Tubule Reabsorption Experiment

The aim of this experiment is to measure the volume of liquid being reabsorbed through the epithelial walls of renal tubule during the filtration process in the kidney.

The overall principle is to perfuse the tube on one end with a saline solution and measure the resulting volume collected at the other end. The difference in volume would tell if the perfusate gained or lost volume traveling along the length of the tubule. In the experiment, the lost volume would go into the bathing saline surrounding the tubule, but in real life it would be reabsorbed back into the body.

Using micromanipulators and a microscope, thin pipettes are inserted into both ends of the tubule. One pipette will inject the perfusate saline and the other, the collecting pipette, is filled with a red-dyed light oil to facilitate seeing the meniscus marking the division between the oil and the perfused saline coming out of the tubule.

An illustration of perfusion fluid flow through a tubule. Where the perfusion fluid starts flowing, there are two labels indicating the concentration and volume that goes in. The flow of the fluid is indicated with an arrow, which point to the other end of the tubule, where the labels "oil" concentration out and volume out can be seen. The tubule's wall is depicted, including the inner endothelial layer. One arrow pointing towards the inside of the tubule's interior and another pointing from it are labeled with a question mark.

Figure 1. Perfusion

However, because it is difficult to measure accurately such small volumes of liquid, the measurement of volumes will be made using a radioactive agent included in the perfusate saline and unable to be reabsorbed by the tubule.

If we know the final volume collected in the collection pipette Vout, the initial and final concentrations of radioactive compound Cin and Cout, using the simple equivalence Cin * Vin = Cout * Vout we can deduce the initial volume Vin necessary to reach a Vout final volume, the initial and final concentrations of radioactive compound, using the simple equivalence initial concentration times initial volume equals final concentration times final volume, we can deduce the initial volume necessary to reach a final volume of 1 nanoliter in the collection tube.

Then simply by subtracting the collected volume Vout from the entering volume Vin from the entering volume, we can measure how much volume was lost in traveling the tubule, sometimes called the reabsorption rate R.

To calculate these concentrations Cin and Cout initial and final concentrations, the radioactivity of the compound in the sample is measured using a liquid scintillation counter.