Cardiology question

[equilibr8]

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I have a question about arterial pressure.

It says in BRS physiology, 4th ed that "The most important determinant of pulse pressure is stroke volume. As blood is ejected from the left ventricle into the arterial system, arterial pressure increases because of the relatively low capacitance of the arteries." (p.73)

However, on the previous page, the book talks about how pressure DECREASES as you go from the arteries to the arterioles to the capillaries all the way to the venules and veins and finally the vena cava.

Can someone clear up my confusion as to whether arterial pressure increases or decreases, and under what conditions?



Also, 1) why does diastolic pressure remain unchanged during ventricular systole and 2) why isn't MAP simply the average of systolic and diastolic pressures?

Thanks. These are probably very simple questions but I'd like to get clarification.

 

[septoplasty]

I have a question about arterial pressure.

It says in BRS physiology, 4th ed that "The most important determinant of pulse pressure is stroke volume. As blood is ejected from the left ventricle into the arterial system, arterial pressure increases because of the relatively low capacitance of the arteries." (p.73)

However, on the previous page, the book talks about how pressure DECREASES as you go from the arteries to the arterioles to the capillaries all the way to the venules and veins and finally the vena cava.

Can someone clear up my confusion as to whether arterial pressure increases or decreases, and under what conditions?



Also, 1) why does diastolic pressure remain unchanged during ventricular systole and 2) why isn't MAP simply the average of systolic and diastolic pressures?

Thanks. These are probably very simple questions but I'd like to get clarification.

I think what you're missing is as blood progresses through arteries -> arterioles it ends up in the capillary bed where compliance is increased so you will see the drop in BP.

Diastolic is proportional to peripheral resistance (veins), if you constrict veins it will increase resistance = inc. return to heart = inc. diastolic pressure. So when you're pumping, blood needs to travel through the whole system (as stated above), so it wont inc. diastolic pressure.

and MAP you are taking into account CO x SVR, not your systolic / diastolic means (misnomer)?

 

[nitpick]

I think what you're missing is as blood progresses through arteries -> arterioles it ends up in the capillary bed where compliance is increased so you will see the drop in BP.

Diastolic is proportional to peripheral resistance (veins), if you constrict veins it will increase resistance = inc. return to heart = inc. diastolic pressure. So when you're pumping, blood needs to travel through the whole system (as stated above), so it wont inc. diastolic pressure.

and MAP you are taking into account CO x SV, not your systolic / diastolic means (misnomer)?

That's totally wrong thing you got. MAP = CO x TPR and CO=HRxSV

 

[atengnr]

I have a question about arterial pressure.

It says in BRS physiology, 4th ed that "The most important determinant of pulse pressure is stroke volume. As blood is ejected from the left ventricle into the arterial system, arterial pressure increases because of the relatively low capacitance of the arteries." (p.73)

However, on the previous page, the book talks about how pressure DECREASES as you go from the arteries to the arterioles to the capillaries all the way to the venules and veins and finally the vena cava.

Can someone clear up my confusion as to whether arterial pressure increases or decreases, and under what conditions?



Also, 1) why does diastolic pressure remain unchanged during ventricular systole and 2) why isn't MAP simply the average of systolic and diastolic pressures?

Thanks. These are probably very simple questions but I'd like to get clarification.

OK, the general principle in fluid mechanics is that friction caused by interaction b/w fluid and wall of tube reduces pressure. So a pressure that begins at say 120 will decrease as one travels away from the pressure source (the pump).

In systole, a volume of blood is ejected into the arterial tree causing a pulsatile increase in pressure. The pressure increases because the aorta/arterial vasculature is stretched by the volume causing an increased pressure (think of adding air to a balloon)

RE diastole (pressure when ventricles are not contracting), I think that this is heavily driven by vascular tone

MAP wouldnt be the average of the two due to the shape of pulse wave - its not a square wave, its more of a triangular shaped pulse.

 

[equilibr8]

OK, the general principle in fluid mechanics is that friction caused by interaction b/w fluid and wall of tube reduces pressure. So a pressure that begins at say 120 will decrease as one travels away from the pressure source (the pump).

In systole, a volume of blood is ejected into the arterial tree causing a pulsatile increase in pressure. The pressure increases because the aorta/arterial vasculature is stretched by the volume causing an increased pressure (think of adding air to a balloon)

RE diastole (pressure when ventricles are not contracting), I think that this is heavily driven by vascular tone

MAP wouldnt be the average of the two due to the shape of pulse wave - its not a square wave, its more of a triangular shaped pulse.

Thank you!! This is great.

 

[septoplasty]

That's totally wrong thing you got. MAP = CO x TPR and CO=HRxSV

Yes, sorry i meant

MAP = CO x SVR

 

[richse]

I think what you're missing is as blood progresses through arteries -> arterioles it ends up in the capillary bed where compliance is increased so you will see the drop in BP.

Diastolic is proportional to peripheral resistance (veins), if you constrict veins it will increase resistance = inc. return to heart = inc. diastolic pressure. So when you're pumping, blood needs to travel through the whole system (as stated above), so it wont inc. diastolic pressure.

and MAP you are taking into account CO x SVR, not your systolic / diastolic means (misnomer)?

I think you are confusing compliance with conductance. Capillaries are not more compliant than arteries (hint: elastic makes things very compliant). However, there is a higher conductance in a capillary network because there are many vessels in parallel despite a higher resistance in each individual capillary. The reason that pressure is less in the capillaries is because of the resistance exerted on blood flow as it travels through the vessels from large to small. If you think about it as a circuit then it makes sense that a going across a resister (vessel) causes a voltage drop (pressure).

 

[Aclamity]

OK, the general principle in fluid mechanics is that friction caused by interaction b/w fluid and wall of tube reduces pressure. So a pressure that begins at say 120 will decrease as one travels away from the pressure source (the pump).

In systole, a volume of blood is ejected into the arterial tree causing a pulsatile increase in pressure. The pressure increases because the aorta/arterial vasculature is stretched by the volume causing an increased pressure (think of adding air to a balloon)

RE diastole (pressure when ventricles are not contracting), I think that this is heavily driven by vascular tone

MAP wouldnt be the average of the two due to the shape of pulse wave - its not a square wave, its more of a triangular shaped pulse.

This post is right on the money. OP is totally correct that friction/conservation of energy/etc cause the pressure to drop as blood goes from arteries-->capillaries. HOWEVER, during systole--for a brief moment--the PULSATILE pressure of the contracting ventricle causes the aortic (arterial) pressure to increase because of the bolus of blood

 

[equilibr8]

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