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 05-06-2012, 09:30 PM #1 Senior Member   Join Date: May 2009 Posts: 297 Applying physics to blood pressure SDN Members don't see this ad. (About Ads) Hi all, Can anyone help explain to me exactly what dictates when there is high blood pressure in the blood vessels versus low? For example, is dilation going to increase or decrease blood pressure. Does increased blood flow cause a decrease in blood pressure? I have been trying to understand by applying the bernoulli and continuity equations, but I am not sure if I am doing it correctly all the time. For example, if the vessels were constricted I would predict that blood velocity (is this same thing has blood flow?) will increase (continuity equation), therefore pressure will decrease. Would this be correct? thanks in advance.
 05-06-2012, 11:12 PM #2 Senior Member   Status: Pre-Medical Join Date: Jun 2010 Posts: 219 Area and velocity are inversly related Q=AV , Q is constant. Now increasing one decreases the other & vice versa. For pressure:Higher v = lower pressure & vice versa. So combining both , this & above,we can say that pressure is greatest where area is greater & vice versa.
05-06-2012, 11:23 PM   #3
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Quote:
 Originally Posted by Aria88997 Hi all, Can anyone help explain to me exactly what dictates when there is high blood pressure in the blood vessels versus low? For example, is dilation going to increase or decrease blood pressure. Does increased blood flow cause a decrease in blood pressure? I have been trying to understand by applying the bernoulli and continuity equations, but I am not sure if I am doing it correctly all the time. For example, if the vessels were constricted I would predict that blood velocity (is this same thing has blood flow?) will increase (continuity equation), therefore pressure will decrease. Would this be correct? thanks in advance.
HR × SV ~= CO ~= MAP / TPR

Heart Rate x Stroke Volume ~= Cardiac Output ~= Mean Arterial Pressure / Total Peripheral Resistance

Resistance is proportional to 1/r^4

Dilatation of the resistance arterioles is going to increase r, which decreases resistance, which means MAP will decrease to maintain same CO (which is what your body cares about: delivery of nutrients and oxygen to tissues).

Increased blood flow is usually precipitated by an increase in SV or HR. An increase in SV is caused by an increase in end-diastolic filling or by an increase in sympathetic, inotropic activity, and leads to an increase MAP.

Yes, you can use Bernoulli's, or you can learn some quick and dirty cardiac physiology, which I think is way more fun (if less applicable for the MCAT).
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05-07-2012, 04:17 AM   #4
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Quote:
 Originally Posted by Morsetlis HR × SV ~= CO ~= MAP / TPR Heart Rate x Stroke Volume ~= Cardiac Output ~= Mean Arterial Pressure / Total Peripheral Resistance Resistance is proportional to 1/r^4 Dilatation of the resistance arterioles is going to increase r, which decreases resistance, which means MAP will decrease to maintain same CO (which is what your body cares about: delivery of nutrients and oxygen to tissues). Increased blood flow is usually precipitated by an increase in SV or HR. An increase in SV is caused by an increase in end-diastolic filling or by an increase in sympathetic, inotropic activity, and leads to an increase MAP. Yes, you can use Bernoulli's, or you can learn some quick and dirty cardiac physiology, which I think is way more fun (if less applicable for the MCAT).
I wouldn't be using bernoulli's with blood.... ever

Bernoulli's is for invicid flow only. If you decrease the radius of a vessel, yes the pressure at that point will decrease due to higher velocity IF the blood flow is the same. That is only true if you increase the pressure upstream, however, because when you restricted the flow you deceased the flow rate. This is the principle of invicid flow. It is very similar to circuits.... and not anything like ideal flow through pipes...

 05-07-2012, 05:38 AM #5 Member     Status: Pre-Medical Join Date: Apr 2010 Posts: 93 up A,down V,down Bp non-ideal
 05-07-2012, 06:17 AM #6 1K Member   Status: Pre-Health Join Date: Aug 2009 Posts: 1,420 Wait, I thought vasoconstriction (the narrowing of blood vessels, lowering A, right?) increased blood pressure?
05-07-2012, 06:34 AM   #7
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Quote:
 Originally Posted by SoulinNeed Wait, I thought vasoconstriction (the narrowing of blood vessels, lowering A, right?) increased blood pressure?
This is where my post comes in... look above.

In non-ideal flow such as with blood, in order to keep the same blood flow as before constriction you must increase the blood pressure upstream. Just like a circuit, if you add a highly resistive resistor, the current goes down as R goes up... if you want the same current across the resistor you must up the voltage (pressure in this case). Thus the voltage upstream from the resistor is higher... pressure before the restricted vessel is higher.

They are assuming that the blood flow before and after constriction is the same and that V x A is constant before and after the change. This can only occur when the BP before the restriction is increased upon restriction of the vessel (the heart is the thing accomplishing this and it is a response not an automatic consequence). I wouldn't go assuming that pressure is lower at the site of restriction as they do, because they are basing that on invicid flow concepts (bernoulli's). On the other hand you can most definitely know what you are saying... that pressure before restriction increases NOT decreases when you have vasoconstriction given the flow rate is the same before and after (heart decided to "increase it's voltage" or pressure output)

Invicid flow is a complicated matter and I urge you all to not use bernoulli's when you see the word blood.

 05-07-2012, 09:33 AM #8 1K Member   Status: Pre-Health Join Date: Aug 2009 Posts: 1,420 Right, vasoconstriction causes increased resistance, which leads to greater MAP. Blood isn't an ideal fluid, though, right? So, Bernoulli's equation wouldn't apply to it.
 05-08-2012, 10:47 AM #9 Senior Member   Join Date: May 2009 Posts: 297 So if you constrict a vessel then you are increasing the blood pressure before the constriction, correct? Then what happens the blood pressure after the constricted area? Also, is blood flow analogous to current while blood pressure is analogous to voltage?

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