Blood pressure question

[Tokspor]

Advertisement - Members don't see this ad

I've been doing some problems about renal circulation and the excretory system and have thought up a few questions about how blood pressure works. Maybe you can help me answer them?

1. If a blood vessel gets constricted, say the renal artery, this mean that the blood pressure at that vessel gets increased right?
2. According to A1v1=A2V2 from physics, a constricted area gives rise to greater velocity. Is it safe to think of higher blood pressure as corresponding to greater velocity?
2. When an artery gets constricted and the blood flow rate increases, what does that say about the flow and pressure of the venule at the other end? What about if the artery is dilated (low blood pressure)?

 

[Tokspor]

bump

 

[Charles_Carmichael]

I've been doing some problems about renal circulation and the excretory system and have thought up a few questions about how blood pressure works. Maybe you can help me answer them?

1. If a blood vessel gets constricted, say the renal artery, this mean that the blood pressure at that vessel gets increased right?
2. According to A1v1=A2V2 from physics, a constricted area gives rise to greater velocity. Is it safe to think of higher blood pressure as corresponding to greater velocity?
2. When an artery gets constricted and the blood flow rate increases, what does that say about the flow and pressure of the venule at the other end? What about if the artery is dilated (low blood pressure)?

The thing about increasing resistance in a blood vessel is that the pressure upstream of the site of constriction would experience increased pressure while the pressure downstream from the site of constriction will experience a decrease in pressure. This is because the increased resistance dissipates more energy and pressure can be thought of as potential energy. Since you mentioned renal physiology, think of it like this: If the afferent arteriole was constricted, the pressure downstream (at the glomerulus) would decrease and thus, GFR would decrease. On the other hand, if the efferent arteriole was constricted, the pressure upstream would increase and thus, GFR would increase. I like thinking about it this way and it makes sense, so hopefully it helps you understand that as well.

Regarding your second question, yes, narrowing of the blood vessel would increase the velocity of the blood flowing through it. Based on the equation dP = Q x R, where dP is delta pressure, Q is blood flow rate, and R is resistance, and substituting for Q as vA (based on the equation v = Q/A) where v is the velocity and A is the total cross-sectional area, you get dP = vAR. So, with greater pressure, a greater velocity or an increase in resistance is possible.

Regarding your last question, blood flow rate (or the cardiac output) is relatively constant at resting conditions at around 5 L/min. So, changes in blood vessel diameter would affect the resistance (obviously) and the change in pressure along the area of constriction while the blood flow rate remains relatively constant. The cardiac output depends on both the heart rate and the stroke volume (CO = HR x SV). So these two factors need to be affected before a change in blood flow rate is observed.

Hope this helps.

 

[thebillsfan]

The thing about increasing resistance in a blood vessel is that the pressure upstream of the site of constriction would experience increased pressure while the pressure downstream from the site of constriction will experience a decrease in pressure. This is because the increased resistance dissipates more energy and pressure can be thought of as potential energy. Since you mentioned renal physiology, think of it like this: If the afferent arteriole was constricted, the pressure downstream (at the glomerulus) would decrease and thus, GFR would decrease. On the other hand, if the efferent arteriole was constricted, the pressure upstream would increase and thus, GFR would increase. I like thinking about it this way and it makes sense, so hopefully it helps you understand that as well.

Regarding your second question, yes, narrowing of the blood vessel would increase the velocity of the blood flowing through it. Based on the equation dP = Q x R, where dP is delta pressure, Q is blood flow rate, and R is resistance, and substituting for Q as vA (based on the equation v = Q/A) where v is the velocity and A is the total cross-sectional area, you get dP = vAR. So, with greater pressure, a greater velocity or an increase in resistance is possible.

Regarding your last question, blood flow rate (or the cardiac output) is relatively constant at resting conditions at around 5 L/min. So, changes in blood vessel diameter would affect the resistance (obviously) and the change in pressure along the area of constriction while the blood flow rate remains relatively constant. The cardiac output depends on both the heart rate and the stroke volume (CO = HR x SV). So these two factors need to be affected before a change in blood flow rate is observed.

Hope this helps.

kaushik, regarding your answer to question 2: this breaks down at the capillary level, right? because there is certainly an increased resistance, but the increased resistance does not translate into an increase in pressure because the Q has gone down so much. and then, as blood moves into the capillaries, the pressure decreases even further, and the resistance correspondingly goes down but the flow rate goes up. wow I think that sounded really convoluted.

P=QR, but how can Q ever change because I thought the continuity equation made it constant?

 

[Charles_Carmichael]

kaushik, regarding your answer to question 2: this breaks down at the capillary level, right? because there is certainly an increased resistance, but the increased resistance does not translate into an increase in pressure because the Q has gone down so much. and then, as blood moves into the capillaries, the pressure decreases even further, and the resistance correspondingly goes down but the flow rate goes up. wow I think that sounded really convoluted.

P=QR, but how can Q ever change because I thought the continuity equation made it constant?

You're right that Q is relatively constant during resting conditions (the two factors that affect it are heart rate and stroke volume). However, the velocity can change:

v = Q/A

where v is the velocity, Q is the blood flow rate, and A is the cross-sectional area. In one particular blood vessel, if the area is reduced due to constriction or some sort of vasculopathy, etc., there would be an increase in velocity (the denominator in the equation decreases while blood flow remains constant). An example of this is a bruit, which is the turbulent sound blood makes as it rushes past some sort of obstruction in a blood vessel. Due to the narrowing of the blood vessel, the velocity increases, which increases Reynold's number (something that I highly doubt would be needed for the MCAT) above 2000 and causes turbulent flow (remember that blood flow is normally laminar, but I doubt you'll even need to know this for the MCAT), which results in the production of the sound.

As resistance decreases, the flow rate doesn't really increase. The change in pressure decreases since less pressure is dissipated along the length of the vessel. Capillary diameter is not regulated, so there shouldn't be any constriction/dilation in them. When blood first enters a capillary, it's hydrostatic pressure will be high (this amount varies depending on the ammount of pressure dissipated by arterioles prior to the blood entering the capillary) but the pressure will be dissipated along the length of the capillary until the blood pops out into the venule. During this time, the blood flow rate will not change; if it changes, it screws up the entire cardiovascular system. Think about it, whatever the heart pumps out per minute, it has to get back; if it gets back only part of what it's been pumping out, something has gone wrong (hemorrhage, etc.). So the flow rate throughout the CV system has to be the same, no matter if it's 5 L/min or 25 L/min. Otherwise, blood will just pool in the periphery or back up in the lungs (depending on whether the left heart or right heart fails). Hope this helps.

 

[TheMightyBoosh]

I have to resurrect this thread because I've been confused about this topic for a while now and I've done extensive searches on it.

So I've gathered from studying the cardiovascular system and reading Kaushik's explanation that increasing resistance (by changing the diameter of blood vessels) will result in changes in pressure (increased upstream of constricted vessel/decreased downstream of constricted vessels), while blood flow Q in systemic circulation remains constant. Thus, P = Q*R.

I get confused in situations like a body's response to a heart attack. Arterial pressure will decrease, baroreceptors will pick up the decrease in pressure and notify the central nervous system to produce a sympathetic response (constriction of arteries). But when I think of this with Kaushik's logic, I would assume that as a result of constriction of arteries and arterioles, the increased resistance would result in a pressure drop as the blood gets to the capillaries. Isn't this the opposite of what the body would want to occur in a situation where organs are getting less blood perfusion than normal? Or is this is a response that would cause overall systemic pressure to increase (including capillary pressure)?

 

[plzNOCarribbean]

Im so lost. someone help.

 

[TheMightyBoosh]

Im so lost. someone help.

Haha. I'll take that as my cue to clarify.

I understand that after a heart attack, the body attempts to compensate for a loss in arterial pressure by constricting those vessels. However, my thought is that by doing this, it should also cause a drop in pressure downstream of arteries/arterioles. Is my thinking correct? How would the body compensate for the "lack of blood flow to organs" problem?

 

[MT Headed]

If a particular arteriole gets constricted, the pressure will drop downstream of that arteriole. If the entire vascular system gets constricted, the overall systemic pressure will rise. You got it.

 

[TheMightyBoosh]

If a particular arteriole gets constricted, the pressure will drop downstream of that arteriole. If the entire vascular system gets constricted, the overall systemic pressure will rise. You got it.

Thanks