Could be a stupid question...

[EmersonAnne]

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but why does venoconstriction increase venous return? I can't make sense of it.

 

[ChiDO]

Majority of blood is in the venous system. If you constrict the venous system, you force the volume to go somewhere. It can't go back (due to valves), thus it goes forward (ie. increasing venous return to the heart).

 

[EmersonAnne]

My hang-up is on the constriction. If it's constricted, how can the blood move forward?

 

[engineeredout]

The veins are a means of storage of blood. The circulatory system is a closed system. When the veins are constricted, its going to force the blood to go elsewhere. Elsewhere after the systemic veins is the vena cavae/right atrium (increased venous return)

Think about toothpaste. If you squeeze a tube of toothpaste, it gets constricted and the toothpaste is pushed out in one direction.

 

[Jolie South]

The venous side of circulation is high capacity. If those vessels constrict from a more dilated state (where they're holding lots of blood), the diameter and the volume that the veins can hold is reduced. Blood is forced forward to the SVC/IVC and the right side of the heart.

 

[EmersonAnne]

Everything you're saying makes sense and I get the toothpaste analogy, but I guess I'm thinking of it as one vein. If it's constricted, I don't see how the blood gets through the constriction in a larger amount.

 

[Jolie South]

Everything you're saying makes sense and I get the toothpaste analogy, but I guess I'm thinking of it as one vein. If it's constricted, I don't see how the blood gets through the constriction in a larger amount.

Think of it as the veins starting really dilated and full. The blood is not moving really fast and is just kind of hanging out. When you add the vasoconstrictor, you decrease the diameter, and the blood can't just hang out any more. It needs to keep moving forward.

 

[ChiDO]

YOu are correct that FLOW decreases through the vein. However, your original question asked about increased venous return.

Veins HOLD the majority of the blood. Thus, when you originally CONSTRICT them, it forces all the stored blood in the veins to move forward towards the heart (increase venous return).

The flow from the arteries to the veins will now be slowed as well due to venous constriction; however, since veins are now constricted, blood will no longer be "stored" and will go back to the heart more rapidly.

 

[turkeyjerky]

The flow from the arteries to the veins will now be slowed as well due to venous constriction; however, since veins are now constricted, blood will no longer be "stored" and will go back to the heart more rapidly.

Not really. Resistance to blood flow through arteries is a function of arteriolar diameter. Veins are downstream to the arterioles and don't offer any resistance to arterial blood flow. (If an analogy to biochem helps, the arterioles are the 'rate-limiting' step of blood flow)

Remember this--downstream to the arterioles there is NO resistance to blood flow. (That's not really true, but functionally it may as well be)

The function of venoconstriction is to mobilize the blood that is normally 'pooled' within the veins.

Thus, the time of greatest blood flow is when the veins are constricted and the arterioles are dilated. Like in active skeletal muscle.

 

[xanthomondo]

My hang-up is on the constriction. If it's constricted, how can the blood move forward?

The constriction isn't 100%. The blood still has room to move forward.

I like to think of it as a garden hose. If you turn the hose on it shoots a certain distance, if you put your thumb over the top (which contricts the diameter) the water goes farther (and faster).

 

[EmersonAnne]

So blood that's pooled in the veins moves faster since the diameter is reduced? And this will eventually lead to an increase in stroke volume, right?

 

[spicedmanna]

No, it's a good question. I'm not sure of the answer. But I'll take a stab at it.

One way of looking at it is that veins are fairly compliant vessels and thus have a fairly large capacitance, unlike arteries, which are impacted and governed more by resistance. The venous system has a good capacity to contain and hold blood. Therefore, if you decrease the capacitance, by decreasing compliance (venoconstriction), you are going to increase the pressure, which will then translate into an increase in venous return as indicated by an increase in right atrial pressure.

Does that make any sense at all?

Reference: http://jap.physiology.org/cgi/reprint/00535.2004v1.pdf

 

[username456789]

It's a dynamic system. Venous return will initially increase since you're basically squeezing the blood that's pooled in the veins back to the heart. This will cause an increased stroke volume. But eventually venous return (speaking in terms of volume/time) will decrease slightly, since more blood is being "stored" on the arterial side. This tends to DECREASE the stroke volume.

It's basically "changing" beat to beat, and it's certainly not a static property/principle. There's a lot more to it, but I'm having trouble channeling M1 cardio.

Basically, it's a back-and-forth between [more blood returning to the heart/INCREASED stroke volume/more blood "stored" on the arterial side] and [more blood on arterial side/so less blood returning to RA/so DECREASED stroke volume].

 

[Sol Rosenberg]

So blood that's pooled in the veins moves faster since the diameter is reduced? And this will eventually lead to an increase in stroke volume, right?

Yes, blood will move slightly faster through the veins because of their reduced diameter (Bernoulli's Principle.) The slightly increased velocity of blood through the venous system does not result in increased stroke volume, per se (most of the energy of the fluid is lost by the time you get to the veins.) The increased venous return results in increased EDV, which causes increased preload, which results in increased contractility (Frank-Starling,) which results in increased stroke volume.

 

[NTF]

Another way to look at it is to view peripheral venoconstriction as increasing the pressure differential between the peripheral venous pool and central venous pool (lungs) thus increasing flow.

 

[HelpingHand]

My hang-up is on the constriction. If it's constricted, how can the blood move forward?

I don't know if your question has been answered to your satisfaction yet, but referring to this quote of yours, I think the problem might be your definition of what venoconstriction is?

Vasoconstriction doesn't mean that you are blocking the veins; it means that you are squeezing the veins. If you squeeze the veins, as someone pointed out, the blood has to move toward the heart due the one-way valves of the veins.

If this doesn't help, then you might be thinking too hard. In that case, just memorize the fact and move on. Somewhere along the line, you will have a light bulb moment, and it will spontaneously make sense to you.

Good luck, friend.

 

[Jolie South]

I don't know if your question has been answered to your satisfaction yet, but referring to this quote of yours, I think the problem might be your definition of what venoconstriction is?

Vasoconstriction doesn't mean that you are blocking the veins; it means that you are squeezing the veins. If you squeeze the veins, as someone pointed out, the blood has to move toward the heart due the one-way valves of the veins.

If this doesn't help, then you might be thinking too hard. In that case, just memorize the fact and move on. Somewhere along the line, you will have a light bulb moment, and it will spontaneously make sense to you.

Good luck, friend.

Isn't that how most learning in med school works? I mean I don't always have all the time to sit and think about things when I'm going over lectures, but then I'll be walking to the metro stop or whatever and the answers will come to me.

 

[Knicks]

Since this thread's about asking "stupid questions" (LoL!), here's one, that for some reason, has always confused me.

I mean, somehow I intuitively "know the answer" but I can't translate it into words, so here it goes:

Can someone remind me again how exactly blood is "stored" in the veins?

Aren't arteries and veins ultimately all connected, and so whatever is flowing in the arteries will pass through the veins (as long as the heart is pumping)?

So how can blood be "stored" in the veins? Does it have to do with things being connected in parallel? Does it have something to do with veins being compliant? Even so, veins are still tubes connected to arteries,,,,, so won't the blood flowing in the arteries "push" the blood that's in the veins (towards the heart)?

What a noob question but, I got nothin' to lose! 😎

 

[Sol Rosenberg]

Since this thread's about asking "stupid questions" (LoL!), here's one, that for some reason, has always confused me.

I mean, somehow I intuitively "know the answer" but I can't translate it into words, so here it goes:

Can someone remind me again how exactly blood is "stored" in the veins?

Aren't arteries and veins ultimately all connected, and so whatever is flowing in the arteries will pass through the veins (as long as the heart is pumping)?

So how can blood be "stored" in the veins? Does it have to do with things being connected in parallel? Does it have something to do with veins being compliant? Even so, veins are still tubes connected to arteries,,,,, so won't the blood flowing in the arteries "push" the blood that's in the veins (towards the heart)?

What a noob question but, I got nothin' to lose! 😎

I'm going to (hopefully) answer your question and, in a different way than I did before, re-answer the OP's question. Think of it this way:

You are in a park. The park has a pond (lake, whatever) with a relatively fast-moving stream flowing into it on one side and out of it on the other. The pond is MUCH wider than the streams such that even though the movement of water through the streams is fast and obvious, the pond looks like it is basically still.

The FLOW of water is set by the width of the STREAMS (assume some constant pressure driving water through the [closed] system.) The with/size of the POND has ALMOST NOTHING to do with the flow through the system. However, the pond holds a lot of water.

So, what you are missing is that during one cardiac cycle NOT ALL of your blood volume makes the round-trip through your blood vessels. If you haven't figured it out yet, the pond is your systemic venous system, the stream flowing into it is a crude approximation of your systemic ARTERIOLAR system (arteries contribute to resistance as well, but remember that the major resistance in the systemic circulation is the arterioles. In reality, there should be millions of tiny slow-moving streams flowing into the lake, but that just makes things more confusing) and the stream flowing out of it is, again, a crude approximation of your pulmonary circulation.

Now, what if I could somehow narrow the size of the pond (but this is still a closed system, so no water spills out.) The amount of water stored in the pond must decrease. This is how venous return is increased -- there is less blood stored int he venous system.

Make sense?

 

[Knicks]

I'm going to (hopefully) answer your question and, in a different way than I did before, re-answer the OP's question. Think of it this way:

You are in a park. The park has a pond (lake, whatever) with a relatively fast-moving stream flowing into it on one side and out of it on the other. The pond is MUCH wider than the streams such that even though the movement of water through the streams is fast and obvious, the pond looks like it is basically still.

The FLOW of water is set by the width of the STREAMS (assume some constant pressure driving water through the [closed] system.) The with/size of the POND has ALMOST NOTHING to do with the flow through the system. However, the pond holds a lot of water.

So, what you are missing is that during one cardiac cycle NOT ALL of your blood volume makes the round-trip through your blood vessels. If you haven't figured it out yet, the pond is your systemic venous system, the stream flowing into it is a crude approximation of your systemic ARTERIOLAR system (arteries contribute to resistance as well, but remember that the major resistance in the systemic circulation is the arterioles. In reality, there should be millions of tiny slow-moving streams flowing into the lake, but that just makes things more confusing) and the stream flowing out of it is, again, a crude approximation of your pulmonary circulation.

Now, what if I could somehow narrow the size of the pond (but this is still a closed system, so no water spills out.) The amount of water stored in the pond must decrease. This is how venous return is increased -- there is less blood stored int he venous system.

Make sense?

*slowly clapping*

 

[Downbytheicu]

Everyone danced around it, and the topic may be a dead horse, but just to satisfy my need for completeness...

In addition to things already mentioned, constricting the veins increases venous pressure creating a larger pressure differential between the vena cavas and the right heart. Increased pressure differential = increased flow. So, by changing resistance, we changed pressure. By changing the pressure (relative to the right heart), we change flow (venous return).

For an excellent cardio book, check out Lilly's Pathophys of Cardiology.

 

[Sol Rosenberg]

Everyone danced around it, and the topic may be a dead horse, but just to satisfy my need for completeness...

In addition to things already mentioned, constricting the veins increases venous pressure creating a larger pressure differential between the vena cavas and the right heart. Increased pressure differential = increased flow. So, by changing resistance, we changed pressure. By changing the pressure (relative to the right heart), we change flow (venous return).

For an excellent cardio book, check out Lilly's Pathophys of Cardiology.

Someone else posted that, and I danced around it because it is not correct.

Think about it for a second: You are saying that increased resistance leads to increased flow.

Flow = Pressure / Resistance

Therefore, increasing resistance will DECREASE flow for a constant pressure. It is true that for a constant FLOW, increasing the resistance will increase the pressure gradient across that resistance, but that isn't really relevant here.

Also, like I mentioned in a previous post -- by the time the blood gets to the venous system, it has lost most of its energy, so changes in the venous resistance cause an almost negligible change in flow (but that change is an almost negligible DECREASE in flow for constriction of the venous system.)

This is all true for an UNCOMPENSATED change in venous resistance. Ultimately, as I mentioned in my last post, the increased venous return increases preload, which increases contractility, which increases the pressure that the ventricle generates. This will cause an overall increase in flow, but not for the reasons that you state above.

 

[greatnt249]

I'm going to (hopefully) answer your question and, in a different way than I did before, re-answer the OP's question. Think of it this way:

You are in a park. The park has a pond (lake, whatever) with a relatively fast-moving stream flowing into it on one side and out of it on the other. The pond is MUCH wider than the streams such that even though the movement of water through the streams is fast and obvious, the pond looks like it is basically still.

The FLOW of water is set by the width of the STREAMS (assume some constant pressure driving water through the [closed] system.) The with/size of the POND has ALMOST NOTHING to do with the flow through the system. However, the pond holds a lot of water.

So, what you are missing is that during one cardiac cycle NOT ALL of your blood volume makes the round-trip through your blood vessels. If you haven't figured it out yet, the pond is your systemic venous system, the stream flowing into it is a crude approximation of your systemic ARTERIOLAR system (arteries contribute to resistance as well, but remember that the major resistance in the systemic circulation is the arterioles. In reality, there should be millions of tiny slow-moving streams flowing into the lake, but that just makes things more confusing) and the stream flowing out of it is, again, a crude approximation of your pulmonary circulation.

Now, what if I could somehow narrow the size of the pond (but this is still a closed system, so no water spills out.) The amount of water stored in the pond must decrease. This is how venous return is increased -- there is less blood stored int he venous system.

Make sense?

Dam up the back end of the pond (valves) while "squeezing" the dimensions of the pond together and you have yourself a pretty good analogy.👍