Transport of other molecules

Question 1

The highest rate (e.g., mg/min) at which the renal tubules can transfer a substance either from the tubular luminal fluid to the interstitial fluid or from the interstitial fluid to the tubular luminal fluid.

Answer 1

Transport maximum

Question 2

T/F: Typically all glucose goes back into the body in the proximal tubule.

Answer 2

True

Question 3

Tx = absorption rate. What is the formula?

Answer 3

Tx = filtered load - excretion rate

Question 4

What is the equation for filtered load?

Answer 4

GFR * Px (concentration in the plasma)

Question 5

What is the equation for excretion rate?

Answer 5

Ux (concentration in the urine) * V (urine flow rate)

Question 6

Reabsorption of glucose continues to increase as plasma concentration increases until it is eventually seen in the ______.

Answer 6

Urine (excretion)

Question 7

So, below the Tm of glucose, is glucose seen in the urine?

Answer 7

No! It is reabsorbed!

Question 8

In a plasma concentration vs. excretion graph of glucose, where does Tm occur?

Answer 8

Above the point where the line curves up linearly and exponentially. Not when the first drop of glucose is seen in the urine. It’s referred to as splay.

Question 9

An example of an antidiabetic drug used to block the reabsorption of glucose so that it may be excreted

Answer 9

Phlorizin

Question 10

Explain the effect of a glucose transport inhibitor

Answer 10

Typically, you don’t begin to excrete glucose in the urine until you reach Tm and max out the transporter for reabsorption. However, with a glucose transport inhibitor, you can get glucose to be excreted at a lower plasma concentration

Question 11

Why is urine flow increased with diabetes?

Answer 11

Because glucose and sodium are transported together. So, if you have a large conc of glucose in the urine, you will also have more salt. As a result, more water will be pulled in. High glucose concentration will retain water in the lumen of the tubule

Question 12

If a molecule has a lesser clearance than inulin, what could explain this?

Answer 12

It is being reabsorbed

Question 13

If a molecule has a greater clearance than inulin, what could explain this?

Answer 13

It is being secreted

Question 14

Why is inulin a flat line on a clearance scale?

Answer 14

If it gets into the tubule, it is excreted. 100% of the filtered load is excreted.

Question 15

When PAH enters a cell, it is accompanied by an _______ going the other direction.

Answer 15

Anion

Question 16

This molecule is used as a transport blocker

Answer 16

Probenecid

Question 17

What’s happens to PAH when concentration is below Tm? What about when it is greater than Tm?

Answer 17

100% of it will be excreted in the urine and secretion will occur; if it is above Tm, maximum amount of PAH is being secreted, so some will go back into the blood stream

Question 18

When PAH enters the plasma, will it be found immediately in the urine?

Answer 18

Yes

Question 19

So, compared to the curve of inulin clearance which is a straight and steady line, explain the clearance curve for PAH as its concentration in the plasma increases.

Answer 19

The curve will have a decreased slope once it hits Tm, because some of the PAH will have to be reabsorbed into the bloodstream.

Question 20

Is clearance higher when the plasma concentration is above or below Tm?

Answer 20

Clearance is higher when plasma concentration is below Tm, because then it is 100%.

Question 21

Inulin becomes _______ as concentrated in the urine as it was in the plasma

Answer 21

125 times

Question 22

PAH becomes ______ as concentrated in the urine as it was in the plasma

Answer 22

585 times

Question 23

In gout, this is blocked by a transporter, so that uric acid cannot get out of body, and will build up to toxic levels

Answer 23

Probenicid

Question 24

T/F: Uric acid is both reabsorbed and secreted in the nephron.

Answer 24

True.

Question 25

About how much of urea is reabsorbed (percentage)?

Answer 25

60%

Question 26

Four factors which affect passive reabsorption of urea?

Answer 26

1. permeability of the tubule
2. tubular area (more room for reabsorption to occur)
3. concentration gradient
4. tubular flow (with too quick of flow, not much is going to be reabsorbed)

Question 27

Explain passive reabsorption of urea in the proximal tubule

Answer 27

So, in the proximal tubule, Na+ is leaving, because it is reabsorbed. Water follows it. This creates a greater concentration of urea in the tubule. Alas, it will be able to move down a gradient to be reabsorbed.

Question 28

The clearance of urea is dependent upon tubular flow, whereas that of __________ is not.

Answer 28

inulin

Question 29

When drinking a lot of water, explain what happens to the concentration of urea in the urine and the reabsorption rate.

Answer 29

With lots of water, there is a high urine flow rate. The concentration of urea in the urine will be lower, but there will be a higher clearance of urea, or less reabsorption. So, more dilute urine, but more effective at clearing it.

Question 30

What happens to urea concentration and clearance when you don’t drink water?

Answer 30

With less urine flow, the concentration of urea in the urine will be high. However, this will create that gradient allowing for more urea to seep back into the bloodstream, leading to a decreased clearance rate.

Question 31

Explain ion trapping. Will a molecule be more likely to be excreted in its ionized or un-ionized form?

Answer 31

Ionized!

Question 32

What is the approximate fractional excretion of the following?
Sodium 
Inulin
Creatinine 
Glucose
PAH 
Uric acid
Potassium

Answer 32

0.4%
100%
120% (secretion)
0% (all reabsorbed)
500% (5 times the filtered load)
10%
10-20%

Question 33

What can be used to calculate GFR?

Answer 33

CLEARANCE of inulin

Question 34

What can be used to calculate RPF?

Answer 34

CLEARANCE of PAH