Gauss44

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Why is it that when the afferent arteriole is constricted, the glomerular filtration rate is reduced?

#43 Answer Key: Sympathetic innervation of the kidneys primarily affects the afferent arterioles. This innervation constricts the afferent arterioles and reduces the glomerular filtration rate. The urine output will be dramatically reduced.

Why reduced and not increased?
 

Ibn Alnafis MD

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Why is it that when the afferent arteriole is constricted, the glomerular filtration rate is reduced?

#43 Answer Key: Sympathetic innervation of the kidneys primarily affects the afferent arterioles. This innervation constricts the afferent arterioles and reduces the glomerular filtration rate. The urine output will be dramatically reduced.

Why reduced and not increased?
Sympathetic PNS is involved in flight-fight mode whereas the parasympathetic is involved in rest and digest, remember?

So, it wouldn't make sense if one is in a life or death situation to have his urinary system functioning at a high levels. This energy would be conserved to more "important" functions ie. heart rate and breathing. Therefore, a sympathetic innervation constrict the blood flow to "less important" parts of the body in order to supply "more important" parts with nutrition and oxygen.

Note: my use of "more/less important is relative to the situation.
 

slz1900

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If you've got constriction, less blood is getting through (remember, blood isn't an ideal fluid), so less filtrate is entering bowmann's capsule, which means a lower gfr.
 
Jul 24, 2012
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Why is it that when the afferent arteriole is constricted, the glomerular filtration rate is reduced?

#43 Answer Key: Sympathetic innervation of the kidneys primarily affects the afferent arterioles. This innervation constricts the afferent arterioles and reduces the glomerular filtration rate. The urine output will be dramatically reduced.

Why reduced and not increased?
Both of the answers above are good, but I have a little to add that might help.

Afferent Arteriole--->Glomerulus--->Efferent Arteriole

Above is the route that blood takes as it flows to and then away from the glomerulus. If you constrict the "pipe" that leads the fluid (blood) into the filter (glomerulus), then less fluid will be filtered = less urine production.

Dr. Leonardo Noto

www.leonardonoto.com
 

Meredith92

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This is a related question but here we say that constriction leads to decreased blood flow to the kidney and that this happens for a sympathetic response

BUT I thought that a vasoconstrictor raises blood pressure which leads to more blood flow?

I may be getting things confused but I was looking up how epinephrine is a vasoconstrictor so that it can increase blood flow/ heart rate. So just to clarify does vasoconstriction increase or decrease blood flow?
 
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This is a related question but here we say that constriction leads to decreased blood flow to the kidney and that this happens for a sympathetic response

BUT I thought that a vasoconstrictor raises blood pressure which leads to more blood flow?

I may be getting things confused but I was looking up how epinephrine is a vasoconstrictor so that it can increase blood flow/ heart rate. So just to clarify does vasoconstriction increase or decrease blood flow?

The sympathetic nervous system (and it's hormonal accomplice in crime, epinephrine) generally causes vasoconstriction but this varies substantially from tissue to tissue. Constriction of blood flow to visceral organs (the digestive tract, kidneys, liver, etc) decreases blood flow to these organs during the fight or flight response so that blood flow can increase to the muscles that you'll be needing to use to run away from the cannonical bear in the woods or to fight it.The effect on the particular tissue depends on the type of receptor that predominates in a tissue: Alpha 1 = vasoconstriction in smooth muscle. Beta 1 causes heart muscle to pump harder and faster. Beta 2 causes the smooth muscle in the broncioles to relax (because you need to breath in more air) and the smooth muscle in the gut to relax (take a break intestines, you can get back to work if we don't get eaten by Mr. Bear)--activation of Beta 2 also causes dilation of blood vessels that lead to skeletal muscles, which is why your muscles get pumped up before you fight or when you're working out. The net effect of sympathetic stimulation on the circulation is to increase blood pressure, but some tissues get an increase in blood flow whereas others (like the gut and blood vessels of the skin) get a huge decrease in blood flow during the fight-or-flight response. Another important aspect of the fight-or-flight response is pupillary dilation (you want to see the bear).

Blood Flow = Change in Blood Pressure/Resistance

Generally speaking, an increase in resistance (constriction of a blood vessel) --> decreased blood flow to that organ. The sympathetic response constricts the blood vessels that lead to organs that are nonessential during a fight, increases blood pressure, dilates the blood vessels of tissues that help you fight--the increase in blood pressure and the dilation of blood vessels that lead to skeletal muscle, etc lead to greatly increased blood flow to the organs that help you to flee or to fight.

Dr. Leonardo Noto
www.leonardonoto.com
 

Meredith92

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Thanks for your post! That was very helpful for understanding where constriction/dilation happens.

But I'm still confused on whether it is constriction of dilation that results in a higher blood pressure. I've been reading past posts and it seems like constriction increases blood pressure because to maintain a constant blood flow, more pressure is needed. However, it seems like you are saying that dilation is what increases blood flow.
 

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Constriction of the afferent arteriole increases the pressure upstream and decreases the pressure downstream.
Think of it as in a pipe.
So both glomerular filtration rate and renal blood flow will decrease (downstream)