CO=MAP/TPR

[AlexBest96]

If we decrease TPR (arteriolodilation) CO will increase (this is the statement of all books), so systolic pressure will increase, but diastolic highly decreased(low TPR) SO if we decrease TPR MAP also will decrease (blood goes to the periphery then returns via VENOUS RETURN and increased CO). SO what is the problem?(i mean blood still circulate at all tissues with low TPR). Can it be that some of the blood don't go to the circulation(flow) because it is trapped in the periphery? this is the problem or all the blood are still circulating and what's then?

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[natemerg]

What situation are you talking about (exercise, medication...)?

One thing to keep in mind is that diastolic pressure does not necessarily decrease.

For example, exercise will result in dilation of skeletal muscle arterioles. This shifts shifts venous function curve clockwise. It also causes more blood to enter the venous compartment. Heart rate and contractility increase to compensate for this (increased sympathetic input from brain), which increases Systolic BP and cardiac output. As you can see, there are multiple factors keeping the blood pumping, so it is possible that diastolic blood pressure does not decrease much, if at all (therefore, MAP increases, not decreases).

In a different situation, giving someone a selective arterio-dilator (Hydralazine, eg), would dilate the arterioles. As a response, the heart would increase in rate and inotropy. There is also less afterload, allowing the heart to pump more blood in a given period of time.

Acute hemorrhage would be a different story, since there is less blood in the tube to push through. This would decrease diastolic blood pressure.

CV Pharmacology | Vasodilator Drugs

 

[AlexBest96]

What situation are you talking about (exercise, medication...)?

One thing to keep in mind is that diastolic pressure does not necessarily decrease.

For example, exercise will result in dilation of skeletal muscle arterioles. This shifts shifts venous function curve clockwise. It also causes more blood to enter the venous compartment. Heart rate and contractility increase to compensate for this (increased sympathetic input from brain), which increases Systolic BP and cardiac output. As you can see, there are multiple factors keeping the blood pumping, so it is possible that diastolic blood pressure does not decrease much, if at all (therefore, MAP increases, not decreases).

In a different situation, giving someone a selective arterio-dilator (Hydralazine, eg), would dilate the arterioles. As a response, the heart would increase in rate and inotropy. There is also less afterload, allowing the heart to pump more blood in a given period of time.

Acute hemorrhage would be a different story, since there is less blood in the tube to push through. This would decrease diastolic blood pressure.

CV Pharmacology | Vasodilator Drugs

Why does Blood pressure important in low TPR? situation:septic shock (decrease TPR increase CO(but no so much, because of trapping some in veins??) decrease MAP)
Also about hydralazine: "Although cardiac output is increased, systemic vascular resistance is reduced relatively more causing arterial pressure to fall."(CV Pharmacology | Vasodilator Drugs[/QUOTE])

 

[ASM student]

I don't know if I was getting your question right but have you considered that plasma volume is limited and it can maintain optimum perfusion at the normal BP and TPR, decreasing TPR in vivo implies vasodilation which will increase the capacity of the blood vessels decreasing the flow within unless the plasma volume increase proportionately, try to blow a certain amount of air in a straw and put your hand at the other end and feel how fast the air comes out then bring a 2 times wider straw and blow the same amount of air with the same effort and feel the flow again.

 

[natemerg]

Right. To add to that... The interesting thing about septic shock is that extremities will be warm and pink, unlike hypovolemic shock where they will be pale and cool. The issue is that your peripheral arteries and veins are totally dilated, which decreases the pressure in your system. The post-capillary venules are so dilated that blood that gets to the organs flies though without having time to properly do gas exchange. I


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