Understanding how the nephrons of the kidney concentrate urea in urine is a particularly challenging concept for students of organismal biology/anatomy. I developed a laboratory exercise using dialysis bags to replicate the progression of filtrate through the nephron. Dialysis bags are permeable to small molecules like water and salt, but not sugar. Since the descending loop of Henle and part of the collecting duct are permeable to water, but not salt, I replaced sugar with salt for this part of the simulation. Students start with a dialysis bag of known volume and concentration of solute (10mL, 10% sugar) placed in 50% sugar water simulating the descending loop of Henle. The new volume and concentration is determined, and a new dialysis bag of the same volume and concentration of solute (salt) is placed in a beaker of 100% water simulate the ascending loop of Henle. The volume is assumed to not change and salt concentration is determined using a salinity probe. The final bag then is of the same volume and solute concentration (sugar) and placed in 50% sugar water to simulate the collecting duct. Students track the concentration of salt and urea (hypothetical) and water volume through the experiment.
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