Cardiac output vs Blood flow and effect of vasodilation on both??

[jmiz]

Hi,

What is the main difference between cardiac output and blood flow? This will be important because according to one article, vasodilation not only increases internal diameter of acting vessels but also the blood flow. If blood flow = cardiac output then I am no longer able to see how vasodilation will correspond to a decrease in blood pressure: Pressure = cardiac output * resistance. If resistance goes down, while cardiac output stays constant then it is evident pressure will decrease. However, if cardiac output is the same as blood volume and blood volume increases with vasodilation, then wouldn't the effect on pressure be dependent on the magnitude of change between cardiac output and resistance?

Here is the source that stated vasodilation will increase blood flow, and decrease TPR:http://omicsonline.org/effects-of-v...cardiac-output-2155-9880.1000170.php?aid=3452

Thanks!

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[EParker37]

Hi,

What is the main difference between cardiac output and blood flow? This will be important because according to one article, vasodilation not only increases internal diameter of acting vessels but also the blood flow. If blood flow = cardiac output then I am no longer able to see how vasodilation will correspond to a decrease in blood pressure: Pressure = cardiac output * resistance. If resistance goes down, while cardiac output stays constant then it is evident pressure will decrease. However, if cardiac output is the same as blood volume and blood volume increases with vasodilation, then wouldn't the effect on pressure be dependent on the magnitude of change between cardiac output and resistance?

Here is the source that stated vasodilation will increase blood flow, and decrease TPR:http://omicsonline.org/effects-of-v...cardiac-output-2155-9880.1000170.php?aid=3452

Thanks!

Couple of quick points, Cardiac Output is defined as Heart Rate X Stroke Volume, in other words, the amount of blood pumped out per unit time. Blood volume actually Decreases when there is vasodilation, think of the vascular system as a giant cup, I have a relatively defined amount of fluid inside this cup, and suddenly I make the cup bigger, my volume has gone down and there is also a corresponding decrease in pressure. But the CO and blood volume are not the same thing. Thus the article is relying on the idea of reduced resistance leads to higher flow, it isn't really explained well in that text though.

 

[StarFall]

Couple of quick points, Cardiac Output is defined as Heart Rate X Stroke Volume, in other words, the amount of blood pumped out per unit time. Blood volume actually Decreases when there is vasodilation, think of the vascular system as a giant cup, I have a relatively defined amount of fluid inside this cup, and suddenly I make the cup bigger, my volume has gone down and there is also a corresponding decrease in pressure. But the CO and blood volume are not the same thing. Thus the article is relying on the idea of reduced resistance leads to higher flow, it isn't really explained well in that text though.

Wouldn't blood volume increase, not decrease? When you increase the radius, there is less resistance so more blood can flow through = more blood volume.

 

[RH8448]

Correct me if I'm wrong: Looking at Poiseuille's Equation we see that increasing the diameter reduces resistance, increasing blood flow. So, organs involved in digestion would have their vessels dilated to allow more blood to be received by those functioning organs. I'm not familiar with the term "blood volume." But I would imagine blood volume would be greater in those tissues? Blood volume I would think would refer to the whole circulatory system which doesn't change. Does the continuity equation have anything to do with this?

 

[EParker37]

Wouldn't blood volume increase, not decrease? When you increase the radius, there is less resistance so more blood can flow through = more blood volume.

Volume does not have anything to do with the flow. Blood volume merely refers to the total amount of plasma + other things that constitute our blood. But if we take the context of volume being a number with respect to the total quantity of space within a container, IE a 2 cup measuring cup would have a volume of ~ 2 cups, then we would imagine that if the actual amount of blood remains constant, but the container suddenly gets larger, that the ratio of occupied vs unoccupied space would get larger, equating to a "smaller" blood volume, or decrease.

A real life example of this would be hypovolemia due to head trauma. If smooth muscle contraction suddenly stops due to trauma, and the vascular system relaxes, the amount of blood present isn't going to be able to fill the container anymore, dropping the blood pressure and causing shock. The actual quantity of blood didn't change, but the amount with respect to total container size did change.

 

[StarFall]

Volume does not have anything to do with the flow. Blood volume merely refers to the total amount of plasma + other things that constitute our blood. But if we take the context of volume being a number with respect to the total quantity of space within a container, IE a 2 cup measuring cup would have a volume of ~ 2 cups, then we would imagine that if the actual amount of blood remains constant, but the container suddenly gets larger, that the ratio of occupied vs unoccupied space would get larger, equating to a "smaller" blood volume, or decrease.

A real life example of this would be hypovolemia due to head trauma. If smooth muscle contraction suddenly stops due to trauma, and the vascular system relaxes, the amount of blood present isn't going to be able to fill the container anymore, dropping the blood pressure and causing shock. The actual quantity of blood didn't change, but the amount with respect to total container size did change.

Thanks for the response. I think I confused the two terms of blood volume and blood flow since I don't think I've seen "blood volume" anywhere.. So vasodilating would increase blood flow to that region, decreasing blood pressure, as you stated above.
But now that I'm thinking about it, when you exercise, your blood vessels in your muscle dilate, allowing for a greater blood flow. So wouldn't blood volume over, say 5 minutes, in your muscles be greater than the blood volume over the same amount of time in blood vessels in say, the stomach? The volume/time has increased from vasodilation.
Am I just over-complicating things?

 

[EParker37]

Thanks for the response. I think I confused the two terms of blood volume and blood flow since I don't think I've seen "blood volume" anywhere.. So vasodilating would increase blood flow to that region, decreasing blood pressure, as you stated above.
But now that I'm thinking about it, when you exercise, your blood vessels in your muscle dilate, allowing for a greater blood flow. So wouldn't blood volume over, say 5 minutes, in your muscles be greater than the blood volume over the same amount of time in blood vessels in say, the stomach? The volume/time has increased from vasodilation.
Am I just over-complicating things?

I think that's an over-complication, it isn't really referring to a localization of volume, that is defined by the entire vascular system, not just a small segment. Flow definitely increases to muscles, allowing for more fuel and oxygen to arrive at the muscle, but the blood isn't pooling there, merely arriving and leaving at a greater rate.

 

[Altius Premier Tutor]

Pressure differences (analogous to voltage across a circuit) is key to conceptualizing both macroscopic and microscopic vascular flow rate (analogous to current in a circuit). Imagine a giant square water tank as representing your entire circulatory system. If you plug a spigot into the side of the tank it will be the distance from the surface down to your spigot (pressure diff) that will determine the flow rate out. Now imagine a tank TWICE as wide, but with water to the same height. Plug the spigot in at the same level and you get the same flow rate, even though volume increased dramatically. Locally, blood vessel dilation works off pressure differences as well. Dilated vessels in a muscle increase volume of the VESSEL without a concurrent increase in the fluid contents, resulting in a lower pressure. The pressure upstream didn't change because those vessels did not dilate...and shazam! - you've got high pressure upstream and low pressure in the muscles...blood hurries down its pressure gradient to the muscle!

This is a very important concept for MCAT-2015 because close to 100% of your fluid question must now be bio-related and so what do you think they are all going to be about?
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[jmiz]

Couple of quick points, Cardiac Output is defined as Heart Rate X Stroke Volume, in other words, the amount of blood pumped out per unit time. Blood volume actually Decreases when there is vasodilation, think of the vascular system as a giant cup, I have a relatively defined amount of fluid inside this cup, and suddenly I make the cup bigger, my volume has gone down and there is also a corresponding decrease in pressure. But the CO and blood volume are not the same thing. Thus the article is relying on the idea of reduced resistance leads to higher flow, it isn't really explained well in that text though.

I did further research, and here is my understanding:

To clarify, Stroke volume is the amount of blood pumped out by the heart to the rest of the body per time. I am presuming the stroke volume will not change despite any sort of vascular distortion (e.g constriction and dilation). If that is true, then cardiac output is INDEPENDENT of the flow rate. And increasing radius at a particular vessel will decrease resistance, and DECREASE pressure. As Altius pointed out, the decrease in pressure at that point will thus create a larger pressure differential, which will create a larger REGIONAL flow rate, between point A (a set point upstream) and point B (point where dilation occurred. This may seem to contradict the law of continuity equation, but I believe it does not once the difference in pressure and thus a difference in total energy is accommodated for.

Moreover, flow rate, which follows the law of continuity (A1v1= A2v2) once accomodating for the pressure gradient established throughout the body (highest aorta --> lowest inferior and superior vena cava), should be constant throughout the body.

 

[Labrat07]

Good answers. I learned quite a few things. Here's another source for futher understanding

http://www.cvphysiology.com/Blood Flow/BF004.htm
http://forums.studentdoctor.net/threads/blood-flow-pressure-bernoullis.526219/