zone 2 in the lungs. When I talk of pressure I mean right atrial or central venous, because they are effectively the same thing. Let me know if you don't understand something I said.

When does the vena cava reach a pressure of 0 or lower?

 

[Hayden2102]

When does the vena cava reach a pressure of 0 or lower?

When you sniff quickly.

Ever seen it on ultrasound? Looks pretty cool.

 

[NRAI2001]

When you sniff quickly.

Ever seen it on ultrasound? Looks pretty cool.

So when you breath in quickly/deeply? Is it bc the intrapleural pressure pulls in pulmonary artery blood lowering pressure all the way back to the RA and vena cava?

 

[Hayden2102]

Yeah very briefly. The gradient doesn't get high enough when you just breathe deeply, it has to be a fast breath in so the lower caval blood doesn't have time yet to fill the (relatively) negatively pressured IVC.