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Stroke volume, Total peripheral resistance and Cardiac output

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coolkid17

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Hey guys so I'm slightly confused about these 3 things.

From what I understand,

Blood pressure= Cardiac output x Total peripheral resistance

Cardiac output= Stroke volume * Heart rate

Stroke volume depends on 3 things- preload, contractility, and afterload.

When baroreceptors detect a low blood pressure, sympathetic stimulation causes vasoconstriction. This increases total peripheral resistance and afterload. Thereby, it increases blood pressure, but at the same time decreases stroke volume, thereby decreasing cardiac output. Isn't this contradictory?

Also, I'm having trouble understanding the distinction between blood pressure and velocity of flow. If resistance decreases then surely the blood must lose some of its kinetic energy as it has to overcome more resistance- so the velocity of the blood flow should decrease (?), thus exerting a greater pressure on the walls of the arteries. But then don't the tissues get less blood supply? So isn't this mechanism counterintuitive?

And lastly, when we look for a measure of tissue perfusion, do we look at blood pressure or velocity of blood flow? What is the difference between the two?

I appreciate any help you can give me!! Thank you!
 

Nans

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When baroreceptors detect a low blood pressure, sympathetic stimulation causes vasoconstriction. This increases total peripheral resistance and afterload. Thereby, it increases blood pressure, but at the same time decreases stroke volume, thereby decreasing cardiac output. Isn't this contradictory?

You are correct, but you are missing one key piece: when the sympathetic kicks in, the decrease in stroke volume is due to the increase in heart rate. The rise in heart rate will decrease the diastolic filling time, thus there is less blood in the ventricles when it contracts, which results in a decrease in stroke volume. Thus, with a rise in HR and decrease in SV, I believe the overall cardiac output remains the same.

I don't think I know enough to comment on the other 2 questions.
 

ugm12

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Also, I'm having trouble understanding the distinction between blood pressure and velocity of flow. If resistance decreases then surely the blood must lose some of its kinetic energy as it has to overcome more resistance- so the velocity of the blood flow should decrease (?), thus exerting a greater pressure on the walls of the arteries. But then don't the tissues get less blood supply? So isn't this mechanism counterintuitive?

And lastly, when we look for a measure of tissue perfusion, do we look at blood pressure or velocity of blood flow? What is the difference between the two?

I'm not sure what you mean by "if resistance decreases", and I've also not heard blood flow described in terms of velocity before (this could be due to my UK training) however:

This link will tell you everything you need to know about blood flow. Of most importance to blood flow is the Poiseuille equation (see link) which basically states that blood flow increases 4-fold as the radius of the tube (or vessel) decreases (like in vasoconstriction).

To answer your final question: it is much more useful to assess tissue perfusion clinically. In critically ill patients "tissue perfusion" is assessed by monitoring end-organ perfusion. For example: urine output. Reduced tissue perfusion = no urine in catheter bag.
Other clinical signs of reduced tissue perfusion are cold and clammy peripheries, reduced capillary refill time and reduced consciousness. Non-invasive methods of monitoring must come first!

Source: final year medical student England, UK
 

Instatewaiter

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Hey guys so I'm slightly confused about these 3 things.

From what I understand,

Blood pressure= Cardiac output x Total peripheral resistance

Cardiac output= Stroke volume * Heart rate

Stroke volume depends on 3 things- preload, contractility, and afterload.

When baroreceptors detect a low blood pressure, sympathetic stimulation causes vasoconstriction. This increases total peripheral resistance and afterload. Thereby, it increases blood pressure, but at the same time decreases stroke volume, thereby decreasing cardiac output. Isn't this contradictory?

Also, I'm having trouble understanding the distinction between blood pressure and velocity of flow. If resistance decreases then surely the blood must lose some of its kinetic energy as it has to overcome more resistance- so the velocity of the blood flow should decrease (?), thus exerting a greater pressure on the walls of the arteries. But then don't the tissues get less blood supply? So isn't this mechanism counterintuitive?

And lastly, when we look for a measure of tissue perfusion, do we look at blood pressure or velocity of blood flow? What is the difference between the two?

I appreciate any help you can give me!! Thank you!

When sympathetic tone increases, you increase contractility, increase chronotropy (heart rate) and increase vasoconstriction. So while the TPR increases, your stroke volume and heart rate increase (just as they do in exercise for the same reason).

The assumption is that flow in the system is constant. So increasing resistance will increase velocity. This is the same reason that holding your finger over a hose will shoot the water faster and farther.

Perfusion is measured by flow (L/min)- so neither blood pressure nor velocity.
 

stevenejohnson

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Also keep in mind that if velocity is increased to a certain extent in the capillary beds this will negatively impact the tissues ability to extract all of the available oxygen.
 
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