THE PEE LAB

HUMAN OSMOREGULATION

When you walked into the lab your kidneys had done their best to ensure that your ECF [Na], total osmolarity, and total volume were optimal. Now we are going to screw things up by having you partake of different beverages. At each bench one person will imbibe one of the the following:

• 800 ml of delicious tap water: imposes a heavy water load which dilutes the ECF and increases its total volume
• 800 ml of 0.9 % NaCl. This matches your total ECF osmolarity precisely so it should not alter your ECF osmolarity when absorbed by the gut. It does however impose both a water load and NaCl load which will act to increase your total ECF NaCl and volume (high blood pressure anyone?).
• 80 ml of 9 % NaCl (or 8 g worth of salt tablets): imposes a heavy salt load, but should not affect your total ECF volume
• 800 ml of the most heavily caffeineated soft drink I can find (Jolt Cola?). Caffeine is a diuretic (mechanism of action?), and should increase urine flow. The soft drink itself contains relatively little NaCl and should not materially affect total body NaCl.

At 40 and 80 minute intervals after taking the drink, you will retire to the john and collect all of your available urine in a beaker. For each of these samples you will determine the urine flow rate (using highly sophisticated apparatus) in ml/min (=total volume of urine collected - measured with a graduated cylinder/40) and osmolarity using the freezing point depression osmometer; this device freezes a 50 µl sample of your urine and then determines the temperature at which it melts = freezing point. The important thing to remember here is that for each osmole/L in a solution, the freezing point is lowered by -1.8 degrees C (colligative properties of solutions). The osmometer will actually readout your urine conentration in mosmoles/L via a digital display. The business end of the osmometer consists of two probes which are immersed in the 50 µl urine sample (which is placed in a 1.5 ml microtube with a 50 µl pipettor). THE THING YOU HAVE TO BE CAREFUL ABOUT IS NOT TOUCHING THE GLASS PROBE TO THE LEFT OF THE FIGURE - IT IS A THERMISTOR WHICH CAN BE EASILY DAMAGED. The other probe is a stir bar which buzzes to set off ice crystal formation in the supercooled urine sample.

You will enter the data (urine flow rate in ml/min and osmolarity) for each of the 40 and 80 minute samples in an Excel spreadsheet table which will be up on one of the computer lab Macs. Once the data are collected for the whole class, I will post them on my website with a link from this lab so that you can put it up on your site. IT IS THEN YOUR JOB TO USE EXCEL TO MAKE GRAPHS AND DO SIMPLE STATISTICS ON THE DATA.

Discussion can be brief and should center on whether the data conform to predictions.

I will post other images on this page which you can download and use for your web site.

• Stull montage for physiology lab notebook
• Data for 11-2 lab
• Data for 2-5 lab
• NOTE: to get a copy of the data for your site, open the data table above in IE and select Save As - when the dialog box comes up, name the page appropriately (e.g., pee_labdata.html) and save as Source rather than text. Put the file in your web site folder and it will behave like a PageMill page file when you open your site.

You can use Microsoft Excel to analyze data and chart it. Here are a few sites which offer tutorials on Excel.

• From MIT. What could be better?
• From Florida Gulf Coast University, 1 and 2. These guys seem to be doing right by their students.
• From the horse's mouth, mighty Microsoft. Click the "analyzing data with Excel" link to download the tutorial.