Measuring GFR with inulin
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Measuring GFR with inulin
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[post_date] => 2024-12-23 18:39:21
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[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Kidney function is determined by measuring the glomerular filtration rate (GFR) – the volume of plasma that the kidneys filter through the glomeruli per unit time. The “gold standard” method for measuring GFR involves the use of inulin, a carbohydrate produced by plants.
Inulin is useful as an indicator of GFR because the kidneys handle it in a unique way. Unlike most other substances in the blood, inulin is neither reabsorbed into the blood after filtration nor secreted through peritubular capillaries. Thus, the amount of inulin cleared through the urine is indicative of the amount of plasma filtered by the body’s glomeruli. GFR can be calculated using Equation 1.
Equation 1 Glomerular filtration rate calculation
Urine concentration refers to the concentration of inulin in a sample of urine, urine flow refers to the amount of urine produced in a given time period, and plasma concentration refers to the concentration of inulin in blood plasma after intravenous injection.
Researchers investigated how GFR was affected by the “loop diuretic” furosemide. Loop diuretics, including furosemide and bumetanide, bind to the co-transporter NKCC2 located in the luminal membrane of the thick ascending limb of the loop of Henle. By inhibiting NKCC2, loop diuretics disrupt this ion transport and reduce the reabsorption of cotransported ions. Researchers observed the clearance rate of inulin in three groups of healthy cats. The results are shown in Table 1.
Table 1 Measurements taken in control and experimental groups
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[question] => The study supports what conclusion about the effects of furosemide on GFR and inulin clearance in cats?
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[answer] => 4
[description] => Reason for Correct Answer:
Inulin is filtered through the glomeruli, but not reabsorbed or secreted. This makes inulin clearance directly proportional to glomerular filtration rate (GFR) (i.e. when GFR increases, inulin clearance increases, and vice versa).
To know how furosemide affected GFR/inulin clearance, compare Group B and Group C.
Without actually performing calculations, we can determine the relationship between GFR for Group B and GFR for Group C. Group B has double the urine concentration of inulin and half the urine output, which, according to Equation 1, would make GFR equal for the two groups.
Therefore, the study suggests that furosemide has no effect on GFR and therefore no effect on inulin clearance in cats. (Note, this does not mean that these results are accurate, and other studies show furosemide having an effect on GFR.)
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[each_answer] => A. It increases inulin clearance by preventing reuptake by the kidney.
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[each_answer] => B. It increases GFR by increasing renal blood flow.
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[2] => Array
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[each_answer] => C. It decreases GFR by decreasing renal blood flow.
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[each_answer] => D. It has no effect on GFR or inulin clearance.
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[question] => Once filtered, through which structures does inulin travel (in order)?
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[answer] => 1
[description] => Reason for Correct Answer:
Inulin is filtered into the Bowman’s capsule of the nephron. The filtrate flows out of the glomerulus (a collection of blood vessels) and into this capsule, which leads into the tubule system.
From there, filtrate flows from the glomerulus to the proximal tubule and then down the loop of Henle.
The filtrate proceeds back up the loop of Henle into the distal tubule and then to the collecting duct.
The correct answer is therefore Bowman’s capsule -> proximal convoluted tubule -> loop of Henle -> distal convoluted tubule -> collecting duct.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Bowman’s capsule -> proximal convoluted tubule -> loop of Henle -> distal convoluted tubule -> collecting duct
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[each_answer] => B. Bowman’s capsule -> proximal convoluted tubule -> distal convoluted tubule -> loop of Henle -> collecting duct
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[each_answer] => C. Efferent arteriole -> proximal convoluted tubule -> loop of Henle -> distal convoluted tubule -> collecting duct
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[each_answer] => D. Efferent arteriole -> proximal convoluted tubule -> distal convoluted tubule -> loop of Henle -> collecting duct
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[question] => In the nephron, blood flows from the afferent arteriole, to the glomerulus, to the efferent arteriole, and to the peritubular capillaries. Its position between the glomerulus and peritubular capillaries makes the efferent arteriole anatomically similar to a:
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The glomerulus is a cluster capillaries.
Typically, there is only one bed of capillaries between two larger blood vessels, but in the vascular system of the nephron, there is a blood vessel between two capillary beds.
https://pressbooks.ccconline.org/bio106/chapter/urinary-structures-and-functions/
When a capillary bed recollects into a larger vessel and then divides into another capillary bed before returning to the heart, this is called a portal system. The efferent arteriole connects two capillary beds and therefore resembles a portal blood vessel. Here is an example of the portal vein connecting lower abdominal capillary beds to capillaries in the liver:
https://www.sciencedirect.com/topics/neuroscience/hepatic-portal-vein
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[each_answer] => A. Portal blood vessel
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[each_answer] => B. Thoroughfare channel
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[each_answer] => C. Sinusoidal capillary
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[each_answer] => D. Fenestrated capillary
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[question] => Based on the measurements in Table 1, which substance(s) could be used to increase GFR?
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[answer] => 2
[description] => Reason for Correct Answer:
In Table 1, Group A received no treatment; Group B received only intravenous saline solution; Group C received both intravenous saline and furosemide.
We can use Equation 1 to calculate the GFR for each group, but doing so would take up precious time. Instead, try to use the basic properties of the equation to see relationships between the GFRs of the three groups.
Since the starting plasma inulin concentration is the same in all three groups, we can ignore this data. Comparing Group B to Group A, notice that urine concentration is cut in half while urine output quadruples. Taken together, this means the GFR doubled when intravenous saline was administered.
The only thing left to determine is whether furosemide also increased GFR, which we can do by comparing the GFRs for Groups B and C. The urine concentration of Group C is cut in half, while the urine output doubles.
This means that furosemide had no effect on GFR in this experiment. Only intravenous saline could be used to increase GFR.
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[each_answer] => A. Inulin
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[each_answer] => B. Intravenous saline
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[each_answer] => C. Furosemide
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[each_answer] => D. Either intravenous saline or furosemide
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[question] => The researchers would also like to measure the effects of diuretics on total renal plasma flow. To approximate renal plasma flow based on a urine sample, they would want to measure the urine concentration of a substance that is treated by the kidneys in which way?
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[description] => Reason for Correct Answer:
Reabsorption is when a substance is taken back into the blood from the filtrate. Secretion is when a substance is transferred into the filtrate after blood has already passed the glomerulus.
When the glomeruli filter plasma, only about 20% goes through, while the rest stays in the blood. Therefore a substance that is only filtered doesn’t give us an accurate approximation of renal plasma flow. The rest of it flows through the peritubular capillary network, which wraps around the nephrons.
Here is a schematic of filtration into the nephron (from the glomerulus), reabsorption (into the blood), secretion (from the blood), and excretion of urine.
To better approximate renal plasma flow, the ideal substance for the researchers would be one that is entirely deposited into the urine after one circulation through the kidneys.
For this purpose, you want the substance to be filtered through the glomerulus and secreted into tubules from the peritubular capillaries.
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[each_answer] => A. Filtered only
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[each_answer] => B. Filtered and reabsorbed
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[each_answer] => C. Filtered and secreted
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[each_answer] => D. Reabsorbed and secreted
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[quiz_unique_key] => 1997864699
[question] => What is a likely side effect of loop diuretics like furosemide?
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[answer] => 2
[description] => Reason for Correct Answer:
The passage states that loop diuretics like furosemide bind to the co-transporter NKCC2 located in the luminal membrane of the thick ascending limb of the loop of Henle, preventing reabsorption of ions.
You should know from your studies that sodium and chloride are both actively reabsorbed in the thick ascending limb of the loop of Henle.
Therefore, you can conclude that this cotransporter is likely involved in the reabsorption of chloride ions and sodium ions. (In fact, it is a Na/Cl/K cotransporter.)
Blocking this cotransporter could lead to low levels of sodium and chloride in the blood. Decreasing reabsorption in the ascending limb would also decrease the ionic gradients that form around the tubules, therefore reducing water reabsorption and effectively reducing blood pressure.
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[each_answer] => A. Hypernatremia, or high sodium levels in the blood
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[each_answer] => B. Hypochloremia, or low chloride levels in the blood
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[each_answer] => C. Increased blood pressure
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[each_answer] => D. Increased water reabsorption in the collecting duct
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Equation 1
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