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Q6 - Renal Clearance and Glomerular Filtration

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Comparative Osmoregulation & Excretion

Inulin is an inert 4 kDa polymer. Dextran is an inert 500 kDa polymer. Both are handled by the kidney as you would expect for non-actively transported molecules of their size.

Para-amino Hippurate (PAH) is unusual because it is completely removed from plasma as it moves past a nephron (i.e., it is actively transported into the urine).

All molecules were tagged with different color fluorescent dyes and injected by intravenous injection into pigs. The injection was a one-off and contained 0.75 milliMoles of each molecule.

Plasma concentration of each molecule over time was then measured with a fiberoptic fluorescence imager inserted into a blood vessel.

Three exponential decay curves on a graph with Y (Concentration in mM) on the y-axis and X (Time in min) on the x-axis. The top curve (dextran) barely declines from 0.1. The middle curve (inulin) crosses 0.05 at t = 231 min. The steepest curve (PAH) crosses 0.05 at t = 34.7 min. Y = Concentration (mM), X = Time (min). Dotted lines mark key half-life reference points at t = 34.7 and t = 231.

The equation for exponential decay is: A = A₀ e^(-kt)

Which can be rearranged as: kt = ln(A₀/A)

Where A = the amount remaining, A₀ = the initial amount, k = the chance that each molecule is lost per unit time, t = time. Note: ln(2) ≈ 0.693.

Q6.1. Calculate the plasma volume of the pig. Give your answer in liters to one decimal place.
Q6.2. Calculate the renal plasma flow of the pig (how much plasma the kidneys receive). Give your answer in ml/min to the nearest ml. Assume plasma volume = 10 L.
Q6.3. Calculate the glomerular filtration rate of the pig (how much plasma the Bowman's capsule excretes). Give your answer in ml/min to the nearest ml. Assume plasma volume = 10 L.
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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF