TBR BIO Ch. 7 pass 3... Circulatory

[EnginrTheFuture]

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Does anyone have any clue as to what these graphs are stating? Their explanations to these questions are atrocious and make no sense to me.


How is it that while cardiac output is increased, venous pressure increases....

yet if venous return is increased, venous pressure decreases?!?!

Yes, I am switching the independent variable/dependent variable... but they have the same relation whether you look at the graph as is or turned 90 degrees.


Are venous return and cardiac output not the exact same thing? Aren't these two x and y axes the exact same thing? How can the venous return not equal the cardiac output without creating a vacuum in the heart and killing the person immediately 😱


This entire passage is just wooshing right over my head... what the hell is going on here? There is something about circulatory that just doesn't click in my brain... I feel there has to be some pointers on how to look at this subject more intuitively. Any insights?

 

[MedPR]

If you increase cardiac output, you are introducing more blood into the system which has a relatively fixed volume, thus leading to greater pressure.

If venous return increases, less blood is in the venous system, thus decreasing the pressure. Blood has to go into the arteries before it goes into the veins.

 

[SaCkO]

Does anyone have any clue as to what these graphs are stating? Their explanations to these questions are atrocious and make no sense to me.


How is it that while cardiac output is increased, venous pressure increases....

yet if venous return is increased, venous pressure decreases?!?!

Yes, I am switching the independent variable/dependent variable... but they have the same relation whether you look at the graph as is or turned 90 degrees.


Are venous return and cardiac output not the exact same thing? Aren't these two x and y axes the exact same thing? How can the venous return not equal the cardiac output without creating a vacuum in the heart and killing the person immediately 😱


This entire passage is just wooshing right over my head... what the hell is going on here? There is something about circulatory that just doesn't click in my brain... I feel there has to be some pointers on how to look at this subject more intuitively. Any insights?

Yes that passage complicated things especially when you dont have a big picture
Try this link it should explain everything.. I loved it
Remember that circulatory system is a loop in the end and their is alot of factors affecting each other

Try this link I promise you will like it
http://www.biosbcc.net/doohan/sample/htm/COandMAPhtm.htm

 

[EnginrTheFuture]

Yes that passage complicated things especially when you dont have a big picture
Try this link it should explain everything.. I loved it
Remember that circulatory system is a loop in the end and their is alot of factors affecting each other

Try this link I promise you will like it
http://www.biosbcc.net/doohan/sample/htm/COandMAPhtm.htm

That is awesome... this link should be sticky'ed to the top of the forum!


Unfortunately this graphs still don't make sense after reading the info.

Everywhere I read says venous return must equal cardiac output.... so how is it that these can be considered two separate entities in these two graphs?

What's nice is that didn't realize that the venous return is a great controller of cardiac output. That the heart simply pumps everything it receives, therefore the more it receives the more it pumps out. This increases arterial pressure and decreases venous pressure.

The second figure doesn't agree with this concept, however. As cardiac output increases, the venous pressure magically increases? Contrast this with venous return... which should be the same relationship as cardiac output and there is a lot of contradiction going on. Isn't the venous return having to pull more blood into the ventricles to increase cardiac output? Wouldn't this decrease the venous pressure?!

 

[EnginrTheFuture]

If you increase cardiac output, you are introducing more blood into the system which has a relatively fixed volume, thus leading to greater pressure.

If venous return increases, less blood is in the venous system, thus decreasing the pressure. Blood has to go into the arteries before it goes into the veins.

Yes, increasing the blood volume in the arteries will no doubt increase arterial pressure, but that blood had to come from the venous system. Pulling that blood into the heart would undoubtedly DECREASE the venous pressure and increase the arterial pressure.... but that is not what the plot states. It states that as cardiac output increases... venous pressure increases.

In how I am viewing things, increased cardiac output is equivalent to increased venous return. You can't have one without the other yet they are somehow trying to do that AND say they have different outcomes. How are they increasing cardiac output and leaving venous return constant?

 

[SaCkO]

Yes, increasing the blood volume in the arteries will no doubt increase arterial pressure, but that blood had to come from the venous system. Pulling that blood into the heart would undoubtedly DECREASE the venous pressure and increase the arterial pressure.... but that is not what the plot states. It states that as cardiac output increases... venous pressure increases.

In how I am viewing things, increased cardiac output is equivalent to increased venous return. You can't have one without the other yet they are somehow trying to do that AND say they have different outcomes. How are they increasing cardiac output and leaving venous return constant?

I see how your thinking and its right .. but if you check that curve you would see that the graph starts at a slope close to zero and this is in a normal person where the heart is pumping.. However what they did is that they stopped the heart from pumping and thus CO becomes zero.. Which means there is no pumping anymore and that the blood will not return any more to the heart thus venous return is dropping down to zero.
The reason we see pressure increasing the veins while that is happening is because fluids move from high to low pressure in an effort to create equilibrium and this is what happens when the blood goes from arteries (high pressure) to the veins (low pressure).. this will create an equilibrium pressure called MCP and it is shown in the graph. At MCP fluid will stop flowing either from veins or from the arteries because it has reached an equilibrium. Just remember that we removed the heart and there is no pump anymore and that the arteries and veins are pipes where fluid moves from high to low pressure. Thus to Sum this up , because the veins have more blood in them after blood moved from the arteries and since we removed the heart and we don't have a pump anymore then venous pressure will be at maximum and we will have a zero venous return
Hope this helps.. It took me some reading too to get the right interpretation

 

[EnginrTheFuture]

Thanks, that helped. This passage really threw me for a loop but I feel I got a net gain from it.

 

[Youknowgo]

Bump, totally thrown off by this question too

So for question 19 are we supposed to consider the two graphs separately, Figure 1 representing a stopped heart while Figure 2 does not?

I still do not understand how for Figure 1 an increase in venous pressure decreases venous return (=cardiac output?) while for Figure 2 an increase in venous pressure increases cardiac output....?

 

[Singhp03]

stumped by this passage too, the explanation made sense though(eventually). If more blood is introduced in to circulation, there must be more blood in the veins and thus would increase venous pressure. The same thing would happen in the case explained by the tbr book, stopping the heart--> leading to increased venous pressure. This is how i understand it anyway.

 

[99dmg]

I was confused on the graphs too and after a bit of searching I found that the curves are called Guyton's curves.

http://forums.studentdoctor.net/showthread.php?t=1007507

The venous pressure is actually the right atrial pressure, which makes sense when you think of it like that.

Graph 1: As the right atrial pressure increases the venous return will decrease due to a decrease in pressure difference between the veins and the atrium (until pressure difference is 0 = equilibrium at 8mmHg)

Graph 2: As the right atrial pressure increases more blood can be pumped into the ventricle thus increasing the cardiac output.

I think it can also be thought of as: as more blood fills the right atrium pressure increases and less blood can enter (venous return) due to the atrium filling up (Graph 1). And if there is more blood in the atrium (higher venous pressure) more can be pumped into the ventricles resulting in more blood being pumped into the body (higher cardiac output)

Let me know if this sounds right. Sorry for reviving an old topic