AAMC 3 Bio #31 question

[coyfish]

Plasma clearance is affected by the tubular transport maximum (Tm) of a substance. The Tm is the maximum rate of transport (mg/min) at which a substance can be reabsorbed by the kidney. That is, if the filtration rate of a substance exceeds its Tm, the substance will begin to appear in the urine. The Tm for glucose averages 320 mg/min in an adult human.

The question: "Tm represents the rate of plasma filtration that just exceeds the . . . "

The answer is "capacity of kidney tubules to reabsorb the substance."

I'm confused because the passage states that the Tm is the maximum absorption rate. That means none will be in the urine unless it is exceeded. So why does the correct answer state that the Tm is the rate that exceeds kidney reabsorption?

It is probably obvious but I have read it so many times it is driving me nuts.

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[MCAT guy]

Plasma clearance is affected by the tubular transport maximum (Tm) of a substance. The Tm is the maximum rate of transport (mg/min) at which a substance can be reabsorbed by the kidney. That is, if the filtration rate of a substance exceeds its Tm, the substance will begin to appear in the urine. The Tm for glucose averages 320 mg/min in an adult human.

The question: "Tm represents the rate of plasma filtration that just exceeds the . . . "

The answer is "capacity of kidney tubules to reabsorb the substance."

I'm confused because the passage states that the Tm is the maximum absorption rate. That means none will be in the urine unless it is exceeded. So why does the correct answer state that the Tm is the rate that exceeds kidney reabsorption?

It is probably obvious but I have read it so many times it is driving me nuts.

I just took this and reviewed it, so I'll chime in...

So the Tm is the maximum rate of transport at which a substance can be reabsorbed by the kidney.

Pay attention to the wording there... it is the MOST I can GET BACK from the kidneys after I dump all my stuff in the kidneys.

Example, I have 600 glucose in my blood, well I will dump all of that into my kidneys, but my Tm will be in effect and I will be able to REabsorb (back into my blood from my kidneys) 320 of the glucose, leaving 280 of the glucose to the urine.

So your question, why does it say "just exceeds the capacity of the kidney tubules to reabsorb the substance"? Well that is exactly what it is, once I surpass the Tm, then my kidneys are done for the day. They can only reabsorb 320... The Tm is really just telling you the max workload of the kidneys.

 

[Isoprop]

the phrase "just exceeds" pretty much means "is equal to"

What value just exceeds 5?
5.1? 5.01? 5.001? For all practical purposes, it's just 5.

 

[MCAT guy]

the phrase "just exceeds" pretty much means "is equal to"

What value just exceeds 5?
5.1? 5.01? 5.001? For all practical purposes, it's just 5.

true dat.

 

[coyfish]

Alright thanks guys I think I see it now.

The Tm is basically right when the substance starts appearing in the urine. Meaning just exceeding the max. Tricky wording . . . was rereading that question for like 20 minutes.

Edit: by the way mcat guy how did you do. I missed 8 questions and I got an 11 . . . seems like if you miss one you are already down to like a 13 lol.