# What is the difference between blood flow and cardiac output

according to this one source, cardiac output (vascular) = blood flow (http://legacy.owensboro.kctcs.edu/gcaplan/anat2/notes/APIINotes5 cardiac_equations.htm)

However, their equations do not match:

Cardiac output (CO)= Stroke volume * Heart Rate
Blood Flow = cross sec area of vessel times velocity

This is important because cardiac output is also in relationship with blood pressure:
change in bP = cardiac output times resistance of vessel

If blood flow equals cardiac output then the equation of blood pressure would also be impacted by blood flow.

The statement that started my curiosity was that one way of regulating bP is with vasoconstriction which leads to an increase in peripheral resistance and a lower BLOOD FLOW. Ultimately, this is a mechanism for bP to be increased, however if the lower blood flow also has an effect on blood pressure then it becomes more complicated. So, I want to make sure that blood pressure is not impacted by blood flow.

Also, the sympathetic nervous system leads to vasoconstriction of vessels and leads to an increase in bP. But, when you are exercising wouldn't vasoconstriction decrease the amount of blood flowing and thus decrease the oxygen supplying rate to muscles?

Thanks for your help.

## Answers and Replies

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I want to make sure that blood pressure is not impacted by blood flow.
The velocity subheading of the blood flow Wiki gives a few examples of areas where cross section and velocity are drastically different. You're probably aware that blood pressure is significantly lower in capillaries than in the aorta. However, multiply their given values and you'll see that flow in all 3 of those instances is more or less the same. Easy counterexample. Now, blood flow may have an effect of pressure, but as you can see, it's not the sole determinant.

As for constriction during exercise, not all of your vessels have the same response to epinephrine. Vessels in your core constrict, but peripheral vessels have a different response due to their respective balance of the different adrenergic receptors.[/quote]

Suraj M
Gold Member
wouldn't vasoconstriction decrease the amount of blood flowing and thus decrease the oxygen supplying rate to muscles?
That's not the only deciding factor for the O₂ reaching the muscles. Epinephrine also acts on your heart, increasing rate and stroke volume. hence ##\uparrow## in cardiac output ⇒ ##\uparrow## O₂ to muscles

The velocity subheading of the blood flow Wiki gives a few examples of areas where cross section and velocity are drastically different. You're probably aware that blood pressure is significantly lower in capillaries than in the aorta. However, multiply their given values and you'll see that flow in all 3 of those instances is more or less the same. Easy counterexample. Now, blood flow may have an effect of pressure, but as you can see, it's not the sole determinant.

As for constriction during exercise, not all of your vessels have the same response to epinephrine. Vessels in your core constrict, but peripheral vessels have a different response due to their respective balance of the different adrenergic receptors.
Thanks. I believe it is the loss of force via friction throughout the pathway from aorta to capillaries as well as the internal higher resistance of aorta that explain for the difference in bP between the capillaries (farther out) and the aorta.

Also accommodating for the example that you have provided, does that mean that blood flow and cardiac output are not the same?

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Vasoconstriction does not merely affect the arteries. Veins are the major blood reservoir. When they contract due to sympathetic activity, all the blood in the veins will be forced to return to the heart, so with the increase in venous return, there will be a subsequent increase in cardiac output.