Blood pressure/kidneys question??

[Ari1584]

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Okay so im a little confused on this topic and im trying to get it down cold...is this true:

1.) When there is an increase in blood pressure, there is an increase in GFR. This causes more flow of filtrate to flow through the tubules, and past the macula densa in the distal conv. tubule. The macula densa senses this, and in response causes the blood vessels in the proximal conv. tubule to constrict, reducing the diameter. This lowers the hydrostatic blood pressure which has a net effect to decrease GFR.

--Is this correct?? And if it is, i don't understand one thing. Vasoconstriction= increase in BP, so why would vasoconstriction here cause a DECREASE in BP?? I thought reducing the area of something causes an increase in pressure, so i dont know why it would cause a decrease in hydrostatic pressure here.

2.) When BP is extremely low, the sympathetic NS can increase BP by vasoconstriction of vessels which causes a decrease in GFR.

--Okay, this was taken from class notes that my professor sent me in his physiology class. I thought that a decrease in BP = decrease in GFR. Now i am just confused?? Help is needed 😳

 

[Airplay355]

Taken from my old APP notes....

Glomerular Filtration – fluid in plasma gets squeezed out into bowmans capsule (filtrate). Capillaries in the glomerular wall are fenestrated and thus 100x more permeable then most other capillaries in the body. Efferent arteriolar sphincter will create resistance to outflow of blood which will increase the hydrostatic pressure of the blood in capillaries. The pressure here is about 55 mm of HG, about 3 times more then it is in regular capillaries. Glomerular Filtration Rate (GFR) = 125ml/minute in an adult male, 115mL/minute in a female. Glomerular capillaries have a hydrostatic pressure of 55mmHg.

...so basically if your BP is too high you will need to if you INCREASE the hydrostatic pressure in the capillaries by INCREASING RESISTANCE TO BLOOD FLOW (with a sphincter, not complete vasoconstriction) more fluid will more out of the blood and into kidney tubules. Less fluid in the blood DECREASES blood pressure. This is done by constricting the efferent arteriolar sphincter and relaxing the afferent arteriolar sphincter.

If your BP is too low then the afferent sphincter constricts and the efferent sphincter relaxes. This DECREASES THE RESISTANCE TO BLOOD FLOW OVER THE GLOMERULUS, WHICH DECREASED HYDROSTATIC PRESURE IN CAPILLARIES AND DECREASES GFR. Fluid will remain in the blood stream and raise BP.

These sphincters are smooth muscle and respond to passive stretching by contracting. So when there's an increase in BP, they constrict and a decrease causes them to relax.

I hope this helps. If you need more help let me know. I hope your thread doesn't get closed on you because it's not an MCAT question lol!

 

[Ari1584]

thankx for explaining airplay...it is an MCAT question though...im trying to get these facts down bc ive seen lots of passages/question pertaining to this topic. I just referenced my physiology notes bc it had a more detailed explanation.

Taken from my old APP notes....

Glomerular Filtration – fluid in plasma gets squeezed out into bowmans capsule (filtrate). Capillaries in the glomerular wall are fenestrated and thus 100x more permeable then most other capillaries in the body. Efferent arteriolar sphincter will create resistance to outflow of blood which will increase the hydrostatic pressure of the blood in capillaries. The pressure here is about 55 mm of HG, about 3 times more then it is in regular capillaries. Glomerular Filtration Rate (GFR) = 125ml/minute in an adult male, 115mL/minute in a female. Glomerular capillaries have a hydrostatic pressure of 55mmHg.

...so basically if your BP is too high you will need to if you INCREASE the hydrostatic pressure in the capillaries by INCREASING RESISTANCE TO BLOOD FLOW (with a sphincter, not complete vasoconstriction) more fluid will more out of the blood and into kidney tubules. Less fluid in the blood DECREASES blood pressure. This is done by constricting the efferent arteriolar sphincter and relaxing the afferent arteriolar sphincter.

If your BP is too low then the afferent sphincter constricts and the efferent sphincter relaxes. This DECREASES THE RESISTANCE TO BLOOD FLOW OVER THE GLOMERULUS, WHICH DECREASED HYDROSTATIC PRESURE IN CAPILLARIES AND DECREASES GFR. Fluid will remain in the blood stream and raise BP.

These sphincters are smooth muscle and respond to passive stretching by contracting. So when there's an increase in BP, they constrict and a decrease causes them to relax.

I hope this helps. If you need more help let me know. I hope your thread doesn't get closed on you because it's not an MCAT question lol!

 

[DrMattOglesby]

ya i had a problem with this too.
I kept on thinking that because we have vasoconstriction--that BP should increase (and thus raise GFR too).
However, I was told by my physio prof to simply think of it as a garden hose...and to imagine pinching the hose. You would notice that the flow of water coming out at the other end of the hose would be a lot lower. And that the pressure on the other side of the hose would be a lot higher. Simple as that!
good job in explaining it AirPlay 👍

 

[Vihsadas]

Nice explanation Ari, and nice analogy BS.

I guess the issue here is thinking about the difference between general and local BP. If you're thinking about "vasoconstrction", well, you have to ask "Do I mean vasoconstriction in a general sense, or vasoconstriction in one specific local area"? Because the answer will be different depending on what's happening.

Now, interms of GRR, it is largely determined by the local effects on the BP of the glomeruli by altering the efferent and afferent arterioles. If you use BS's garden hose analogy then you can figure out what will happen depending on what you do to each side of the hose:
Vasodilate efferent: less fluid in the filtering area = less GFR
Vasoconstrict efferent: more fluid in the filtering area = greater GFR
Vasodilate afferent: more fluid in the filtering area = greater GFR
Vasoconstrict afferent: less fluid in the filtering area = less GFR

In terms of general effects on BP, you should think what the body is going to try to do in a general sense to maintain BP homeostasis, and furthermore how the kidneys will help to that effect.

i.e. If you have low overall blood pressure, the body is going to want to retain more fluid to increase BP. It will do this by decreasing GFR. A reduction in GFR can happen two ways: Vasodilating the efferent arteriole (more blood exits the glomeruli capillary before it can be filtered), or constricting the afferent arteriole (less blood enters the glomeruli so less is filtered).

If you have high overall blood pressure, then the body will want to remove some blood volume by increasing GFR. Then there will be the opposite effects on the afferent and efferent arterioles.

 

[DrMattOglesby]

...and nice analogy BS. ...If you use BS's garden hose analogy then you can ...

hey!
Im MrMatt!
not bloody surgeon!

 

[Vihsadas]

hey!
Im MrMatt!
not bloody surgeon!

oh lol. Stupid avatars confusing me. (Not that that excuse makes any sense.) 😛

 

[90210]

Nice explanation Ari, and nice analogy BS.

I guess the issue here is thinking about the difference between general and local BP. If you're thinking about "vasoconstrction", well, you have to ask "Do I mean vasoconstriction in a general sense, or vasoconstriction in one specific local area"? Because the answer will be different depending on what's happening.

Now, interms of GRR, it is largely determined by the local effects on the BP of the glomeruli by altering the efferent and afferent arterioles. If you use BS's garden hose analogy then you can figure out what will happen depending on what you do to each side of the hose:
Vasodilate efferent: less fluid in the filtering area = less GFR
Vasoconstrict efferent: more fluid in the filtering area = greater GFR
Vasodilate afferent: more fluid in the filtering area = greater GFR
Vasoconstrict afferent: less fluid in the filtering area = less GFR

In terms of general effects on BP, you should think what the body is going to try to do in a general sense to maintain BP homeostasis, and furthermore how the kidneys will help to that effect.

i.e. If you have low overall blood pressure, the body is going to want to retain more fluid to increase BP. It will do this by decreasing GFR. A reduction in GFR can happen two ways: Vasodilating the efferent arteriole (more blood exits the glomeruli capillary before it can be filtered), or constricting the afferent arteriole (less blood enters the glomeruli so less is filtered).

If you have high overall blood pressure, then the body will want to remove some blood volume by increasing GFR. Then there will be the opposite effects on the afferent and efferent arterioles.

Omg...I love u for this post. I was trying to figure out why the hell someone with increased blood pressure would want an increased glomerulus pressure/GFR (and vice versa) and what you said totally makes sense. I think I get it now but I have an additional question. So for someone with low BP for instance and therefore, decreased GFR (to retain blood plasma) - the macula densa cells will also detect a decrease in NaCl and the JGC's a decrease in flow rate causing release of renin and eventual activation of angiotensin II. AII has multiple effects to increase blood pressure. (On the kidney, it also causes efferent arteriole constriction to increase GFR. I don't know if this is a feedback mechanism or what because it seems contradicting). Also AII stimulates release of aldosterone which allows for increased retention of sodium (excretion of potassium) ..increasing the osmolarity of blood and eventually activating osmo receptors in hypothalamus to release ADH and thereby allow retention of water, which increases pressure. I hope that's right.