Diastolic pressure vs. "mean systemic pressure"

[Ruban]

That's mean systemic pressure, not mean arterial pressure (2/3 diastolic + 1/3 systolic).

Costanzo makes a big deal out of mean systemic pressure, which is apparently the pressure in the vessels if the heart was stopped. But I thought this was diastolic pressure (with systolic pressure being the pressure when the heart contracts).

Anyone have a good explanation of mean systemic pressure?

Thanks!

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[azzarah]

If you are talking about mean systemic filling pressure that is the pressure in the vascular bed when flow throughout the vascular bed is zero....Here is what my class notes say about it. HTH.

"[Consider] the case when the heart is not pumping at all. Under this condition, there is enough blod volume in the entire system to stress the vascular walls of both the arterial and venous capacitances (a volume at a given pressure) and create a static pressure in both the arterial and venous capacitances. These two pressures are equal to one another since there is no flow within the systemic vascular system. Therefore, the pressure gradient--arterial pressure minus venous pressure--must be zero. This static pressure first drew Starling's attention, then that of Starr and Rawson. It was finally measured by Guyton to be approximately 7 mmHg. This static pressure depends on the total compliance of the arterial and venous beds and the total blood volume within them. This baseline static pressure has been defined by Guyton as the mean systemic filling pressure. It is the pressure in the vascular bed when flow throughout the vascular bed is zeor."

 

[Idiopathic]

Mean systemic pressure is the pressure that would force blood back into the right atrium if the heart quit beating, it is usually fairly low, relatively.

 

[Ruban]

Cool, thanks!

 

[bigfrank]

Know the formula and you'll be fine. No question will force you to distinguish between the two.

 

[holabuster]

Hey, guys, I'm going crazy trying to understand part of the mean systemic pressure concept.

In First Aid, it says that when total peripheral resistance increases, the venous return curve slope decreases but there is NO change in mean systemic pressure. However, this does not make sense to me. If mean systemic pressure is blood volume relative to vessel capacity, an increase in TPR would leave blood volume unchanged and decrease vessel capacity, resulting in an INCREASE in mean systemic pressure.

I tried to look up different resources and came upon this guide:
http://www.ursa.atsu.edu/Department/LectureNotes/COVRTut.htm

This indeed corroborates my logic, under "Two things will change mean systemic filling pressure:":
"An increase in sympathetic tone constricts the veins, increases mean systemic pressure, and moves the hinge to the right."

Anyone have insights into this? Is First Aid wrong on this?

 

[seminoma]

FA is correct - when TPR changes there is a compensatory rotation of the venous return curve and a shift (up or down) in the cardiac function curve. The mean systemic pressure (MSP) is the "hinge" point of the venous return curve.

What you quoted from that website doesn't corroborate with your logic. MSP changes when blood volume changes and/or when the distribution of blood changes. As you correctly pointed out, changes in TPR do not change the blood volume. The only other option now is a change in the distribution of blood, which TPR also does not control. Why not? Recall that TPR is mainly determined by the arterioles and that most of the blood volume is held in the veins.

TPR represents the slope of the venous return curve. This makes sense because an increase in venous return results in a proportional increase in preload --> LVEDP --> SV --> CO --> TPR. A change in TPR can only change the slope of the curve (and not the x-intercept of the curve).

 

[holabuster]

Seminoma, thank you so much for your explanation. I think I get it now. MSP = blood volume/vascular container size. And since the venous system is what controls the vascular container size, an increase in TPR mediated by arteriolar constriction will not change the vascular container size. I so appreciate your help!!!

 

[Onigiri]

Hello Seminoma,

I was wondering why the distribution would not be changed when arterioles are constricted?
"Recall that TPR is mainly determined by the arterioles and that most of the blood volume is held in the veins." -Seminoma
Even though the veins hold more blood, there might be a small change through arteriole constriction that causes even more blood to be held by the veins and not so much in the arteriole side, therefore there would be less stressed volume and causing the MSP to decrease?

Please, can someone help me understand this issue?

Thank you.