Please Help! Cardiac and Vascualr Fxn Curves

As constituted the graph makes no sense. The key to understanding them is that there are TWO graphs superposed on each other, and each one has its own different axis title. A properly labeled graph would have both "EDV" and "Right Atrial Pressure" on the x axis, which should hopefully make it a lot more clear that the curves are representing two different things.

Look at the two lines separately

The upward slope line = CO
If the slope increases, the contractility of the heart increases
If the slope decreases, the contractility decreases


The downward slope line = venous return
If the x-int increases, it means that the blood volume increased (aka mean systemic pressure increased)
If the x-int decreases, it means that the blood volume decreased (aka mean system pressure decreased)
If preload increases without a change in blood volume, then the slope will increase while the x-int will remain the same
If the preload decreases without a change in blood volume, then the slope will decrease

Now look at them together,
if the intercept point increases without a change in x-ints then there is a decrease in TPR resulting in an increase in cardiac output and venous return
If the intercept point decreases without a change in x-ints then there is an increase in TPR resulting in decreased cardiac output and venous return.

I think that is right 😳

Now look at them together,
if the intercept point increases without a change in x-ints then there is a decrease in TPR resulting in an increase in cardiac output and venous return
If the intercept point decreases without a change in x-ints then there is an increase in TPR resulting in decreased cardiac output and venous return.


My reasoning for decreased venous return due to increased TPR is that there is less blood being pumped into arterial side which translates into less blood that can go to venous side. Would you agree?

Blesbok said:

Look at the two lines separately

The upward slope line = CO
If the slope increases, the contractility of the heart increases
If the slope decreases, the contractility decreases


The downward slope line = venous return
If the x-int increases, it means that the blood volume increased (aka mean systemic pressure increased)
If the x-int decreases, it means that the blood volume decreased (aka mean system pressure decreased)
If preload increases without a change in blood volume, then the slope will increase while the x-int will remain the same
If the preload decreases without a change in blood volume, then the slope will decrease

Now look at them together,
if the intercept point increases without a change in x-ints then there is a decrease in TPR resulting in an increase in cardiac output and venous return
If the intercept point decreases without a change in x-ints then there is an increase in TPR resulting in decreased cardiac output and venous return.

I think that is right 😳

Click to expand...

I thought if you increase TPR, you increase venous return regardless of the x-intercept.

And if you decrease TPR, you decrease venous return (i.e. less blood is being pumped to the heart thus decreased return)

I followed you up until those last two points. Correct me if I'm wrong.

overfiend06 said:

I thought if you increase TPR, you increase venous return regardless of the x-intercept.

And if you decrease TPR, you decrease venous return (i.e. less blood is being pumped to the heart thus decreased return)

I followed you up until those last two points. Correct me if I'm wrong.

Click to expand...

yeah you make more sense than the other guy.. i think he/she has it backwards.


increasing tpr could happen during sympathetic stimulation for example and that's done to raise bp so that more blood can return to the heart to be oxygenated and pumped out to the tissues. decreasing tpr as in vasodilators will cause blood to pool in the viens and less blood to return to the heart and thus a lower BP. MAP=CO X TPR, and CO= SV X HR, the body tries to keep MAP constant so if TPR increases (for example volume overload, or vasoconstriction, like from exogenous sympathetic stimulation like pheochromocytoma for example) then CO will drop via a drop in HR (by vagus stimulation) to keep the map constant or as constant as possible.

To the OP the graph in your link is actually pretty well labeled. there are pretty much two graphs like everyone says, the on the right is heart muscle function ( the higher up the lines the higher the contractility or contraction force of the heart, this could be due to digoxin or sympathetic action, the lower the line the less contractility, most likely due to myocyte damage as in an MI or CHF) If you stay on that same line then the contractility doesn't change i.e. heart muscle function doesn't change, but only blood volume changes.

The lines on the left are venous return to the heart as in blood volume, it can increase due to fluid retention as in CHF or fluid overload as in too much IV fluid or it can get smaller as in bleeding or dehydration. thus if the volume increases that line will cross the x-axis at a greater number, if it decreases it will cross the x-axis at a smaller number, and that change in blood volume will result in either increase or decrease in right atrial pressure. if you stay on the same line but move the point up or down that means the blood volume doesn't change but the cardiac function i.e. cardiac muscle function changes.

The part where the two graphs cross if extrapolated to the y-axis will give you the cardiac output, if extrapolated to the x-axis will give you the EDV.

Capiche?

obiwan said:

Now look at them together,
if the intercept point increases without a change in x-ints then there is a decrease in TPR resulting in an increase in cardiac output and venous return
If the intercept point decreases without a change in x-ints then there is an increase in TPR resulting in decreased cardiac output and venous return.


My reasoning for decreased venous return due to increased TPR is that there is less blood being pumped into arterial side which translates into less blood that can go to venous side. Would you agree?

Click to expand...

Since it is a closed circuit that has to be the reason.

Just remember that arteriolar constriction always causes decreased blood flow to anything down stream. If all of the arterioles constrict then it is going to cause decreased blood flow down stream (aka the venous side)

overfiend06 said:

I thought if you increase TPR, you increase venous return regardless of the x-intercept.

And if you decrease TPR, you decrease venous return (i.e. less blood is being pumped to the heart thus decreased return)

I followed you up until those last two points. Correct me if I'm wrong.

Click to expand...

You are backwards. If you increase TPR, you increase arteriolar constriction, which decreases blood delivered to the venous side, which decreases blood delivered back to the heart. Also, you get reflex bradycardia which causes a drop in cardiac contractility and cardiac output

bt9099 said:

Thanks so much for the help guys...I really really appreciate it!

-Briana

Click to expand...

any time sweet thing.

Blesbok said:

You are backwards. If you increase TPR, you increase arteriolar constriction, which decreases blood delivered to the venous side, which decreases blood delivered back to the heart. Also, you get reflex bradycardia which causes a drop in cardiac contractility and cardiac output

Click to expand...

I stand corrected...

i understand that the venous return curve is shifted downwards but x intercept stays same for increase in TPR... however, why does the cardiac output curve shift down... BRS says that there is increased afterload but i thought that you would shift the actual curve up or down if there are changes in contractility...

See? Procrastinating on SDN is still some form of studying for boards.

.

WOW!! Thanks Guys! Russian, et all...Yal just explained this better than any of my teachers! I always have trouble with graphs! Thanks for taking the time!

why the mean systemic pressure is not changed with increased TPR?

pico

changes in Vascular fxn curve:
1.blood volume - hemorrhage, IV fluid overload..etc
2.compliance of veins - more compliant veins will hold a bunch of blood
3.tpr

only changes in 1 and 2 can give you changes in Mean systemic pressure.
remember msp is "theoretically derived" pressure in Right Atrium when there is NO FLOW what so ever, so no CO, no VR, blood flows from arteries to the veins until the pressure in the whole system is the same. You are looking at the blood in the WHOLE system indepenent of Cardiac function. Also note that the majority of the blood is held in your veins not the arteries.

Changes in Cardiac Output curve:
1. Changes in Inotropy ( drugs, MI..etc)
2. TPR

Keep it simple.

Ah. it makes sense. thx