BRS Renal question

[Oh_Gee]

At plasma concentrations of glucose higher than occur at transport maximum (Tm), the

(A) clearance of glucose is zero
(B) excretion rate of glucose equals the filtration
rate of glucose
(C) reabsorption rate of glucose equals the
filtration rate of glucose
(D) excretion rate of glucose increases with
increasing plasma glucose concentrations
(E) renal vein glucose concentration equals
the renal artery glucose concentration

answer below



The answer is D [III B; Figure 5-5]. At concentrations greater than at the transport maximum
(Tm) for glucose, the carriers are saturated so that the reabsorption rate no longer
matches the filtration rate. The difference is excreted in the urine. As the plasma glucose
concentration increases, the excretion of glucose increases. When it is greater than the
Tm, the renal vein glucose concentration will be less than the renal artery concentration
because some glucose is being excreted in urine and therefore is not returned to the
blood. The clearance of glucose is zero at concentrations lower than at Tm (or lower than
threshold) when all of the filtered glucose is reabsorbed, but is greater than zero at concentrations
greater than Tm.

why is B not the right answer too? check out the graph here (http://images.slideplayer.com/31/9741800/slides/slide_23.jpg) the filtration and excretion slopes are equal to each other after Tm is reached.

also i feel like D is not that correct. even if plasma [glucose] increases, would excretion rate really increase? i think that excretion rate would depend on how much glucose can be filtered (filtration rate). would a higher plasma [glucose] increase filtration rate?

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[68PGunner]

Yup, B makes perfect sense to me.

Tm is around 375 for glucose. That means that any filtrated # over 375 will be excreted. Let's say that you have 400 glucose filtered through the glomerulus. Renal threshold for glucose is around 200, so you might see minimal glucose over 200 due to the heterogeneity of diff glucose transporters. However, at 375, all of these glucose transporters will be fully saturated. So, your urine is have 25 glucose excreted assuming the filtrated load being 400.

Excretion = filtrated load - reabsorption load

 

[68PGunner]

At plasma concentrations of glucose higher than occur at transport maximum (Tm), the

(A) clearance of glucose is zero
(B) excretion rate of glucose equals the filtration
rate of glucose
(C) reabsorption rate of glucose equals the
filtration rate of glucose
(D) excretion rate of glucose increases with
increasing plasma glucose concentrations
(E) renal vein glucose concentration equals
the renal artery glucose concentration

answer below



The answer is D [III B; Figure 5-5]. At concentrations greater than at the transport maximum
(Tm) for glucose, the carriers are saturated so that the reabsorption rate no longer
matches the filtration rate. The difference is excreted in the urine. As the plasma glucose
concentration increases, the excretion of glucose increases. When it is greater than the
Tm, the renal vein glucose concentration will be less than the renal artery concentration
because some glucose is being excreted in urine and therefore is not returned to the
blood. The clearance of glucose is zero at concentrations lower than at Tm (or lower than
threshold) when all of the filtered glucose is reabsorbed, but is greater than zero at concentrations
greater than Tm.

why is B not the right answer too? check out the graph here (http://images.slideplayer.com/31/9741800/slides/slide_23.jpg) the filtration and excretion slopes are equal to each other after Tm is reached.

also i feel like D is not that correct. even if plasma [glucose] increases, would excretion rate really increase? i think that excretion rate would depend on how much glucose can be filtered (filtration rate). would a higher plasma [glucose] increase filtration rate?

BTW, check your units. slope of the line would not be rate. it would be mg/ml. y axis is rate.

 

[Oh_Gee]

Yup, B makes perfect sense to me.

Tm is around 375 for glucose. That means that any filtrated # over 375 will be excreted. Let's say that you have 400 glucose filtered through the glomerulus. Renal threshold for glucose is around 200, so you might see minimal glucose over 200 due to the heterogeneity of diff glucose transporters. However, at 375, all of these glucose transporters will be fully saturated. So, your urine is have 25 glucose excreted assuming the filtrated load being 400.

Excretion = filtrated load - reabsorption load

so you also disagree with the book's answer?

 

[68PGunner]

so you also disagree with the book's answer?

No I agree w/ the book answer.

 

[68PGunner]

Think of it this way:

Glomerulus filtrates glucose at the rate of 400 mg/min. Glucose transporters at your PCT can only absorb at the max of 375 mg/min. Therefore, excretion rate is 25 mg/min. Given your answer B, it would be 400 mg/min. Do you see my point now? Your answer B would say that excretion rate would be 400 mg/min which is incorrect.

Therefore, increasing glucose filtrate rate from a higher plasma conc of glucose will increase glucose excretion rate. Let's say that your glucose filtrate rate is now 600 mg/min. Reabsorption rate is still 375 mg/min. Excretion rate would then be 225 mg/min. Your answer B would say that excretion rate would be 600 mg/min, which is incorrect.

 

[Aerus]

B makes absolutely no sense. The only time excretion equals filtration is if there is no reabsorption at all. We KNOW glucose is reabsorbed, so B should be the first answer you eliminate.

Think of the Glucose transporters like a "train" and think of Tm like its "train capacity". The goal of the train is to bring passengers to their destination, just like the goal for Glucose transporters is to bring Glucose back into the body. You don't want to excrete valuable glucose.

So up to around its Tm, it will reabsorb 100% of glucose. Makes sense. As long as the train has seats, it will take 100% of the waiting passengers. But once the [Glucose] reaches Tm, it's maxing out all the Glucose transporters for reabsorption. Your train is at capacity and cannot accept further passengers. Any more passengers past the train capacity who arrive at the waiting station will have to leave. Likewise, any further increase in Glucose will not be able to fit on the Glucose transporters (AKA your transporters are fully saturated) and it will be excreted proportionally with the additional increase past the Tm capacity.

 

[Instatewaiter]

At plasma concentrations of glucose higher than occur at transport maximum (Tm), the

(A) clearance of glucose is zero
(B) excretion rate of glucose equals the filtration
rate of glucose
(C) reabsorption rate of glucose equals the
filtration rate of glucose
(D) excretion rate of glucose increases with
increasing plasma glucose concentrations
(E) renal vein glucose concentration equals
the renal artery glucose concentration

answer below



The answer is D [III B; Figure 5-5]. At concentrations greater than at the transport maximum
(Tm) for glucose, the carriers are saturated so that the reabsorption rate no longer
matches the filtration rate. The difference is excreted in the urine. As the plasma glucose
concentration increases, the excretion of glucose increases. When it is greater than the
Tm, the renal vein glucose concentration will be less than the renal artery concentration
because some glucose is being excreted in urine and therefore is not returned to the
blood. The clearance of glucose is zero at concentrations lower than at Tm (or lower than
threshold) when all of the filtered glucose is reabsorbed, but is greater than zero at concentrations
greater than Tm.

why is B not the right answer too? check out the graph here (http://images.slideplayer.com/31/9741800/slides/slide_23.jpg) the filtration and excretion slopes are equal to each other after Tm is reached.

also i feel like D is not that correct. even if plasma [glucose] increases, would excretion rate really increase? i think that excretion rate would depend on how much glucose can be filtered (filtration rate). would a higher plasma [glucose] increase filtration rate?

B is wrong because of the filtred glucose, some of the is re-absorbed, some is excreted in the urine. Thereforethe excretion rate doesn't equat the filtration rate (*b/c some is reabsorbed. )

D is right. As the plasma glucose increases, more is excreted (although some is still reabsorbed)

 

[Brucelaa67]

I think you're getting confused between 'excretion rate of glucose equals the filtration rate of glucose' and 'excretion rate of glucose is proportional to the filtration rate of glucose' Latter is true based on the graph.
-To answer you last question, yes glucose is a freely filtered substance, so filtration rate of glucose is proportional to the plasma conc.. So D is correct. Its just another way of saying 'excretion rate of glucose increases with increasing filtration rate'.