Question about filtration/osmotic pressure

[deleted647690]

I got this answer correct, but I'm not sure why C isn't also correct. Here is my reasoning for each option.

A was wrong because if you increase the input of fluid, it will increase the hydrostatic pressure in the dialysis chamber, thus increasing outflow.

D was wrong because if there is increased pressure in the dialysis unit, there will be higher pressure pushing against the walls, thus there will be more fluid flow out into the drainage pipe, carrying toxins and thus filtering to a greater extent

I figured B was the answer, because if hydrostatic pressure in the dialysis chamber is increased, and more fluid volume is flowing out into the drainage pipe/dialysate fluid pipe (see figure), that will decrease the concentration of proteins because the amount of solute is remaining the same while the amount of solvent is increasing. Thus B is the option that is not true.


For C, wouldn't the concentration of proteins in the outflow decrease due to the fact that A is true? If there is higher fluid outflow, that would decrease the concentration of proteins, thus, why isn't C also correct?

Their reasoning for why C wasn't the option was that proteins are too large to pass through the membrane, and thus the osmotic concentration wouldn't change. But still, wouldn't the increased fluid volume passing into the outflow still decrease the osmotic concentration, even if the amount of protein isn't changing? The amount of solvent is changing, thus the concentration should change in my opinion.

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[NextStepTutor_3]

Hi there, good question! The renal system is always tough.

The reason why C is wrong is that the question stated that the "blood inflow rate" was increased. This means that more blood - including both more fluid and more proteins (plus more dissolved solutes/toxins/ions) is moving through the blood inflow and through the dialysis apparatus. In other words, if we doubled the blood inflow rate (for example), we'd expect twice the amount of fluid and twice the amount of proteins to enter through the blood inflow. Before we reach the dialysis membranes, then, the protein concentration should still be the same as before even though the amount of fluid has increased.

As we pass the dialysis membranes, the high pressure forces significant amounts of fluid and small solutes through the membranes and out via the drain. Like you said, proteins are too large to pass through the membranes, so they remain. Thus all of the original proteins should reach the blood outflow, but since some of the fluid leaves through the membranes, the protein concentration effectively increased.

In summary - I think your mistake was thinking that just more fluid was entering the apparatus, so this larger amount of solvent would lower protein concentration. But we are pumping more "whole blood" through the apparatus, not just more fluid, so we are effectively sending in more protein as well.

Let me know if any of this was unclear! And good luck

 

[deleted647690]

Thanks for the response!

So the protein that gets retained on the dialysis membranes in the chamber and not passed into the dialysate tube (leading to the waste pump) goes back into the body? For some reason, I thought the protein would remain stuck on the dialysis membranes. I'm a little confused because the passage says that the "multiple dialysis membranes" are there to prevent protein from passing. How is the protein passed back into the patient if these membranes are present? I attached the relevant part of the passage.

And I thought B was the correct answer (IE, it was not true) because the proteins won't pass into the dialysate fluid tube anyway, so that is independent of the inflow going into the dialysis apparatus

Would D be true just due to the fact that with a higher pressure in the dialysis chamber, there would be more toxins/waste being passed with the fluid into the dialysate tube to the drain?

I think I'm totally confused by the diagram. Which way are the "multiple dialysis membranes" retaining things from passing? I thought they were preventing passing of solutes directly going left from the inflow to the outflow, and there was technically a 'second membrane' that was regulating passage of waste products into the dialysate fluid tube.

 

[deleted647690]

And their wording confuses me, "After the solutes have passed through the membrane, they are washed away in the dialysate fluid; excess fluid and waste products are thus removed from the patient's bloodstream." If they "pass through the membrane", that makes me think the waste products are being cleared past the membrane, into the outflow, back into the patient's blood. Shouldn't it say something like, "the solutes are retained by the membrane, which is then washed into the dialysate fluid."

 

[NextStepTutor_3]

These are great questions!

I see how the diagram could be confusing on this point, but the dialysis membranes aren't there to block anything horizontally, including protein. When the passage mentions "passing through the membrane," they mean passing into the dialysate fluid (which then leads to the drain and thus leaves the patient's body permanently). Large molecules are prevented from doing this, so they do not enter the dialysate fluid and are not lost from the body. But even large molecules can pass horizontally through the membranes from the inflow to the outflow.

The part of the passage that best tells us this is the sentence "After the solutes have passed through the membrane, they are washed away in the dialysate fluid." So making it through the membrane must refer to entering the dialysate fluid, NOT passing through horizontally to the outflow.

Alternatively, even if the diagram/passage are confusing, we can think about the purpose of such a procedure. We use this apparatus for patients with kidney failure, who cannot properly clear toxins from the body. We would thus want this apparatus to remove toxins or solutes present in excessive concentrations, while sending everything present in healthy blood back into the patient. The apparatus must not remove all proteins because proteins (for example, albumin) are a necessary component of blood. So proteins must pass into the outflow and get returned to the patient.

The entire apparatus resembles how you might picture a capillary moving through tissue. As blood passes through the capillary, excess small solutes move down their concentration gradient, through the capillary wall and out into the tissue. Proteins are too large to pass through capillary walls, so they remain within the blood vessel.

Does this help answer your questions? If anything is still unclear, let me know!

 

[deleted647690]

These are great questions!

I see how the diagram could be confusing on this point, but the dialysis membranes aren't there to block anything horizontally, including protein. When the passage mentions "passing through the membrane," they mean passing into the dialysate fluid (which then leads to the drain and thus leaves the patient's body permanently). Large molecules are prevented from doing this, so they do not enter the dialysate fluid and are not lost from the body. But even large molecules can pass horizontally through the membranes from the inflow to the outflow.

The part of the passage that best tells us this is the sentence "After the solutes have passed through the membrane, they are washed away in the dialysate fluid." So making it through the membrane must refer to entering the dialysate fluid, NOT passing through horizontally to the outflow.

Alternatively, even if the diagram/passage are confusing, we can think about the purpose of such a procedure. We use this apparatus for patients with kidney failure, who cannot properly clear toxins from the body. We would thus want this apparatus to remove toxins or solutes present in excessive concentrations, while sending everything present in healthy blood back into the patient. The apparatus must not remove all proteins because proteins (for example, albumin) are a necessary component of blood. So proteins must pass into the outflow and get returned to the patient.

The entire apparatus resembles how you might picture a capillary moving through tissue. As blood passes through the capillary, excess small solutes move down their concentration gradient, through the capillary wall and out into the tissue. Proteins are too large to pass through capillary walls, so they remain within the blood vessel.

Does this help answer your questions? If anything is still unclear, let me know!

Thank you for your help and detailed responses. I think I understand now.
A is wrong because increased inflow would increase hydrostatic pressure and thus cause downstream increases in pressure
B is correct because proteins do not pass into the dialysate fluid, since they are retained by the membrane
C is wrong because, with higher inflow, there is more hydrostatic pressure. Thus, we should expect to lose more water/fluid into the dialysate? And retain the new, larger amount of protein. Thus, if anything, the concentration of protein would increase.
D is wrong because the movement of toxins would increase along with the hydrostatic pressure, which is higher. I guess since this is true, this also addresses my question in the line above this. We should expect more water loss with higher hydrostatic pressure. Since we are talking about whole blood, the relative amount of protein to water ratio is the same coming into the inflow at the start of the dialysis apparatus; therefore, with more water loss, we should expect higher concentration of protein on the outflow end.